Combined Radiations: Biological Effects of Mixed Exposures Across the Radiation Spectrum

1. Introduction

Different types of radiation can be combined in various contexts, whether therapeutic or experimental; they may be applied to biological or non-biological targets, simultaneously or sequentially, with intervals between irradiations ranging from minutes to weeks. The dose of each combined radiation type is another important factor, as are other dosimetric features of a given experimental configuration. For biological targets, a number of endpoints (specific measurable biological outcomes) can be utilized to determine the induced effects, the specification of which is typically the main aim of most related research articles. “Combining different radiation types” thus constitutes a broad set of practices, involving a variety of methods developed and applied for distinct purposes.

Yet despite this diversity, no comprehensive review has systematically assembled the relevant literature according to clearly defined criteria for combined radiation exposures to date. In many respects, this review pursues a rather ambitious aim: to create a concise map of the published studies on the radiobiological and therapeutic utilization of combined radiation exposures, discussing and analyzing the qualitative and quantitative characteristics of this highly promising, long-standing set of practices. It aims to provide a succinct depiction of the diverse processes and outcomes that define the use of mixed irradiation in biological systems, reflecting our current scientific understanding of this multifaceted field.

2. Methodology

The challenges impeding an instantaneous compilation of all relevant publications required for such an extensive review arise from a range of factors, including the dispersion of publications across a wide variety of databases, shifting terminology and evolving epistemological frameworks. The selection criteria applied during the literature review are presented here to clarify the rationale behind the inclusion of each study in the compiled database. First, we determined which types of combined radiations to include—specifically, which wavelength and/or energy ranges were relevant. Our focus aligned with conventional and space radiobiology, as well as several combined therapeutic modalities. Consequently, other forms of mixed exposure, such as environmental exposure to extremely low-frequency electromagnetic fields (ELF-EMF) or microwave radiation in combination with other types, were intentionally excluded from this review. Similarly, combinations involving radiation and non-radiation agents, such as chemicals or microgravity, were also excluded, as they fall outside the scope of radiation–radiation interactions. In rare cases where non-radiation factors were present (e.g., the NASA Twins Study), the inclusion of those publications was primarily justified by their focused investigation of combined radiation exposures, with any additional variables considered secondary. Regarding the combinatorial nature of the exposures, studies involving the sequential application of radiations with time intervals longer than those typically defining combinatorial exposure (e.g., >24 h) were mostly excluded.

The radiation types included in the search were near-infrared (NIR), visible (VIS), ultraviolet (UV), X-rays, gamma rays (γ), alpha radiation (α), beta radiation (β), protons (p), neutrons (n), and heavy ions. In addition, space-relevant mixed fields—such as high atomic number and energy (HZE) particles and simulated ion beam combinations (e.g., 5-ion or 33-ion beams)—were incorporated to reflect realistic space radiation exposures. Therapeutic modalities considered included conventional radiotherapy (RT), intensity-modulated radiation therapy (IMRT), boron neutron-capture therapy (BNCT), carbon ion radiation therapy (CIRT), radionuclides therapy (RNT), proton therapy (PT), photodynamic therapy (PDT), photothermal therapy (PTT), near-infrared photoimmunotherapy (NIR-PIT), helium ion radiation therapy (HIRT), electron beam radiation therapy (EBRT), fast neutron therapy (FNT), megavoltage X-ray therapy (megavoltage XRT), and californium-252 neutron therapy (Cf-252 NT).

The methodological approach for the literature review involved several phases. Initially, databases and search engines—such as PubMed, Google Scholar, Semantic Scholar, bioRxiv, and Europe PMC—were searched manually using combinations of keywords, Boolean operators, and filters, covering publications available up to June 20, 2025. Titles were extracted and categorized based on radiation combinations. The “related articles” or “related results” features were also used to locate additional relevant publications. In categories that initially included only one or very few studies, targeted searches were conducted to retrieve further publications, aiming to achieve a more representative sample for each radiation combination. Despite these efforts, some combinations remain represented by a single study; however, their inclusion was preferred over omitting a radiation combination entirely from the review.

A master table was created, consisting of columns detailing the combined radiation types, doses, biological systems, biological endpoints, and effects recorded for each study. Abstracts and relevant sections of the selected publications were reviewed to extract key data, which were organized into structured entries corresponding to the table’s columns.

In the final phase, the master table was split into separate tables, each corresponding to a specific combination of radiation types as studied across the selected publications. Rows were redistributed accordingly and reviewed manually. No specific order—chronological or otherwise—was applied to either the newly created tables or their rows. The tables were then regrouped into three overarching categories: “Radiobiological Studies” (with the sub-categories Non-Ionizing–Ionizing, Non-Ionizing–Non-Ionizing, and Ionizing–Ionizing), “Therapeutic Studies,” and “Space Radiation Studies”. This regrouping resulted in five new tables, which were inserted into the manuscript. Subsequently, the finalized dataset underwent analysis, processing, and discussion.

3. Results

Over 170 studies were retrieved and reviewed. The compiled data spanned a wide spectrum of exposures, including combinations of ionizing and non-ionizing radiation, as well as high- and low-LET (Linear Energy Transfer) modalities. In this review, synergy refers specifically to biological effects that exceed additive expectations based on responses to individual radiation types. These include outcomes such as increased chromosomal aberrations, reduced cell survival, or gene expression changes that cannot be explained by a simple summation of single-exposure effects. Quantitatively, synergy was inferred when the combined effect was statistically greater than the predicted additive response, or when modeled interaction terms (e.g., interaction index > 1) indicated non-linear enhancement. Studies showing that the effect of combined exposure depended on irradiation order or specific DNA repair pathways were also considered synergistic, provided the damage exceeded what would be expected from the individual exposures alone. The following tables summarize key findings across the reviewed studies, organized by radiation type and dose, biological systems, endpoints and observed effects. These are presented first, followed by detailed commentary on the results.

3.1. Radiobiological Studies

3.1.1. Combinations of Non-Ionizing and Ionizing Radiation

Table 1 compiles several studies on combined exposures, with many reporting significant synergistic effects from pairing UV radiation with X-rays, γ-rays, β-particles, or protons [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15]. In one such study, no detectable synergy between UV and X-rays was observed among investigated G₁-phase human lymphocytes, as dicentric chromosome yields were equivalent to those from X-rays alone. This suggests that G₁-phase cells may possess sufficient DNA repair capabilities to independently manage lesions induced by each modality, preventing interactive effects [1]. In contrast, when G₀-phase human lymphocytes were examined, a pronounced ~2-fold increase in dicentric chromosome formation was observed when UV and X-rays were applied within 30 seconds of each other. The synergy was independent of exposure order, indicating a likely mechanistic overlap in lesion processing or repair pathway inhibition [2].

Expanded analysis of the role of timing revealed that when UV irradiation preceded X-rays, the synergistic induction of dicentrics remained constant across time intervals up to 90 minutes. In contrast, when X-rays were administered prior to UV, the synergistic effect decayed exponentially with a half-life of approximately 20 minutes. This suggests that short-lived DNA lesions generated by X-rays may be critical for synergistic exchange formation and are subject to rapid repair or dissipation. Lesion persistence and repair kinetics thus play an important role in controlling the extent and nature of interactive radiation responses [6].

In human peripheral blood lymphocytes, sequential exposures to UV and X-rays induced dicentric chromosome yields up to two-fold higher than X-rays alone, with the order and timing of exposure critically modulating the magnitude of the effect [7]. In HeLa cells, UV and X-rays were shown to induce distinct but complementary forms of damage—X-rays causing chromosomal fragmentation, and UV leading to DNA synthesis and elongation of chromosomes. Their combination intensified cellular damage profiles, supporting the concept that synergy can result not only from lesion overlap but also from convergence of disparate stress responses [9].

Altogether, these studies demonstrate that synergistic interactions between UV and X-rays are highly dependent on factors such as cell cycle stage, DNA repair competence, order of exposure, and the biochemical nature of radiation-induced lesions. These parameters must be carefully considered when evaluating the risks or therapeutic potential of mixed radiation exposures.

Beyond mammalian systems, synergistic interactions were also noted in microbial models. For instance, in bacteria, UV pretreatment sensitized Escherichia coli to X-ray–induced lethality, particularly in wild-type and uvr mutants. In contrast, no synergy was detected in recA, recB, and polA mutants, implicating DNA repair mechanisms in mediating interaction [3,4]. In yeast models, UV exposure prior to gamma irradiation resulted in more than 100-fold reductions in survival beyond additive predictions [10], whereas UV pretreatment enhanced the lethality of β-radiation in E. coli and Salmonella, with synergy abolished upon photoreactivation [14]. The combination of UV and proton radiation also produced synergistic decreases in bacterial survival when UV preceded protons, suggesting order-dependent DNA repair interference [15].

In Schizosaccharomyces pombe, gamma and UV-C exposure produced strong synergy in wild-type cells but not in recombination-deficient mutants, implicating homologous recombination in mediating interaction [10]. Applied microbial studies further demonstrated that combined UV and gamma radiation increased antifungal activity and mutagenesis in Bacillus strains used in bioindustrial screening, including enhanced secondary metabolite profiles [12].

While many of these interactions amplify biological damage, some combinations exhibit the opposite effect. Notably, in a full-thickness human skin model, pre-irradiation with water-filtered infrared-A radiation (wIRA, a subtype of near-infrared radiation) prior to X-ray exposure significantly attenuated the DNA damage response, as evidenced by reduced γH2AX and 53BP1 foci. This was accompanied by lowered pro-inflammatory cytokine expression and preserved tissue viability [16]. Thereby, wIRA may be of a certain importance in activating adaptive or protective signaling pathways, offering a potential avenue for radioprotection in normal tissues. Mixed radiation exposures therefore carry a dual potential—to either amplify or mitigate biological damage depending on wavelength, timing, and cellular context.

3.1.2. Combinations of Non-Ionizing Radiation Types

In Table 2, synergistic effects were reported for UV–UV and UV–visible light combinations. For instance, simultaneous UV-A and UV-B exposure in E. coli produced 100-fold greater inactivation than UV-B alone, mediated by s⁴U modifications in tRNA [18]. In human skin tissue models, simultaneous exposure to UV-B, UV-A, and near-infrared light produced marked synergistic effects on photoaging biomarkers, oxidative stress, and gene expression. These responses were distinct from those induced by single or sequential exposures, suggesting molecular cross-talk across photoreactive pathways, including MAPK signaling and DNA repair [21].

In a study involving ragweed-allergic patients, combined UV and visible light exposures significantly inhibited allergen-induced wheal formation, whereas neither UV-A nor visible light alone produced this effect. The mixed exposure (mUV/VIS) showed a strong, dose-dependent suppression of mast cell–mediated responses, even at suberythematous doses [33].

Additional synergistic effects were observed in diverse biological systems. In tomato plants, UV-B and UV-C exposure enhanced disease resistance and antioxidant gene expression [20]. Marine invertebrate models exposed to long-term UV-A and UV-B irradiation exhibited reduced reproductive and feeding capacity [22], demonstrating chronic toxicity. In environmental disinfection studies, complete microbial inactivation of Enterococcus faecalis and E. coli was achieved in synthetic water matrices using dual-UV systems, with synergy modulated by fluence and pH [27]. Furthermore, predictive modeling of microbial inactivation revealed that combining UV-C radiation from excimer lamps (222–285 nm) with low-pressure mercury lamps (254 nm) produced the highest energy efficiency, supporting a dose- and sequence-dependent basis for synergy [29].

These results indicate that non-ionizing radiation combinations can, at times, amplify biological effects through complementary damage mechanisms or overlapping signaling responses. The diversity of observed outcomes across bacterial, plant, invertebrate, and human systems accentuates the broad applicability of these combinations—from environmental sterilization to therapeutic and dermatological contexts. Additive and supra-additive effects, depending on exposure configuration and biological context, reinforce the importance of wavelength interaction in determining damage outcomes across both experimental and applied settings.

3.1.3. Combinations of Ionizing Radiation Types

Table 3 presents extensive data on combinations of ionizing radiation types—particularly alpha particles, X-rays, gamma rays, protons, and neutrons. These studies reveal a broad range of interaction outcomes, with synergistic effects being frequently observed across multiple biological endpoints.

A recurring finding involves persistent or amplified DNA damage responses. In human peripheral blood lymphocytes, combined alpha and X-ray exposure induced a significant increase in complex chromosomal aberrations, with evidence of non-additive interactions and a linear-quadratic dose–response relationship at higher combined doses [39]. In U2OS osteosarcoma cells, alpha and X-ray co-exposure resulted in delayed decay of small and large 53BP1 foci, along with prolonged ATM and p53 signaling. These findings suggest a synergistic disruption of DNA repair kinetics, with the most pronounced effects observed at lower total doses [42]. Similar synergy was observed in other beam combinations, including alpha followed by gamma radiation, where foci were larger and more persistent than in the reverse sequence [49].

Mixed-exposure effects on clonogenic survival varied substantially by system and irradiation parameters. In V79 Chinese hamster lung fibroblasts, neutron–gamma co-exposure produced supra-additive reductions in survival, particularly under simultaneous delivery. Survival curves fitted better with quadratic models, indicating interaction between damage types [71]. In rat lung epithelial cells (LECs), combined alpha (high-dose, 1 Gy) and X-ray exposure eliminated the survival curve shoulder and reduced clonogenic survival beyond additive expectations. Micronuclei frequencies were also elevated under combined exposure, consistent with non-linear interaction effects [41]. In contrast, a study in Chinese hamster ovary (AA8) cells using mixed alpha and X-ray beams reported no evidence of synergy, with survival data aligning closely with predictions from mathematical additivity models [36]. This difference illustrates the context-dependent nature of mixed-radiation effects, where factors such as cell type, DNA repair capacity, dose composition, and exposure geometry critically shape biological responses. It shows why system-specific investigations are essential when evaluating potential synergy in combined radiation settings.

At the transcriptional level, combined radiation exposures modulated key stress and damage response genes across multiple systems. In human lymphocytes, combined alpha–X-ray exposure increased FDXR, GADD45A, and MDM2 expression beyond levels induced by alpha radiation alone in most donors, with synergy confirmed in 3 out of 4 cases using envelope-of-additivity analysis. Furthermore, ATM inhibition reduced this response, implicating checkpoint signaling [38]. Neutron–photon mixtures elicited strong transcriptomic effects even at low neutron fractions. In murine models, as little as 5% neutron contribution suppressed EIF2/mTOR signaling and ribosomal protein expression—alterations not observed with X-rays alone [55]. In human peripheral blood, increasing neutron percentages led to enhanced TP53 signaling and broader immune dysregulation [60]. Other studies reported dose-dependent modulation of BAX, DDB2, and FDXR expression following neutron–gamma co-exposure [75]. Mixed radiation fields can thus reshape transcriptional networks even in the absence of overt cytotoxicity, reflecting subtle but potentially consequential molecular responses.

Beyond transcriptional responses, several studies show that mixed-field irradiation can also trigger broader cellular and physiological adaptations. In murine blood cells, neutron–gamma exposure altered membrane architecture and lectin-binding patterns, with lymphocytes and platelets showing the most marked ultrastructural remodeling [73]. In separate mouse models, combined exposures impaired hippocampus-dependent memory and shifted neuroimmune profiles toward an anti-inflammatory state [74]. These organism-level effects reinforce observations at the cellular and molecular levels, and underline the significance of mixed-field research in clinical and spaceflight contexts, where heterogeneous radiation fields are the norm.

A rare entry in the dataset examined neutron–proton co-exposure in human breast cancer cell lines, offering insight into mixed-beam effects on cancer stem cell (CSC) populations. The response varied by cell line: CSC fractions declined additively in MCF-7 cells but antagonistically in MDA-MB-231, with no significant changes in canonical stemness gene expression [80]. This study stands out as one of the few to explore neutron–proton co-exposure, emphasizing the need to investigate underrepresented beam combinations that may yield distinct biological responses relevant to radiation protection and therapy.

Additional underrepresented combinations include alpha–beta inhalation in rats, which elicited additive impairments in pulmonary function [52], and mixed radionuclide exposure in plants, where barley grown in contaminated soil accumulated mutations at rates exceeding those predicted by dose alone [53]. Though differing in species and context, these models reflect real-world exposure scenarios—from internal contamination to chronic environmental irradiation—and illustrate why investigating complex mixtures that challenge conventional assumptions about radiation risk matters.

3.2. Therapeutic Studies

The therapeutic potential of mixed radiation modalities is explored in a wide variety of publications, ranging from clinical trials to preclinical and in vitro studies. These studies, compiled in Table 4, reveal nuanced biological outcomes depending on beam combinations, timing, sequencing, and tumor context.

Clinical investigations involving mixed proton–carbon ion therapies have demonstrated encouraging results in managing difficult-to-treat malignancies. In locally advanced pancreatic cancer (LANC), a mixed-beam regimen of proton therapy (50.4 GyE) and a carbon ion boost (12–18 GyE) yielded a 1-year overall survival of 80% and median survival of 17.3 months without dose-limiting toxicities, suggesting carbon ions may enhance local control over protons alone [86]. Similarly, in skull base and cervical spine chordomas and chondrosarcomas, a combined Intensity Modulated Proton Therapy (IMPT) and Intensity Modulated Carbon Therapy (IMCT) approach achieved 2-year local control of 86.2% with low-grade toxicities, although outcomes declined with larger tumors or re-irradiation [87]. For small cell lung cancer, a regimen combining protons and carbon ions with concurrent chemotherapy proved feasible, with a 2-year OS of 81.7% and manageable toxicities [88].

Preclinical models offer mechanistic insights into these effects. In Chinese hamster fibrosarcoma cells, sequential carbon-to-proton irradiation with a 45% carbon contribution induced significant synergy, while reversing the order diminished the effect or led to antagonism, indicating how dose fraction, timing, and sequence modulate therapeutic efficacy [89]. A related study using mixed 16O/28Si/proton beams demonstrated that elevated high-LET contributions suppressed cell survival more effectively, especially when delivered before protons. Notably, a proton-to-high-LET sequence permitted partial recovery, reinforcing the importance of beam order [92].

Several studies evaluated neutron-based combinations. Fast neutron therapy followed by proton boost in salivary gland tumors produced high local control (89.7%) with acceptable toxicity, albeit with notable late effects such as vision and hearing loss [91]. In another, murine study, neutron–proton sequencing influenced both toxicity and tumor outcomes in solid Ehrlich ascites carcinoma. Neutron-first exposures exacerbated skin damage and reduced survival, while the reverse sequence was better tolerated [93]. These findings were echoed in vitro, where neutron-before-proton combinations consistently outperformed proton-first in reducing clonogenic survival, supporting the sequence-dependent efficacy of mixed-beam regimens [94].

Efforts to combine external beam therapy with biologically targeted or nanotechnology-enhanced modalities have also shown promise. In mouse xenograft models, combining proton irradiation with 177Lu-labeled targeted radionuclides produced additive or synergistic tumor suppression, depending on cancer type [90]. Photodynamic therapy (PDT) integrated with photon radiotherapy (RT) has shown synergistic antitumor effects across diverse preclinical models. Studies using bladder cancer organoids, pancreatic co-cultures, and xenograft systems demonstrated enhanced tumor suppression via multimodal cell death, improved DNA and membrane damage, and—in some cases—increased immune infiltration. PDT efficacy was further potentiated using nanoparticle carriers or photosensitizers such as indocyanine green (ICG), supporting its radiosensitizing potential across tumor types [97,98,99,100,101]. X-ray-triggered UV-C via nanoscintillators further potentiated DNA damage and cell killing, particularly in UV-sensitive fibroblasts, suggesting a potential radiosensitization strategy [102,103].

In prostate and nasopharyngeal cancer, clinical application of mixed beams combining photon-based IMRT with carbon ions or protons reduced toxicity and preserved efficacy. These regimens yielded favorable PSA kinetics and quality-of-life scores, and in some cases reduced radiation doses to organs at risk compared to IMRT-only protocols [105,106,107]. Similar approaches using photon-electron VMAT combinations improved sparing of cardiac and pulmonary tissues in breast cancer radiotherapy, highlighting dosimetric advantages of multi-modality planning [108].

Some studies focused on biological outcomes in the context of boron neutron capture therapy (BNCT), which typically combines high-LET components such as thermal, epithermal, or fast neutrons with accompanying low-LET gamma or X-ray radiation. Across in vitro models, these mixed-field exposures generally produced additive effects, particularly in survival outcomes following alpha–gamma or neutron–gamma co-irradiation [120,121]. However, one study reported sequence-dependent synergy when alpha particles and X-rays were delivered simultaneously at higher alpha doses, suggesting interference with DNA repair capacity [122]. Additionally, BNCT-associated mixed-beam exposures induced spatially clustered and persistent DNA damage, particularly in wild-type but not repair-deficient cell lines, emphasizing damage complexity as a key determinant of biological response [123].

Taken together, these studies show that combining radiation modalities can improve tumor control, reduce side effects, and help elucidate mechanisms of cell damage. The choice of beams, their sequencing, and the tumor environment all play a decisive role in shaping outcomes—supporting the case for ongoing research into mixed-field radiotherapy.

3.3. Space Radiation Studies

The biological complexity of space-relevant radiation exposures has been increasingly modeled using mixed-field simulations of galactic cosmic rays (GCRs), incorporating protons, helium, and high-LET heavy ions such as oxygen, silicon, titanium, and iron. These ion combinations, delivered either acutely or fractionated over time, approximate the heterogeneous radiation profile encountered beyond low Earth orbit.

A central point discussed in related publications presented in Table 5 is the vulnerability of the central nervous system (CNS) to structural, behavioral, and synaptic disruptions following GCR-like exposures. Multiple studies using simplified and complex GCRsim beams reported dose- and sex-specific deficits in short-term memory, spatial learning, and cognitive flexibility at total doses as low as 15–50 cGy [129,130,136]. These effects were associated with dendritic simplification, reduced mushroom spine density, and altered expression of synaptic proteins such as GluR1, NR2A, and Synapsin-1. In another 5-ion beam study, male-specific cognitive impairments were causally linked to activated microglia and hippocampal synaptic remodeling, with early circulating monocyte levels serving as predictors of late-stage spatial learning deficits [137].

Other studies focused on complex behaviors, including grooming, burrowing, and risk-based decision-making, where low-dose GCRsim exposure produced sex-dependent performance shifts and increased risk-taking in females [140,144]. In related work using a touchscreen-based switch task, female rats exposed to 10 cGy of GCRsim showed reduced task-switch accuracy (~20% below controls), increased perseverative errors, and a higher failure rate during training stages that required greater cognitive load [147].

Larger ion fields, including 33-ion GCR simulations, reinforced these CNS findings. Chronic and acute exposures impaired memory updating and object recognition in a sex-dependent manner. Long-term potentiation (LTP) was reduced, and changes in postsynaptic density and axonal myelination were observed [156]. Complementary touchscreen-based testing showed that male mice developed attentional deficits and slower response times, alongside disrupted prefrontal dopamine signaling and altered neurotransmitter network organization [157]. Importantly, one study utilizing 6-ion exposures demonstrated that pharmacological HDAC3 inhibition reversed LTP deficits and restored behavioral performance, identifying an epigenetic pathway of functional recovery [152].

Parallel investigations assessed cancer risk and tissue remodeling under space-relevant mixed radiation. In K-rasLA1 mice, chronic 33-ion GCRsim exposure increased lung adenocarcinoma incidence, an effect further enhanced by a delayed neutron dose [159]. In Apc^1638N/+ mice, combined gamma and silicon ion exposure elevated gastrointestinal tumor burden, senescence-associated signaling (p16, IL-6), and β-catenin pathway activation, with these effects mitigated by senolytic treatment [170]. Mammary gland studies in Apc^Min/+ mice showed increased ductal hyperplasia and overgrowth, along with elevated expression of ERα, ERRα, and SPP1—markers associated with estrogen signaling and tumor development [155,162].

Mechanistic studies in primary human fibroblasts and epithelial cells revealed that exposure timing critically shapes biological outcomes following mixed-beam irradiation. When proton exposure preceded titanium or iron ions by as little as 15 minutes, a strong synergistic increase in anchorage-independent growth was observed—even at near-threshold doses—an effect not replicated by same-ion split doses or reversed sequences [163]. This pattern was reinforced in neonatal fibroblasts, where transformation rates were up to three times higher than additive predictions when protons were followed by HZE ions within a 2.5-minute to 6-hour window, but not when the order was reversed [164]. In human mammary epithelial cells, chromosome aberration frequency peaked when iron was delivered 30 minutes after protons, revealing the role of unresolved proton-induced DNA lesions in sensitizing cells to complex damage from subsequent high-LET ions [165].

Radiation-induced cardiovascular and systemic effects were documented across multiple models. In WAG/RijCmcr rats, whole-body exposure to a mixed field of protons, silicon, and iron ions led to perivascular cardiac fibrosis, increased systolic blood pressure, and infiltration of CD68+ macrophages in the heart and kidneys—effects not observed with single-ion exposures [133]. In BALB/c and CD1 mice, 5-ion GCRsim caused sex- and strain-specific changes in cardiac structure, including increased collagen deposition, altered capillary density, and modulation of immune markers such as CD2 and TLR4 [139]. Another study using a myocardial infarction model showed that the sequence of proton and iron irradiation significantly influenced myocardial fibrosis and infarct size, with proton-first exposure exacerbating cardiac damage [166]. Peripheral immune alterations included changes in leukocyte profiles and phagocytic activity, with notable sex-specific transcriptional shifts [135]. Additional GCRsim studies reported reduced circulating cytokines and genotype-dependent inflammatory responses, reinforcing the relevance of immune modulation under space radiation [143].

Studies also implicated the microbiome as a sensitive and interactive component. In several mouse models, microbiota composition changed after GCRsim in a sex- and dose-dependent manner and correlated with cognitive and affective outcomes [131,151]. Gut–brain signaling may thus amplify radiation effects on neural circuitry and behavior.

Other systemic interactions included viral reactivation, endocrine modulation, and altered intercellular communication. In latently infected human myeloblasts, GCRsim and high-LET ions reactivated CMV and induced viral gene expression in a dose- and LET-dependent fashion [154]. In co-culture systems, proton pre-exposure suppressed the bystander response to iron ions in human fibroblasts, underscoring the impact of prior radiation history on intercellular signaling dynamics [167].

A uniquely translational lens is provided by the emblematic NASA Twins Study, which evaluated one astronaut exposed to chronic spaceflight stressors, including ~76 mGy of estimated GCR, over 340 days. Multi-omic profiling revealed persistent chromosomal inversions, immune dysregulation, cognitive decline, altered microbiome signatures, and telomere dynamics (elongation followed by rapid shortening). These effects—many of which mirror patterns in rodent and cellular GCR models—support the idea that space radiation interacts synergistically with other flight stressors, such as microgravity, circadian disruption, and isolation.

Finally, several studies evaluated countermeasures for radiation-induced damage. For instance, the antioxidant CDDO-EA mitigated cognitive flexibility deficits and restored neurogenesis following GCRsim in female mice, although its effects on social behavior were mixed and context-dependent [160,161]. Furthermore, HDAC3 inhibition was shown to reverse synaptic plasticity impairment [152], while low-dose X-ray pretreatment reportedly improved viability and reduced ROS in cardiomyocytes subjected to GCRsim [150]. Conversely, the use of aspirin failed to confer benefit and even worsened cognitive performance in control animals [169], stressing the need for carefully validated interventions.

Viewed collectively, the compiled studies of this section reveal that space-relevant mixed radiation fields elicit widespread, tissue-specific, and often synergistic biological responses. As is common in combined radiation exposures, these effects are shaped by radiation quality, order of exposure, dose rate, biological sex, genetic predisposition, and environmental co-factors. With the convergence of rodent GCRsim models, human cellular systems, and longitudinal astronaut studies, the field is now poised to generate predictive frameworks for long-duration spaceflight risk and precision countermeasures. The integration of mechanistic insight, translational relevance, and systems-level biology marks the next frontier in space radiation research.

4. Discussion

The systematic collection, classification, and synthesis of studies on combined irradiation of biological systems conducted in this review provided critical insights into the current state of the field. The research landscape is consistently structured around three main contexts: Radiobiology, Therapeutic Applications, and Space Radiation Research. This tripartite framework reflects fundamentally distinct objectives—mechanistic elucidation, clinical application, and astronaut health risk mitigation, respectively.

4.1. Mechanistic Basis of Mixed Radiation Effects

Radiobiological studies have consistently revealed that combinations of ionizing and non-ionizing radiation produce distinctive biological effects compared to single-modality exposures. These effects are shaped by variables such as radiation type, dose, timing, and cellular context. As summarized in Figure 1, such interactions often lead to complex DNA damage, saturation of repair capacity, and the emergence of genomic instability.

Multiple studies involving UV and ionizing radiation exposures in mammalian systems reported synergistic increases in dicentric chromosome formation, particularly when UV preceded X-rays or gamma rays. For example, in human G₀-phase lymphocytes, combined UV and X-ray exposures produced synergistic increases in dicentric chromosome yield, particularly when UV preceded X-rays and the interval between exposures was short [2,6,7]. This synergy diminished when the order was reversed, suggesting that repair interference plays a role in amplifying damage. In contrast, this effect was absent in G₁-phase cells, indicating a key role for the DNA repair landscape in dictating interactive outcomes [1]. Further support for this comes from HeLa cells synchronized in G₁, for which detailed chromosomal condensation analysis showed that UV and X-rays triggered fundamentally distinct responses: while X-rays induced dose-dependent fragmentation, UV elongated chromosomes into an "S-like" morphology and triggered unscheduled DNA synthesis—suggesting complementary, rather than overlapping, mechanisms of genomic stress [9].

More insights have emerged from microbial systems, where genetic manipulation enables precise dissection of DNA repair pathways. In E. coli, synergistic lethality from combined UV and X-ray exposures was observed in wild-type and polA mutants, but abolished in recA, recB, and recC mutants, implicating homologous recombination (Type III repair) as essential for synergy. The partial effect seen in polA mutants suggests that base excision or Type II repair may also modulate the response, though they are not sufficient on their own [3,4]. Additional work confirmed that UV pretreatment sensitizes E. coli to ionizing radiation by interfering with repair of strand breaks. Using B/r and Bs-1 strains, it was shown that synergistic X-ray sensitization occurred only in wild-type B/r cells, and was abolished when DNA metabolism was perturbed via 5-bromouracil substitution or purine starvation—conditions known to inhibit DNA repair [5]. In human leukocytes, combined UV and X-ray exposure led to greater chromosome damage in both healthy and Down syndrome donors, with diminished DNA repair synthesis observed in the latter. It can thus be deduced that overlapping repair pathways mediate synergistic outcomes and are modulated by cellular repair competence [6].

Critically, the temporal arrangement of UV and ionizing radiation exposures emerged as a key determinant of synergistic response in mammalian cells. In peripheral human lymphocytes, a substantial, two-fold increase in dicentric chromosome yield was consistently observed when UV exposure preceded X-rays—even with inter-exposure intervals extending to 90 minutes. This stability suggests that UV-induced lesions can persist long enough to interfere with the repair of subsequent ionizing damage. In contrast, reversing the order of exposure led to a rapid decay in synergistic effect, with a half-life of ~20 minutes, implicating short-lived intermediates—such as unrepaired single-strand breaks or stalled replication forks—in facilitating synergy [7]. Therefore, the interaction between lesions is time-sensitive, with DNA repair kinetics playing a central role in shaping the outcome.

Moreover, this temporal dependence aligns with cell cycle–specific differences in DNA repair dynamics. In G₀-phase lymphocytes, synergy was consistently observed, whereas in G₁-phase cells, no increase beyond additive expectations was detected [1]. This shift likely reflects the relative abundance and efficiency of lesion-processing pathways between quiescent and actively cycling cells. Further support comes from HeLa cells synchronized in G₁, where simultaneous UV and X-ray exposure triggered fundamentally distinct chromosomal responses: X-rays produced dose-dependent fragmentation, while UV induced elongation and unscheduled DNA synthesis—pointing to complementary, rather than overlapping, stress responses [9]. Analysis of this dataset leads to the inference that synergy may not simply reflect lesion quantity but rather the convergence of lesion type, repair timing, and cellular repair state.

Temporal modulation of synergistic effects appears to be governed not only by repair capacity but also by the kinetic properties of radiation-induced lesions. Parameters such as lesion half-life and inter-exposure interval critically shape the outcome of combined exposures. When interactions occur within narrow time windows, unrepaired lesions or repair intermediates from the first radiation may persist and interfere with responses to the second, thereby amplifying biological damage. Conversely, extended intervals may allow for lesion resolution and pathway recovery, reducing the potential for synergy. Intriguingly, certain mixed exposures can even attenuate damage. For instance, as mentioned previously, pretreatment with water-filtered infrared-A radiation (wIRA) before X-ray exposure was shown to reduce DNA damage foci and inflammatory signaling in full-thickness skin models, suggesting activation of protective or adaptive stress responses [16].

In non-ionizing radiation combinations, synergistic effects have been observed across both environmental and biological systems, often involving complementary mechanisms of photodamage. In E. coli, simultaneous UV-A and UV-B exposure resulted in a nearly 100-fold increase in inactivation compared to UV-B alone, driven by UV-A absorption through thiouridine (s⁴U) residues in tRNA, which impaired translation and thereby reduced the cell’s capacity to repair subsequent DNA damage [18]. In human skin models, co-exposure to UV-A, UV-B, and near-infrared radiation produced a distinct biomolecular response characterized by heightened oxidative stress, elevated MMP-1 expression, and activation of MAPK signaling—effects not reproduced by individual or sequential exposures, suggesting wavelength-dependent crosstalk between damage and repair pathways [21]. In microbial systems, sequential UV-A and UV-C exposure produced persistent translational arrest and synergistic inactivation in wild-type E. coli, an effect absent in strains lacking thiI-dependent tRNA modifications, highlighting the role of non-DNA targets in modulating radiation sensitivity [23]. In a human skin model of allergen challenge, mixed ultraviolet and visible light (mUV/VIS) exposure produced dose-dependent suppression of mast cell–mediated wheal formation at suberythematous doses—an effect not seen with UV-A or visible light alone—indicating that non-ionizing combinations can modulate immune responsiveness and may hold translational potential for therapeutic or immunomodulatory strategies [33]. A key deduction from these findings is that synergy in non-ionizing radiation exposures arises from converging damage mechanisms affecting DNA, RNA, and proteins, as well as from temporally coordinated activation of cellular stress signaling pathways.

Mechanistic insights from combinations of ionizing radiation types reveal that synergy frequently stems from the intersection of distinct lesion types and the breakdown of repair coordination. When alpha particles are combined with X-rays or gamma rays, the resulting DNA damage is not merely greater in magnitude but also altered in quality—evidenced by persistent 53BP1 foci, complex chromosomal aberrations, and sustained ATM and p53 signaling [39,42]. These features suggest that repair mechanisms become overwhelmed or misregulated under combined irradiation—especially at lower total doses, where synergy tends to be most apparent.

Transcriptional reprogramming offers insight into how cells perceive and prioritize competing damage signals. Co-exposures such as alpha–X-ray and neutron–photon combinations induced distinct expression signatures, including upregulation of DNA damage response genes (e.g., FDXR, GADD45A, MDM2) and suppression of translation-related pathways such as EIF2/mTOR signaling [38,55]. Importantly, these effects occur even in the absence of cell death, highlighting how transcriptional signatures may serve as early markers of synergistic stress before overt damage is visible—crucial for diagnostics and biodosimetry.

Clonogenic survival data link these molecular changes to functional outcomes. Supra-additive losses in colony formation under combined neutron–gamma or alpha–X-ray exposure reflect cumulative disruptions to proliferation and genome stability [41,71]. However, not all systems respond synergistically; for instance, in AA8 cells, survival aligned with additive predictions [43]. These discrepancies reinforce the role of cell-specific repair competence and exposure sequence in shaping whether synergy manifests—an essential consideration in therapeutic planning and radioprotective modelling.

Beyond the nucleus, mixed-field exposures have been shown to impact both membrane integrity and neuroimmune function. In erythrocytes and lymphocytes, neutron–gamma irradiation altered membrane architecture and lectin-binding profiles in a dose- and time-dependent manner [73]. Furthermore, in hippocampal circuits, similar exposures disrupted memory performance and shifted neuroinflammatory tone, marked by reduced pro-inflammatory cytokines and glial activation following gamma priming [74]. These organism-level effects show that synergy is not limited to genotoxic endpoints but can propagate through structural and signaling networks—especially relevant in complex exposures such as spaceflight or nuclear incidents.

Interestingly, even rarely tested combinations such as neutron–proton co-exposure have revealed tumor-specific effects on cancer stem cell populations, suggesting that unconventional radiation pairings may uncover selective biological vulnerabilities not detected under standard protocols [80]. Likewise, environmental models involving complex or atypical exposure scenarios—such as mixed radionuclide contamination in plants or alpha–beta inhalation in rodents—demonstrate that chronic, low-dose synergy can lead to mutation accumulation and functional tissue impairment over time [52,53]. It thus becomes evident that radiation synergy is not merely a byproduct of experimental design but a genuine biological phenomenon that occurs in real-world contexts. As such, mixed-field effects must be integrated into predictive models of environmental risk, long-term health outcomes, and radiological protection—especially in scenarios dominated by chronic, low-dose exposures.

Radiobiological findings such as those discussed here indicate that combined radiation exposures frequently lead to amplified or qualitatively distinct biological outcomes, driven by interactions at the levels of DNA damage formation, repair kinetics, and stress response signaling. A deeper understanding of these dynamics is essential for refining risk assessment models and for effectively harnessing combinatorial effects in both therapeutic interventions and protective strategies in space exploration.

4.2. Determinants and Mechanisms of Synergy

Synergy—defined in this review as a biological effect exceeding the sum of individual radiation exposures—emerged as a recurring observation across diverse radiation combinations. These interactions spanned ionizing and non-ionizing sources and were seen in endpoints such as DNA damage, cell survival, chromosomal aberrations, transcriptomic shifts, and organ-level function.

A key determinant of synergy was exposure sequence and timing. For instance, in studies concerning the combination of UV and X-rays, synergistic increases in dicentric chromosome yields were most pronounced when UV preceded ionizing radiation, such as X-rays or gamma rays, suggesting that early UV-induced DNA lesions or repair processes sensitize cells to subsequent damage [2,6,7]. Similar sequence sensitivity was observed in fibroblast and tumor models exposed to carbon–proton [89], heavy-ion–proton, and neutron–proton combinations [92,94]. In each case, initiating exposure with high-LET radiation—such as carbon, oxygen/silicon, or neutrons—resulted in more pronounced reductions in survival or increased cytotoxicity compared to reverse sequences, underscoring the influence of damage complexity and repair kinetics on synergistic outcome.

Cell state also modulated synergy potential. G₀-phase lymphocytes showed strong synergistic dicentric formation following UV–X-ray exposure, while G₁-phase cells did not [1,2], and microbial models lacking key repair genes (e.g., recA, polA) lost their synergistic response altogether [3,4,10]. Deductively, it can be stated that DNA repair capacity and the cell cycle context are of great importance in determining whether mixed exposures interact.

At the molecular level, co-exposures frequently led to persistent DNA damage signaling, including prolonged 53BP1 and ATM/p53 activity [42,49]. Transcriptionally, synergy was reflected in non-linear expression shifts, with combined alpha–X-ray or neutron–photon exposures enhancing genes like FDXR, GADD45A, and MDM2 in lymphocytes [38,55,60].

Finally, synergistic effects were shown to extend beyond cellular systems. Mixed β–γ, α–X-ray, or GCRsim combinations in mice and plants induced non-additive impairments in behavior, tissue function, and mutagenesis [123,124,125,126,127,129,130,131,132]. In these studies, synergy is shown to be a system-wide phenomenon driven by lesion interplay, repair saturation, and signaling crosstalk—not a fixed trait of any single radiation type. As such, it offers both a mechanistic lens for radiation biology and a potential tool for optimizing radiotherapy or predicting complex exposure risks.

4.3. Therapeutic and Protective Strategies

Mixed radiation exposures are not only of biological interest but also hold considerable translational potential in therapeutic and protective contexts. Clinical studies using combinations of particle beams—such as protons and carbon ions—demonstrate that beam mixing can improve tumor control while maintaining or reducing toxicity. As presented earlier, dual-beam protocols in pancreatic, skull base, and lung cancers have been shown to achieve high local control and survival rates, with tolerable side effect profiles, suggesting synergistic gains in therapeutic index when carbon ion boosts are added to proton regimens [86,87,88].

Preclinical and in vitro studies deepen mechanistic understanding. Here, the timing and order of beam delivery emerge as critical factors. In fibrosarcoma cells, carbon-first protocols induced more effective cytotoxicity than reverse sequences, implicating LET-driven modulation of repair kinetics [89]. Similarly, mixed high-LET and proton beam combinations show that reversing the order can blunt or amplify survival outcomes depending on how cells process clustered damage [92,94]. To add to these, neutron-based regimens—used either alone or with protons—have shown promise in both human tumors and murine models, though late toxicity and sequence sensitivity remain concerns [91,93].

Beyond external beams, hybrid approaches that integrate radiation with nanotechnology or molecular targeting are being explored. PDT combined with photon RT enhances tumor killing in multiple models, and radiosensitization via targeted radiopharmaceuticals or nanoscintillator-triggered UV damage suggests that combinatorial designs can exploit biological vulnerabilities in novel ways [97,98,99,100,101,102,103].

Protective strategies related to combined radiation exposures are also gaining interest. One notable approach involves adaptive conditioning: in cardiomyocyte models, low-dose X-ray pretreatment prior to GCRsim exposure improved cell viability and reduced oxidative stress, suggesting that pre-exposure can induce protective pathways mitigating subsequent high-LET damage [150]. Similar protective mechanisms have been observed across diverse radiation types. In particular, photobiomodulation with near-infrared (NIR) or laser pretreatment prior to X-ray or gamma exposure improved leukocyte counts, enhanced antioxidant enzyme activity, and increased tissue oxygenation in full-thickness human skin models. These interventions also reduced DNA damage and apoptosis in keratinocytes and fibroblasts [16,17].

Altogether, mixed-field approaches offer significant promise for increasing treatment specificity, minimizing collateral damage, and revealing tumor-selective response windows. Future strategies may benefit from personalizing beam selection and delivery sequence based on tumor type, molecular profile, and repair capacity. In parallel, the development of radioprotective interventions—such as photobiomodulation or adaptive preconditioning—may help safeguard healthy tissues across a range of exposure contexts, from clinical radiotherapy to occupational, environmental, and spaceflight scenarios. As mechanistic understanding of these interactions deepens, such approaches could become integral components of comprehensive radiation management strategies.

4.4. Space and Environmental Risk Contexts

Space radiation studies increasingly demonstrate that mixed-field exposures—particularly GCRsim combinations of protons and high-LET ions—trigger system-wide biological disruptions that go beyond linear dose effects. The central nervous system is especially susceptible, with low-dose exposures impairing cognition, synaptic structure, and behavior, with sex-dependent outcomes observed in several studies[131,157]. The induced effects are frequently synergistic, shaped by radiation sequence and timing, emphasizing the role of unresolved DNA damage and repair interference [163,164,165]. Similar order-dependent sensitivities have also emerged in cancer models; in one study, proton-first regimens was shown to amplify tumorigenesis compared to reversed sequences [132]

In space-related research, chronic exposure has emerged as a key determinant of biological risk. For instance, in K-rasLA1 mice, chronic GCRsim regimens increased lung tumor incidence more than acute exposures, with the addition of neutrons further exacerbating malignancy [159]. In the cardiovascular domain, proton-first exposure sequences worsened myocardial fibrosis and infarct size compared to iron-first regimens, highlighting how radiation order governs tissue-level outcomes in the context of space radiation [166]. Meanwhile, microbiome alterations were found to parallel these physiological disruptions, with GCRsim exposures producing sex- and dose-dependent dysbiosis that influenced cognitive and immune phenotypes [131,151].

The NASA Twins Study provided translational support, revealing telomere elongation followed by rapid shortening, chromosomal inversions, immune dysregulation, cognitive performance changes, altered microbiome profiles, and widespread transcriptomic remodeling in the flight-exposed astronaut. Many of these effects mirrored those seen in rodent and cell-based GCR simulations, reinforcing that space radiation acts not in isolation, but synergistically with other spaceflight stressors—such as microgravity, circadian disruption, and confinement—to drive complex, system-wide biological responses.

These insights reflect a pivotal turning point in space radiation biology, enabled in large part by NASA's strategic investment in GCRsim technology and the development of multibeam irradiation platforms like those at the NASA Space Radiation Laboratory (NSRL). The ability to deliver rapid, sequential exposures of biologically relevant ions—mirroring real cosmic ray compositions—has transformed the field from single-ion approximations to nuanced, systems-level modeling. Coupled with omics-driven analyses, behavioral assays, and translational studies like the NASA Twins Study, this technological leap allows for integrative frameworks that capture the interactive, nonlinear nature of spaceflight stressors. As long-duration missions to the Moon and Mars approach, future research will increasingly depend on refining these models, identifying individual susceptibility markers, and testing personalized or adaptive countermeasures that address the cumulative burden of mixed-radiation and environmental exposures.

While the spaceflight context has catalyzed some of the most advanced modeling of mixed radiation effects, such scenarios are not directly representative of terrestrial conditions. Nevertheless, the principles uncovered—such as synergistic interactions, timing-dependent outcomes, and system-level disruption—stress the broader need to investigate combined exposures on Earth. Although everyday environments do not involve showers of galactic ions, people are routinely subjected to overlapping or sequential low-LET sources, radionuclide mixtures, and co-exposures with chemical, thermal, or biological stressors. The insights from GCRsim studies thus should act as a scientific impetus, urging environmental radiation research to move beyond isolated-dose models and account for real-world exposure complexity.

On Earth, mixed radiation exposures can arise in various contexts, such as ultraviolet radiation in combination with background gamma fields in high natural radiation areas, or beta–gamma emissions associated with radionuclide contamination from industrial or military activity. Although these exposures differ in scale and composition from those encountered in space or laboratory models, they may still present layered biological challenges. As evidence suggests that interactions between radiation types can diverge from simple additive expectations, closer examination is in order. Continued investigation into such terrestrial scenarios is important for refining our understanding of mixed exposure effects, supporting environmental protection strategies, and strengthening risk assessment and remediation frameworks.

4.5. Methodological Challenges and Experimental Gaps

This review identifies major gaps in combinatorial radiation research. Some radiation pairings—such as protons with UV or beta ([13]–[15], [168])—remain virtually unstudied despite potential biological significance. Even for well-explored combinations like X-ray – neutron or gamma – heavy ions, systematic mapping of dose-effect relationships and mechanistic modeling is often lacking.

Moreover, although timing, dose ratio, and radiation order have emerged as critical parameters modulating synergy, many studies treat these variables as secondary rather than primary design features. Systematic investigations of inter-exposure intervals, particularly in time-resolved or cell cycle–synchronized systems, are rare but urgently needed. Cross-comparative work that spans cell type, species, or tissue specificity is also scarce, making it difficult to generalize findings across biological contexts.

Another under-addressed area involves combinatorial exposures including more than two radiation types. While many studies have tested complex mixtures such as 5- or 33-ion beams in space radiation models, terrestrial analogs involving triplet exposures (e.g., UV – gamma – alpha/beta) are largely missing. The field also lacks integrative studies combining ionizing and non-ionizing modalities in realistic exposure sequences, such as low-dose UV exposure followed by diagnostic imaging or therapeutic irradiation

Finally, many existing studies are limited to acute, high-dose exposures, whereas real-world scenarios—whether medical, environmental, or space-related—increasingly involve chronic or fractionated mixed exposures. This disparity suggests the need for experimental paradigms that mirror complex human exposure patterns more closely. Addressing these gaps will be essential to construct predictive models of mixed-radiation effects, optimize therapeutic combinations, and refine risk estimates for spaceflight and environmental contamination scenarios.

To bridge these gaps, interdisciplinary collaboration and standardization of experimental protocols will be critical. Advances in dosimetry, high-throughput assays, and computational modeling now offer the tools to systematically dissect combinatorial radiation effects across biological systems and exposure conditions. Harnessing these capabilities will allow the field to evolve from descriptive observations to mechanistic understanding and predictive accuracy.

4.6. Limitations and Future Directions

One of the main challenges in mixed radiation research lies not only in the scarcity of data, but also in how current studies are designed, conducted, and contextualized. Investigations into radiobiological, medical, and space-related exposures are often pursued within separate research domains, each shaped by its own priorities, methodologies, and regulatory frameworks. This disciplinary isolation makes it difficult to compare results, integrate findings, or construct unified frameworks for understanding biological response. As a result, efforts to build predictive models—whether theoretical, computational, or mechanistic—face significant hurdles. Such models are essential not only for estimating health risks, but also for developing effective radioprotection strategies. Yet their development is hindered by fragmented data, inconsistent endpoints, and a lack of shared experimental standards. Compounding this, most studies prioritize acute or short-term outcomes, with far fewer addressing how mixed radiation effects evolve over time, across generations, or within realistic ecological and clinical contexts.

To move the field forward, we need better coordination between disciplines, along with shared databases that connect radiation types, biological systems, and outcomes. These could support larger comparisons and help identify patterns across experiments. Building international collaborations, agreeing on common protocols, and using simulation platforms will also be important. Collectively, these steps can help the field go beyond scattered case studies and work toward a deeper, more predictive understanding of how different kinds of radiation interact with life.

5. Conclusions

This systematic review—drawing on over 170 studies and encompassing an exceptional variety of radiation pairings—demonstrates that the biological effects of combined radiation exposures are neither trivial nor reliably additive. From fundamental radiobiology to medical applications and spaceflight scenarios, evidence consistently shows that interactions between different radiation types can yield emergent effects, including both synergistic and antagonistic responses. These outcomes are often shaped by radiation quality, dose ratio, sequence, and the biological system in question.

Crucially, the findings challenge the assumption that total dose alone can predict biological risk. Instead, they point to the need for a better understanding of how different radiation types interact over time and across levels of biological organization—from molecular repair mechanisms to whole-organism physiology. Although the field remains fragmented and methodologically uneven, the available data support the development of integrative models and coordinated research strategies that more accurately reflect the complexity of mixed radiation exposures.

Conclusively, the evidence presented in this review reflects a pressing need to rethink how we approach radiation exposure in both research and practice. Whether in medical, space, or environmental contexts, the biological impact of radiation cannot be fully understood without accounting for interactions across type, order, timing, and biological scale. This complexity is not merely theoretical—it is central to designing safer therapies, preparing for deep space missions, and protecting life on Earth. Moving beyond reductionist models is essential to predict, mitigate, and ultimately manage the multifaceted nature of mixed radiation effects.