MRI and PET Alterations in Adult Skull-Base Tumors: A Narrative Review of Proton versus Photon Radiotherapy

Background: Radiotherapy is an essential component for the treatment of skull skull‑base tumours. However, radiation‑induced brain injury (RIBI) appearing on magnetic resonance imaging (MRI) as pseudoprogression, radiation‑induced contrast enhancement (RICE), radiation necrosis, and other contrast‑enhancing brain lesions—remains an important late toxicity which is not very well characterized. Clarifying adult‑specific patterns, the role of positron emission tomography (PET), and differences between proton and photon therapy is clinically relevant. Methods: Narrative review of peer‑reviewed literature to 1 October 2025 focused on adult chordoma, chondrosarcoma, meningioma, adenoid cystic carcinoma, pituitary adenoma, craniopharyngioma, undifferentiated carcinoma of nasopharyngeal type (UCNT), and germinoma. Results: Radiation-induced brain injury (RIBI) may range from early transient brain oedema/enhancement to late necrosis. Lesions cluster in temporal and frontal lobes after skull‑base irradiation. Proton therapy generally reduces high‑dose exposure versus photons and is associated with fewer severe necrotic events, though asymptomatic RICE is common. Advanced MRI (diffusion, perfusion, spectroscopy, ASL, amide proton transfer) and positron emission tomography (PET) with Fluorodeoxyglucose (FDG) and amino‑acid tracers aid distinction between treatment effect and progression. Risk reflects dose–volume metrics, prior irradiation, and patient comorbidities. Conclusions: MRI/PET alterations after skull‑base radiotherapy vary by modality, dose, and tumor context. Integrating multimodal imaging with clinical factors should guide surveillance and management; prospective validation is needed.