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

Background: Radiotherapy is essential for skull base tumor management but carries the risk of radiation-induced brain injury (RIBI). This spectrum ranges from transient radiation-induced contrast enhancement (RICE) to irreversible necrosis. Distinguishing these entities from tumor progression is critical, particularly with the increasing adoption of proton therapy. Methods: A comprehensive narrative review of the peer-reviewed literature was conducted up to October 1, 2025. The search strategy focused on adult patients treated for skull base malignancies, synthesizing data on dose-volume metrics, incidence rates, and modality-specific toxicity profiles. Results: RIBI represents a pathophysiological continuum. (a) Descriptive imaging patterns: In prospective proton therapy series, focal RICE occured in 15% of patients, typically at a median of 12 months, and often resolved spontaneously. (b) Modality comparison: Although proton therapy reduces integral brain dose versus photon therapy, elevated linear energy transfer (LET) at the distal Bragg peak may contribute to focal radiation-associated image changes (RAIC), particularly in the temporal lobes. (c) Risk stratification and diagnosis: Risk increased when >1% of the healthy brain received >57.6 Gy (Relative Biological Energy (RBE)) or when V67Gy exceeded 0.17 cc. Advanced MRI and amino acid positron emission tomography (PET) improved differentiation between radiation effects and tumor recurrence. Conclusions: Post-radiation imaging changes are common and often benign. Distinguishing RICE from progression requires multimodal imaging and adherence to specific dose constraints. Management should prioritize surveillance for asymptomatic lesions.