RADT-58. IMPACT OF PROTON VERSUS PHOTON RADIATION ON INTRATREATMENT TUMOR DYNAMICS MEASURED BY INTERVAL MAGNETIC RESONANCE IMAGING IN CRANIOPHARYNGIOMA

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Abstract BACKGROUND Craniopharyngioma frequently demonstrates volumetric fluctuations during fractionated radiotherapy. This necessitates interval imaging to monitor and guide adaptive strategies, which is more critical in proton beam therapy (PBT), which is associated with sharper dose gradient. METHODS We conducted a retrospective audit of craniopharyngioma treated between 2021 and 2024. The dose delivered was 54 Gy in 30 fractions (or Cobalt Gray Equivalents) over 6 weeks. The interval imaging protocol included 3-D T1-weighted contrast, 3D T2-FLAIR, and 3D T2-propeller sequences every 1-2 weeks. All images were co-registered in the treatment planning system and assessed for adaptive treatment based on geometric changes. Volumetric change in the tumor was calculated as percentage change in gross tumor volume (GTV) relative to baseline planning MRI. RESULTS The cohort included 46 patients with a median age of 17 years (IQR: 9–25), and 34 patients received photon and 12 PBT. The mean baseline GTV was 21.8 cc. GTV changes on interval MRI ranged from –45% to +288%, with a mean increase of 27%. A GTV increase of >10% was observed in 23 patients (50%), while 7 patients had >10% reduction (15%). Cystic morphology and hypothalamic involvement were significantly associated with larger volumetric changes (mean: 33.6% vs, 2.5%, p=0.021; and 39.9% vs, 0.03%, p=0.002, respectively). Patients treated with PBT showed higher changes than photons (57% vs 16%, p= 0.1). Volumetric increase of residual disease was more frequent in PBT compared to photon 58% vs 44%. Adaptive replanning was required in 28 patients (58%), including 13 who needed multiple adaptations (cystic expansion 22, shrinkage 6). CONCLUSION Proton therapy was more frequently associated with interval increases in target volume due to dynamic changes in cysts. Interval monitoring of target volume with weekly 3D volumetric MRI should be considered standard in PBT with a low threshold for adaptive PBT.

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