P09.20.A NEW RADIOLOGICAL FINDINGS AND EARLY TOXICITY OUTCOMES FOR IDH MUTANT ADULT-TYPE DIFFUSE GLIOMAS TREATED WITH INTENSITY MODULATED PROTON THERAPY

Abstract BACKGROUND New radiological changes have been one of the newly appreciated findings post proton therapy treatment. One of the possible hypothesis in literature is the high LET and consequently high RBE at the end of the bragg-peak due to possible disruption of the blood brain barrier. We report radiological changes and clinical toxicities for patients treated with intensity modulated proton therapy (IMPT) in high grade gliomas. MATERIAL AND METHODS Clinical records of 25 adults (>18 years) with IDH-mutant gliomas treated with IMPT were obtained from the electronic database. Acute toxicities were graded using common terminology criteria for adverse events (CTCAE) version-5.0 and documented during weekly reviews and at completion. Follow-up imaging with magnetic resonance imaging (MRI) was done 1-month post-radiotherapy and at intervals of 3-6 months. Follow-up MRIs were reviewed for any new contrast enhancement or changes noted in T2/FLAIR sequences. MRI was registered with the planning-CT-scan, and new enhancement/hyperintensity was delineated and correlated with different dosimetric parameters and spatially classified based on distance from 95% CTV (within, marginal <5mm, distal >5mm) An LET-volume histogram (LVH) was generated. Mcnamara variable RBE model-based calculation using alpha/beta ratio of 2. RESULTS All patients completed scheduled treatment without any major interruptions. The highest grade of toxicity was grade 2 dermatitis in 12%. Majority (82%) had grade 1 dermatitis. Grade 1 alopecia in 36% and grade 1 neurological toxicities in 12%. Within a median follow-up of 4 months (range 1-15 months), 41 MRIs were reviewed. Five (20%) patients were detected to have developed new onset radiological findings at a median time of 7 months, with one having radiation-induced contrast enhancement (RICE), one having radionecrosis (only patient with symptoms), and three patients with increase of T2-weighted hyperintensity. 80% of the lesions were located within or <5mm from the high-dose(95%) volume region. In one patient, the RICE was overlapping with the high linear energy transfer(LET) region. The median of max LET & RBE in the lesions were 3.1(2.6-6.1)KeV/um and 62.26Gy respectively. CONCLUSION Proton beam therapy is well tolerated in patients with IDH-mutant gliomas. New radiological findings, which are majorly asymptomatic, are commonly appreciated within the first year of treatment. Implications of using multiple beams and LET/RBE-based optimizations should be explored to reduce the likelihood of radiation-induced complications.