Radt-08. Basal Ganglia Toxicity Following Proton Therapy of the Pineal Region With Bilateral Action-Induced Myoclonus

Abstract INTRODUCTION We sought to evaluate the toxicity profile for children receiving 54Gy to the pineal region with a significant dose delivered to posterior thalamus and basal ganglia. METHODS We performed a retrospective review of patients treated at a single institution with intensity modulated proton therapy (IMPT) receiving 54Gy to the pineal region. Multiple beam angles were utilized for planning. Patient characteristics and dosimetric data were correlated with late-toxicity and imaging. RESULTS From 2021-2023, six children were treated, and at a median follow-up of 13 months 9-26, all patients remain disease-free. Three patients had non-germinomatous germ cell tumors, 1 low-grade glioma, 1 atypical teratoid rhabdoid tumor (ATRT), and 1 embryonal tumor with multilayered rosettes. Age range was 2.4 to 16.8 years. All patients received chemotherapy prior to IMPT. Neuropsychological evaluation indicated range of intellectual functioning (mean = 98.3 83-118). One patient, a 4-year-old female with ATRT, developed T2 changes within the globus pallidus and thalamus on 3 month follow-up. At 5 months, she developed bilateral action-induced myoclonus with corresponding enhancing lesions in the globus pallidus bilaterally. Neuropsychological evaluation at 5 months post-treatment indicated memory impairments. She was treated with dexamethasone without improvement. Between 912 months post-treatment, tremors improved with physical therapy and imaging showed near complete resolution of enhancement within globus pallidus. CONCLUSION One of six children developed grade 3 toxicity manifesting as bilateral action-induced myoclonus with imaging changes in the globus pallidus bilaterally following IMPT and exhibited related neuropsychological deficits. While brainstem toxicity from IMPT is well documented, globus pallidus toxicity is not well characterized. Given the contiguous neuronal tracts with the brainstem, it is logical to apply similar treatment planning techniques, i.e. minimizing hotspots within the globus pallidus and thalamic “core” structures, and being cognizant of LET/RBE effects for treatment involving the pineal region in young children.