REPEATED CRANIOSPINAL IRRADIATION IN PEDIATRIC PATIENTS WITH RECURRENT MEDULLOBLASTOMA: PROTON THERAPY EXPERIENCE AT THE TREATMENT AND DIAGNOSTIC CENTER OF THE INTERNATIONAL INSTITUTE OF BIOLOGICAL SYSTEMS NAMED AFTER SERGEY BEREZIN

Repeated craniospinal irradiation (CSR) is one of the options in the treatment of multifocal recurrences of medulloblastomas (MB), however, it is prescribed to a limited extent due to its toxicity. The use of proton therapy can reduce the toxicity of treatment. Materials and methods used: a single-center retrospective cohort study was conducted in 25 patients (18 (72%) boys/7 (28%) girls) aged 7 to 18 y/o (median 13 (10-14) y/o) in 2019-2023. Repeated CSR was performed for leptomeningeal MB recurrence using a proton beam at the Treatment and Diagnostic Center of the International Institute of Biological Systems named after Sergey Berezin (Saint Petersburg, Russia). The median dose of primary CSR was 35.2 Gy (24-35.2). The median dose of repeated CSR was 30.6 Gy (24-35.2). 23 patients after CSR were given a local boost on the area of metastatic lesions and local recurrence (if any). 7 had previously received courses of repeated radiation therapy for recurrent MB. Results: during the course of the treatment thrombocytopenia was noted with a statistically significant decrease by the 3rd week of treatment (M=97x109/l, p<0.001). The median follow-up was 12.5 months (8-19.5). In 8 patients, progression of the disease was observed, which resulted in the death of 7 of them. In one patient, the appearance of radiation necrosis in the local boost area (left cerebellar peduncle) was noted 5 months after the completion of the repeated irradiation course. The median progression period was 17 months (95% CI 10.8-23.1). Conclusions: proton therapy makes it possible to deliver therapeutic doses for CSR with an acceptable level of hematological toxicity. The obtained data on relapse-free and overall survival indicate the need for further study of the role of repeated CSR and the development of a methodology for calculating the permissible maximum effective dose during the repeated exposure.