ATRT-04. RETROSPECTIVE ANALYSIS OF 271 RELAPSED AND REFRACTORY ATYPICAL TERATOID RHABDOID TUMORS – COMBING DATA FROM INTERNATIONAL COHORTS IN A COLLABORATIVE EFFORT

BACKGROUND Relapse or refractory (r/r) disease is common in atypical teratoid rhabdoid tumors (ATRT), yet little is known about clinical patterns, prognostic factors, and outcomes beyond individual case reports. Large representative cohorts are missing, impeding the interpretation and design of clinical trials for r/r ATRT. METHODS We retrospectively collected data on r/r ATRT from centers in the US, Europe, Hong Kong, and Australia, including patients previously enrolled in clinical trials (e.g., COG ACNS0333, DFCI 02-294) or the prospective EU-RHAB registry. Inclusion criteria were relapse (recurrence after complete remission (CR)) or progression (tumor growth or new metastasis without previous CR) following at least one course of chemotherapy. RESULTS 271 patients were analyzed; median age was 16 months (m) at diagnosis (0-233). Molecular subgroup status was available in n=118. Relapse (44%) or progression (54%) occurred after a median of 8m. Only 34% of events were localized at the primary tumor site; distant metastasis only (31%) and combined events (31%) were more common. Salvage treatment was multimodal with systemic pharmacotherapy (n=176), surgery (n=71), high-dose chemotherapy (n=19), and/or radiotherapy (n=110). Targeted treatment included epigenetically targeted drugs (n=45), alisertib (n=11), immunotherapy (n=10) and metronomic therapy (n=45). 1-year overall survival (OS) following an event was 29.6±2.8%. Log-rank test suggested subgroup status as a prognostic factor. (TYR vs. non-TYR: 30 vs. 21w OS, p< 0.05; MYC vs non-MYC: 14 vs. 25w OS, p<0.05). OS was significantly inferior after progression compared to relapse (19 vs. 33w, p<0.05) and in patients with rhabdoid tumor predisposition syndrome (18 vs. 25w, p<0.05). CONCLUSION Our data confirm the poor prognosis of r/r ATRT. However, intense multimodal treatment is feasible, and molecular characteristics are potentially clinically relevant. Results from this analysis offer a robust foundation for prospective clinical trials and therapeutic algorithms for r/r ATRT.