Proton beam therapy in nonmetastatic rhabdomyosarcoma: Outcome, prognostic factors and the effect of timing of radiation therapy.
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PURPOSE: We report survival and disease-specific outcomes, prognostic factors, and impact of radiotherapy timing in patients with non-metastatic rhabdomyosarcoma (RMS) treated with definitive proton beam therapy (PBT) at a national UK centre. PATIENTS AND METHODS: We retrospectively reviewed patients with non-metastatic, pathologically confirmed RMS treated with definitive PBT at the XX Proton Beam Therapy Centre between December 2018 and May 2025. All patients were treated according to RMS 2005 or FaR-RMS protocols. Time-to-event outcomes included local control (LC), disease-free survival (DFS), distant metastasis-free survival (DMFS), and overall survival (OS). Kaplan-Meier methods were used for survival estimates. Prognostic factors were assessed using Firth-penalised Cox regression due to limited event numbers. Multivariable models adjusted for dominant prognostic variables. RESULTS: Eighty-four patients were included with a median age of 7.6 years (range, 1-70). Parameningeal primaries accounted for 51% of cases, and 75% of patients had high or very high-risk disease. Median time from chemotherapy initiation to PBT was 13 weeks (range, 10-22). After a median follow-up of 48 months, 4-year OS, DFS, LC, and DMFS were 81.2%, 70.8%, 75.4%, and 89.7%, respectively. Fifteen patients developed local failure, all occurring in-field within the high-dose region. On univariate analysis, parameningeal primary site and advanced T stage (T3-4) were significantly associated with inferior OS and DFS. Radiotherapy initiation at ≥14 weeks was not independently associated with worse outcomes after adjustment for other variables. Within the parameningeal subgroup, intracranial extension was associated with significantly inferior OS. CONCLUSIONS: Definitive PBT for non-metastatic RMS resulted in disease control and survival outcomes comparable to those reported in cooperative group trials and other proton therapy series. Prognosis appears to be primarily driven by disease-related factors, particularly parameningeal location, intracranial extension, and advanced T stage. Radiotherapy timing was not independently associated with outcomes after adjustment for these factors; however, these findings should be interpreted with caution, particularly in relation to disease-free survival. Further validation in larger, multi-institutional datasets is warranted.