Clinicopathological Analysis of a European Cohort of MYOD1 Mutant Rhabdomyosarcomas in Children and Young Adults.
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BACKGROUND: Patients with PAX3/7-FOXO1 fusion-negative rhabdomyosarcomas (fnRMS) harbouring the rare L122R MYOD1 mutation have significantly poorer prognosis than other fnRMS. We undertook a detailed clinicopathological evaluation of a cohort of patients with MYOD1 mutated fnRMS in order to improve risk stratification and treatment options. PROCEDURE: Histological, mutational and clinical data from a cohort of patients with MYOD1 mutant RMS treated in Europe were analysed. RESULTS: Thirty-two cases with mutant MYOD1 RMS were identified from patients enrolled in sequential European rhabdomyosarcoma clinical trials from 1992 to 2022 (n = 22) and non-trial cohorts (n = 10). Thirty cases had the recurrent L122R missense mutation, one case harboured a K124E mutation and one case had a truncating mutation (S63X). Increased MyoD1 and reduced MYF4 immunostaining were consistent features of MYOD1L122R-mutated RMS. Applying the risk stratification of the European paediatric Soft tissue sarcoma Study Group (EpSSG) RMS2005 trial, among 20 localised RMS cases that could be assigned a risk category, one was Very High Risk, 13 were High Risk and six were Standard Risk. Eight patients had distant metastases at diagnosis. Of the 25 patients with adequate clinical follow-up data, 15/25 (60%) patients had an event at a median time of 9 months (12/15 included failure of local control) and 13/25 (52%) died of disease. CONCLUSION: This MYOD1 mutant cohort demonstrates increased MYOD and reduced MYF4 immunostaining, high risk of local failure and poor survival in agreement with other studies. Increased treatment intensity and improved local control should be considered for these patients.