Paratesticular alveolar rhabdomyosarcomas do not harbor typical translocations: a distinct entity with favorable prognosis?

The alveolar subtype of rhabdomyosarcoma (RMA) is a strong risk factor. Cases of RMA located in paratesticular sites have however been reported to have similar outcomes to those of embryonal rhabdomyosarcoma (RME). We wanted to re-evaluate the impact of subtype in paratesticular rhabdomyosarcoma (PT-RMS). Patients from a population-based cohort diagnosed with paratesticular RMA in 1990-2013 were analyzed. All tumor samples were re-reviewed using conventional morphology, immunohistochemistry, and molecular testing. Seven patients were eligible. Four tumors showed focal areas morphologically compatible with RMA (mixed RMA/RME). One case was undifferentiated, with a solid round-cell morphology which had to be reclassified as poorly differentiated RME. Two cases had a "microalveolar" morphology which is today regarded as sclerosing RME. No tumor showed the characteristic gene fusion of RMA. Five children had localized disease, one bone metastases, and another lymph-node involvement. All primaries were grossly resected. One locoregional relapse occurred. At a median follow-up of 7 years, all patients were alive disease-free. PT-RMS can show a focal alveolar histology combined with typical features of RME. In current morphological classifications, all rhabdomyosarcomas qualify for the alveolar subtype if typical features of RMA are realized at least focally. Rhabdomyosarcomas consisting of pure RMA morphology were however not found in our patients with PT-RMS. The mixed RMA/RMEs identified in our population-based study did not show a translocation typical for RMA and had a good prognosis. Further prospective studies need to evaluate if mixed RMA/RMEs have a similar favorable outcome in non-paratesticular sites as well.