BACKGROUND: Rhabdomyosarcoma (RMS) is an aggressive malignancy, and 20% of children present with metastases at diagnosis. Patients presenting with disseminated disease very occasionally have no clear evidence of a primary tumor mass. Since these patients have rarely been investigated, we report on a series of patients with RMS and unknown primary tumor site registered in the MTS 2008 protocol (October 2008 - December 2016) coordinated by the European pediatric Soft tissue sarcoma Study Group. METHODS: Patients were administered 9 cycles of induction chemotherapy, and 48 weeks of maintenance chemotherapy. Surgery and/or radiotherapy was planned after the first assessment of tumor response, and implemented after six cycles of chemotherapy. If feasible, radiotherapy to all sites of metastasis was recommended. RESULTS: We identified 10 patients with RMS and unknown primary site, most of them adolescents (median age 15.8 years, range 4.6-20.4). Nine had fusion-positive alveolar RMS. Multiple organ involvement was identified in 7 patients, 2 only had bone marrow disease, and 1 only had leptomeningeal dissemination. All patients were given chemotherapy, 4 were irradiated, and none had surgery. Three patients underwent allogeneic bone marrow transplantation. At the time of this analysis, only 2 patients are alive in complete remission: 1 had received radiotherapy; and 1 had a bone marrow transplant. CONCLUSIONS: RMS with unknown primary tumor occurs mainly in adolescents and is typically fusion-positive alveolar. Radiotherapy may be important, but survival is poor and patients should be offered enrollment in investigational trials.
