Uncommon presentation of perineal rhabdomyosarcoma: A case report

Rhabdomyosarcoma (RMS) is the most common soft tissue sarcoma among children in North America. Early diagnosis and treatment are important to prevent disease progression and improve prognosis. A 6-year-old-boy presented with a 1cm perianal lesion that was tender and erythematous. Presumed diagnosis was perianal abscess: incision & drainage was performed and complicated with significant bleeding requiring arterial embolization. Previous computed tomography (CT) had reported a large abscess cavity. Significant growth of tissue at surgical site and swelling of the right buttock and perineal area prompted further imaging with MRI which demonstrated suspicion for a pelvic soft tissue mass with extension to the perineum. Biopsies confirmed the mass was an embryonal rhabdomyosarcoma (RMS). He was diagnosed with intermediate risk Group III, Stage III translocation-negative rhabdomyosarcoma. He was treated with VAC/VI. Discussion at province-wide tumor boards recommended no further resection given the inability to achieve negative margins. The patient underwent proton beam therapy for local control. There was reduction in size of the pelvic mass, the perineal wound healed, and the defect completely resolved while on active therapy. The patient remains well now eight months into active follow up. Perineal RMS is a rare disease and difficult to diagnosis as it often presents very similarly to perianal abscess. While it is not reasonable to screen all painful perianal masses with MRI, reflection of this case demonstrates some delineating features that can be useful to suggest an alternative diagnosis in order to minimize morbidity and shorten time to diagnosis.