Answer: Embryonal rhabdomyosarcoma

Histology: The lesion displays polypoid architecture with a fibrotic central core and edematous polypoid folds. The overlying squamous epithelium is benign with appropriate maturation. The edematous stroma contains spindled to round cells with hyperchromatic nuclei and scant cytoplasm; occasionally the cells display eosinophilic cytoplasm. The spindled cells focally condense underneath the overlying epithelium to form a cambian layer.

Discussion: Embryonal rhabdomyosarcoma is the most common subtype of rhabdomyosarcoma, with alveolar rhabdomyosarcoma being second most common. Embryonal rhabdomyosarcoma is a “small round blue cell tumor” which resembles primitive, embryonic skeletal muscle; histologically, the lesional cells may be round or spindled, and they may display cross-striations or eosinophilic cytoplasm representing myoid differentiation. In contrast, alveolar rhabdomyosarcoma consists of small round blue cells that line fibrous septae and are loosely discohesive resembling the pattern of lung alveoli.

Clinically, embryonal rhabdomyosarcoma most often occurs in children. The botyroid subtype is characterized by a “grape-like” clusters that may be seen grossly (hence the polypoid appearance of the tumor), and it may project from the vagina. A classic histologic finding in botryoid rhabdomyosarcomas is the presence of a cambian layer, where the malignant cells condense underneath the intact overlying benign epithelium. The botryoid subtype actually has an excellent prognosis, with a 10 year survival of >90%. The alveolar rhabdomyosarcomas have the worst prognosis.

The differential diagnosis of embryonal rhabdomyosarcoma includes other small round blue cell tumors, and the list varies depending upon the anatomic site of the tumor; the differential would include desmoplastic small round cell tumor, lymphomas, neuroblastoma, and Ewings/PNET. By immunohistochemistry, rhabdomyosarcomas label with muscle markers (desmin, actin, myogenin MyoD1, and myoglobin). Alveolar rhabdomyosarcomas are translocation tumors, with translocations between the PAX3 (chromosome 2) or PAX7 (chromosome 1) and FOXO1 (formerly known as forkhead); the most common translocations is t(2;13) followed by t(1;13).

Reference(s):
– Leuschner I, Harms D, Mattke A, Koscielniak E, Treuner J. Rhabdomyosarcoma of the urinary bladder and vagina: a clinicopathologic study with emphasis on recurrent disease: a report from the Kiel Pediatric Tumor Registry and the German CWS Study. Am J Surg Pathol. 2001 Jul;25(7):856-64.
– Fisher C. Soft tissue sarcomas with non-EWS translocations: molecular genetic features and pathologic and clinical correlations. Virchows Arch. 2010 Feb;456(2):153-66.