Spindle cell rhabdomyosarcoma of the retroperitoneum: an unusual case developed in a pregnant woman but obscured by pregnancy.

Spindle cell rhabdomyosarcoma (RMS) is an uncommon histiologic variant of RMS that has spindle cell morphology. This tumor occurs almost exclusively in childhood and more rarely in adults. Only a few adult cases, including two retroperitoneal cases in male patients, have been documented previously. We describe a rare case of spindle cell RMS of the retroperitoneum in a 37-year-old woman developed during pregnancy and incidentally discovered after vaginal delivery. Computed tomography showed a huge tumor mass, measured 20 × 20 × 15 cm in size, arising in retroperitoneal space. Histologically, the tumor consisted of spindle cells arranged in a fascicular or herringbone growth pattern, morphologically mimicking adult fibrosarcoma, intermingled with scattered rhabdomyoblasts. Mitotic activity ranged from 20 to 28 mitoses per 10 high-power fields and tumor necrosis was evident. Immunohistochemically, tumor cells were stained diffusely positive for muscle specific actin, desmin, and vimentin, scattered positive for myogenin, MyoD1 and myoglobin, with a Ki-67 (MIB-1) proliferative labeling index of 46.11%. This tumor also stains positively for CD99, strong cytoplasmic WT1, and nuclear p53. Other markers such as S100 protein, smooth muscle specific actin, CD34, cytokeratin, and epithelial membrane antigen were all negative in the tumor cells. On the basis of the findings, a spindle cell RMS was diagnosed. The neoplasm was incompletely excised because of encasement of major vessels and invasion to adjacent structures, and additional chemotherapy was given.