PAT-1. An unusual case of synovial sarcoma arising from the chest wall.

A 25-year-old male presented with a rapidly enlarging left lateral chest wall mass. The chest radiograph showed minimal expansion of the left sixth posterolateral rib with questionable pleural thickening on the left side of the chest. Routine hematoxylin and eosin stained sections were examined. Immunoperoxidase and immunohistochemical stains were performed. Gross examination demonstrated a circumscribed, soft tan mass measuring 9 cm in greatest dimension, present on the anterior surface of ribs four through seven. Thin fibromembranous tissue separated the tumor from the serratus anterior. The cut surface of the tumor was pink-tan, fleshy and lobulated. Histologically, the tumor was composed of pleomorphic, spindle-shaped malignant cells arranged in small fascicles, in a background of wiry, collagenous stroma. The tumor transitioned into more poorly differentiated areas with a round cell morphology and high mitotic rate. A focal hemangiopericytoma-like vascular pattern was also present in areas. The malignant cells were multifocally immunopositive for EMA, focally positive for muscle-specific actin, desmin, CD99, and myoglobin, and were strongly positive for vimentin. The malignant cells were non-reactive for AE1:3, S100 protein, CD34, CD31, MART-1, and GFAP. We report a case of synovial sarcoma arising from the chest wall, an unusual location. Synovial sarcoma commonly occurs in the extra-articular regions of the long bones, particularly the knee. Rarely (approximately 2%), these tumors arise in the chest wall. Synovial sarcomas pose a diagnostic dilemma, especially when poorly differentiated. As is characteristic of poorly differentiated tumors, the typical immunohistochemical staining pattern is lost. When considering synovial sarcoma in the differential diagnosis of a soft tissue tumor, cytogenetics is most helpful. The characteristic chromosomal translocation is t(X:18).

Candice Frederick, MD, and Armine Baltayan, MD. Pitt County Memorial Hospital/ECU School of Medicine, Greenville, NC.