The patient underwent ultrasound guided core biopsy.
MICROSCOPIC DESCRIPTION: Sections show a diffusely cellular lesion comprised of uniform bland spindle cells with plump nuclei arranged in loose fasicles and storiform patterns. The cells are set in a background of thick eosinophilic collagen bundles. The stroma also shows focal myxoid appearance. Occasional trapped skeletal muscle cells are seen, and there are scant extravasated lymphocytes and erythrocytes. Mitoses and necrosis are not seen. A small amount of skeletal muscle is present in the biopsy, and the cellular lesion focally involves the muscle. By immunohistochemistry the spindle cells are positive for SMA and negative for S100, Desmin, and CD34.
DIAGNOSIS: Nodular fasciitis.