Subcutaneous Leiomyosarcoma in a Smad3^+/- Mouse

Subcutaneous leiomyosarcoma is a malignant tumor that originates from smooth muscles in the skin. Herein, we report a case of subcutaneous leiomyosarcoma that occurred in the ear pinna of a 19-month-old male Smad3^+/- mouse. On gross examination, the mass extended to the whole ear auricle and obscured the ear opening. The tumor was approximately 10x10 mm in size, ill-circumscribed, and firm with superficial hemorrhage. The cut surface of the mass was grayish-white and solid with focal necrosis and hemorrhage. Histologically, spindle tumor cells with cigar-like nuclei and plump cells with round nuclei formed broad interlacing fascicles. Additionally, the tumor cells invaded adjacent muscles. Immuno-histochemically, the tumor sections had positive reactions for desmine and a-SMA and were partially positive for vimentin while negative for S-100 and myogenin. Based on the results of the examination, we diagnosed this case as subcutaneous leiomyosarcoma.