Aggressive Spindle Cell Rhabdomyosarcoma in an 11-Month-Old Boxer Dog

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Rhabdomyosarcoma (RMS) is a malignant neoplasm derived from mesenchymal tissue with a tendency toward myogenic differentiation associated with the embryogenesis of skeletal muscle. According to the histological features, it can be classified in embryonal, botryoid, alveolar, and pleomorphic, which usually correspond to clinical behavior and prognosis. The spindle cell (SCRMS) variant is a rare subtype of the embryonal RMS and is considered a less aggressive lesion. The aim of the present paper is to report an unusual case of SCRMS in an 11-month-old male boxer dog diagnosed with extensive SCRMS that affected the frontal region of the skull. Due to the aggressive nature of the lesion and poor clinical prognosis the dogs owners elected euthanasia as a treatment. A full postmortem analysis was carried out. Microscopically, the lesion was composed of a highly cellular proliferation of spindle cells arranged in long and intersecting fascicles. After performing the immunohistochemical study (HHF-35, smooth muscle actin, desmin and MyoD1) of the lesion the diagnosis of SCRMS was undertaken.