Case 4 180126 (17B2786)
Conference Coordinator: Wesley Siniard.
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Eleven-year-old, female spayed boxer dog
This animal presented for a 4- to 6-week history of a firm, 8 x 7 x 8 cm, firm, movable mass at the level of her right iliac crest, which was slowly increasing in size. Ultrasound of the mass revealed a well-demarcated, heterogeneously hypoechoic mass overlying the right iliac wing. The was close to the bone, but the bone margin was smooth and normal. Fine needle aspirates were obtained and cytology revealed a malignant neoplasia which was likely suggestive of neuroendocrine origin. A biopsy of the mass was then taken.
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Two sections of skeletal muscle are examined, in which an unencapsulated, densely cellular neoplasm composed of sheets and short streams of neoplastic round to spindle cells infiltrates into the skeletal muscle. Neoplastic cells have indistinct cells borders and a moderate amount of eosinophilic cytoplasm. Nuclei are round to ovoid with finely stippled chromatin and one variably distinct nucleolus. Anisocytosis and anisokaryosis are moderate, and there are 20 mitotic figures per ten 400x fields. Neoplastic cells frequently have prominent cytoplasmic invaginations. Moderate numbers of lymphocytes and small numbers of hemosiderin-laden macrophages infiltrate the neoplastic population and adjacent skeletal muscle.
Immunohistochemistry stain for desmin: Neoplastic cells exhibit variably strong cytoplasmic and membranous immunoreactivity.
Right iliac crest mass: Rhabdomyosarcoma
Immunohistochemistry confirmed the neoplastic population to be a rhabdomyosarcoma, which is most consistent with an embryonal subtype based on morphology of the cells. Relatively few cases of these tumors in domestic species have been reported in the literature; and therefore, there is no established criteria for prognosis with these tumors. However, these neoplasms should be regarded as invasive with potential to metastasize. An increased mitotic rate such as is seen in this case has been associated with an unfavorable prognosis.
Meuten DJ. Tumors in Domestic Animals. Fifth Edition. Wiley Blackwell. 2017.
