Malignant ovarian granulosa cell tumour of corpus luteum origin in a California sea lion (Zalophuscalifornianus).

A 19-year-old female California sea lion (Zalophus californianus) in managed care was euthanized following a 3-week history of hyporexia, diarrhoea, lethargy, abdominal discomfort, dyspnoea and tremors. Ante-mortem ultrasound confirmed splenomegaly with multifocal infiltrates and splenic aspirates revealed malignant cells of undetermined origin. Necropsy confirmed a malignant trabecular to diffuse granulosa cell tumour (GCT) that had metastasized to the liver, spleen, kidneys, adipose tissue, pancreas, adrenal glands, lungs, lymph nodes, brain and bone marrow. The GCT was arising within a corpus luteum and was not causing ovarian enlargement. A reticulin stain was useful to distinguish neoplastic granulosa cells from non-neoplastic cells of the corpus luteum. All neoplastic granulosa cells expressed vimentin and cytokeratin but few expressed inhibin alpha. The GCT had a mitotic count of 17 per 2.37 mm2 and a Ki67 index of 85%. The sea lion delivered a stillborn pup 5 months prior to euthanasia, indicating the interval from tumour onset to severe metastatic disease was less than 5 months. The aggressive behaviour, high proliferation indices, origin of the neoplasm in a corpus luteum and lack of ovarian enlargement are unusual features for a GCT.