A granulosa cell tumor
Keywords: GCT, neoplasia, equine, mare, ovaryGranulosa cell tumors (GCTs) such as this one are common forms of ovarian neoplasia in mares. Although they have been related to both male and female behavior in the affected animal they are also a common cause of pathological anestrus.
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Granulosa cell tumors produce a multitude of sex steroids that may cause negative feedback and suppress hypothalamic-pituitary function. High serum concentrations of testosterone are sometimes diagnostic of these tumors but GCTs often produce little testosterone. When they do produce testosterone, clitoral enlargement may be seen as shown in the inset at lower right.
Although their steroid production is variable, GCTs almost invariably produce the complex and closely related polypeptide hormones inhibin and mullerian inhibiting hormone (MIH). In fact, MIH is more consistently produced by these tumors than inhibin, so MIH assays have become the test of choice for the diagnosis of GCTs.
MIH, inhibin and androgens all suppress the production of FSH production. Therefore follicle growth on the contralateral ovary is usually suppressed as well (see inset image on the right). This is a useful diagnostic feature of the condition helping to differentiate GCTs from other forms of neoplasia and hematomas as well.
Suppression of follicle growth on the contralateral ovary can continue for long periods of time after the removal of the tumor. Owners should be warned of this possibility.
The cut surface of a GCT usually has a honeycomb appearance as shown above. This is obvious on pre-operative ultrasonography as is suppression of activity on the contralateral ovary.
As shown below, GCTs can also be solid but this is less common than the honeycomb appearance.
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As shown elsewhere in LORI, GCTs may also consists of only one or two major cystic cavities.
Fortunately (unlike the situation in humans) GCTs are almost always benign in mares and surgical removal can be delayed for months if required. In aged mares, this author has elected not to remove the neoplastic ovary because operative risk can outweigh the value of treatment.
