Granulosa Cell Tumors

By Jos Mottershead

"Granulosa-thecal cell" or "granulosa cell" tumors (GCT's) are one of the most common ovarian tumors to be found in equines.

Granulosa cell tumors are usually benign steroid-producing tumors that may grow very large. A Granulosa Cell Tumor, with a ruler indicating it's sizeOften they are initially found during a routine reproductive examination of the mare, when one ovary is discovered to be excessively larger than the other. The tumor shown at left is six inches in length, but larger examples are found. Typically the opposite ovary is found to be small and displaying no follicular activity.

A GCT may also be suspected if the mare's attitude suddenly changes. For example she may become very "studdy", displaying stallion attitudes and postures, and may become aggressive, and even start to develop the typical stallion cresty neck. This is as a result of the granulosa cell tumor secreting testosterone. Alternatively, she may display continuous estrus ("nymphomania"), or even only very occasional estrus.

GCT being removed through a flank incisionConclusive diagnosis of a GCT can be difficult to determine without the correct evaluation of multiple factors. Ultrasound, which may initially appear to be a sensible diagnostic tool to use, is actually frequently not of great assistance, other than indicating that the affected ovary is of excessive size. It should be noted that there are ovarian conditions other than GCT's that may present similarly per ultrasound, especially annovulatory hemorrhagic follicles. Combined blood-inhibin and testosterone assays have proven reliable for diagnosis of GCT's, however marginal results can again fail to be conclusive. Recent research1 has replicated work done by the same group in 2011, and confirmed another assay to have excellent reliability for identification of GCTs. This assay looks at anti-Müllerian hormone (AMH). This research compared the various assays for accuracy among 403 samples and found the AMH assay correctly identified (98%) of affected mares, compared to only 80% for inhibin and 48% for testosterone assays.

Treatment of granulosa-thecal cell tumors is achieved by removal of the affected ovary and the tumor. A return to reproductive normality may be expected six to twelve months after surgery. Removal can be carried out trans-vaginally or through a flank incision (seen at right), depending upon the size of the tumour, and other circumstances.

GCT cut in half, showing the definitive honeycomb effect



When cut in half, the GCT exhibits a definitive "honeycomb" pattern on the interior, as seen here on the left.




Ball B.A., Almeida J., Conley A.J. (2013) Determination of Serum Anti-Müllerian Hormone Concentrations for the Diagnosis of Granulosa Cell Tumours in Mares. Equine Vet J. 45(2):199-203

All photos on this page courtesy of Terry Ready of Oakwood Quarter Horses, the attending anesthesiologist for the surgery.

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