Granulosa Cell Tumour
Granulosa cell tumours (GCTs) are the most common ovarian tumour in horses and can occur in
mares of any age. They may be composed of granulosa cells alone or contain theca cells (granulosa-
theca cell tumours). They are almost always unilateral, functional and benign.
CLINICAL SIGNS
The hormonal pattern is inconsistent and determines the clinical signs observed. These may include:
Persistent, irregular or absence of oestrus
Masculinisation: stallion-like behaviour including mounting, aggressiveness, squealing,
striking, and in chronic cases, increased muscle deposition, cresty neck and an enlarged
clitoris.
RECTAL EXAMINATION
One ovary is normally enlarged with no ovulation fossa being palpable. GCTs are slow growing
tumours but have been reported up to 40cm in diameter. The contralateral ovary is usually small,
firm and inactive. If there is asymmetry in ovary size yet the smaller ovary is cycling, then a granulosa
cell tumour is less likely and the enlarged ovary is more likely to have a haematoma or teratoma.
ULTRASONOGRAPHIC EXAMINATION
Granulosa cell tumours typically have a multicystic honeycomb appearance with some areas of solid
tissue. A minority will have a dense homogenous appearance and appear as a solid ovarian mass,
with a single anechoic fluid-filled cyst. Haematomas or regions of necrosis within the tumour are
common. The variable ultrasonographic appearance of GCTs hampers differentiation from other
possible diagnoses including haematoma, cystadenoma, lymphoma and germ cell tumours.
Since clinical signs, rectal examination findings and ultrasonographic appearance are not consistent
in all GCT cases they do not predict the presence of a GCT reliably. Therefore, measurement of
serum hormone concentrations are recommended for diagnosis.
ANTI-MÜLLERIAN HORMONE CONCENTRATION
Serum levels of anti-Müllerian hormone (AMH) have been shown recently to be a reliable means of
identifying the presence of granulosa cell tumours (GCT). In mares, AMH is produced by granulosa
cells and in healthy mares serum AMH concentrations are similar throughout the oestrus cycle and
pregnancy. The proliferation of granulosa cells which occurs with GCTs is associated with a marked
increase in serum AMH concentration.
In a recent investigation (Ball et al. 2013), the diagnostic accuracy of AMH, inhibin and testosterone
for the diagnosis of GCT was compared in 44 mares in which GCTs were confirmed with
histopathology. Of these mares with GCTs, AMH significantly outperformed all other tests and was
increased in 98% of cases; compared with inhibin in 80%, testosterone in 48% and either inhibin or
testosterone in 84%. Values in mares of differing reproductive status are in the table below.
AMH (ng/mL)
median interquartile range
Non-pregnant mares 0.3 0.0 – 0.5
Pregnant mares 0.2 0.0 – 0.5
Mares with GCT 66.0 27.1-184.5
Ball, B.A., Almeida, J. & Conley, A.J. (2013) Determination of Serum Anti-Müllerian Hormone
Concentrations for the Diagnosis of Granulosa-Cell Tumors in Mares. Equine Veterinary Journal 45:
199-203.