Granulosa cell tumors (GCT; also known as granulosa-theca cell tumors) are the most common of the sex cord-stromal tumors of ovaries. They are usually unilateral and nonmalignant in any species. In mares, they are the most common reproductive tract neoplasm. There is no breed or age predilection. Because of their variable hormone production, they are associated with three behavioral patterns: anestrous, continuous or intermittent estrus, and male behavior. Abnormal behaviors coupled with rectal examination findings of an enlarged ovary with a small contralateral ovary are highly indicative. Endocrine testing is required to differentiate a GCT from other types of ovarian tumors or causes of ovarian enlargement. Endocrine hormone panels have included levels of testosterone, inhibin, and progesterone and more recently anti-Müllerian hormone (AMH). AMH is secreted by granulosa cells, and levels do not vary with the estrus cycle or pregnancy as inhibin and testosterone levels do. AMH concentrations > 4.0 ng/mL are highly correlated with granulosa cell tumors. The tumors are generally benign, but surgical removal is recommended because complications (eg, lameness, colic) can be associated with a large abdominal mass. For masses less than 15 cm in one dimension, laparoscopic surgery through the paralumbar fossa reduces risks associated with general anesthesia, shortens recovery time, and costs less. Prognosis for return to reproductive function is good, usually by the following spring.
Links to sections in The Manual:
Cystic Ovary Diseases in Mares
References
Ball BA, Almeida J, Conley AJ. Determination of serum anti-Müllerian hormone concentrations for the diagnosis of granulosa-cell tumours in mares. Equine Vet J. 2013;45:199-203. doi: 10.1111/j.2042-3306.2012.00594.x
Ball BA, Claes AN. Biological Functions and Clinical Applications of Anti-Müllerian Hormone in Stallions and Mares. Vet Clin North Am Equine Pract. 2016 Dec;32(3):451-464. doi: 10.1016/j.cveq.2016.07.004