# Ovarian Fibrothecoma in a Mare—Case Report

## TL;DR

A 6-year-old mare was found to have an ovarian fibrothecoma, a rare tumor, during surgery for colic symptoms.

## Contribution

This paper reports a rare case of ovarian fibrothecoma in a mare, emphasizing the importance of histopathology for accurate diagnosis.

## Key findings

- The tumor showed a mixed histological appearance with features of both fibroma and thecoma.
- The normal ovarian structure was completely destroyed, with no follicles or stroma detected.
- Histopathology confirmed the diagnosis of fibrothecoma, a rare sex-cord stromal tumor in mares.

## Abstract

A 6-year-old Dutch Warmblood (KWPN) mare was taken to the Lithuanian University of Health Sciences Large Animal Clinic with colic signs. During a diagnostic laparotomy, an altered right ovary was found and removed. Macroscopically, one part of the ovary was hard and gray. The other parts were smaller, soft, and yellowish, with abundant hemorrhages. Ovarian neoplasia was suspected. During the microscopical examination, the tumor showed a mixed histological appearance. The predominant population of cells showed ovoid to round nuclei and a pale-gray, abundant, oval, round cytoplasm, and indistinct cell membranes imparted a syncytial appearance. A diffuse growth pattern was observed. The cell nucleus showed minimal atypia with large, distinct nucleoli and a moderate mitotic index (10 mitosis). The mentioned features were consistent with fibrothecoma. The tumor was mixed in part of the thecoma in the region of the borders or showed a sharp border in places. The fibroma showed a very fine storiform pattern, which was composed of uniform spindle-shaped cells with an oval nucleus and minimal atypia. The normal histological structure of the ovary was fully destroyed—no primordial, primary, secondary, or tertiary follicles were found. The usual ovarian stroma also was not detected. Based on the histological findings, the tumor was consistent with ovarian fibrothecoma.

Ovarian tumors in mares are uncommon in comparison to other neoplasms and are classified into three categories: gonadal stromal tumors, coelomic epithelium surface tumors, and germinal cell tumors. Some ovarian neoplasms histologically show a mixture of multiple cell types in the same tumor, such as fibrothecoma; therefore, the differentiation between fibroma and thecoma is often difficult. According to the World Health Organization, fibrothecomas are classified as sex-cord stromal tumors (pure stromal tumors). Neoplasms such as fibrothecoma present with limited morphological, clinical, ultrasonographic, and endocrine profile characteristics. To diagnose this type of tumor, a broad clinical examination is needed, but histopathology remains the most accurate. Herein, we report a case of incidentally found ovarian fibrothecoma during a diagnostic laparotomy in a 6-year-old Dutch Warmblood (KWPN) mare who presented to the clinic with colic symptoms. After a unilateral ovariectomy, the altered right ovary was diagnosed as fibrothecoma based on histopathological features.