Brenner tumors are uncommon surface epithelial-stromal tumors of the ovary, frequently presented in women between their fifth and seventh decades of life. The majority of Brenner tumors are benign, and these tumors predominantly present as solid on imaging and pathological examination, although in <30% of cases, serous and mucinous-associated cystadenomas may account for the cystic appearance when the Brenner tumor itself is very small or visually inseparable from the coexisting cystic neoplasm (1). There is little information regarding their appearance on computed tomography (CT) examination (1,2). Although ovarian tumors have similar clinical and radiological features, predominant or specific imaging features may be present in certain types of ovarian tumors. Characterization of an ovarian mass is of the utmost importance by CT in the preoperative evaluation of an ovarian neoplasm (3). It enables the surgeon to anticipate carcinoma of the ovary prior to surgery so that adequate procedures are planned. Therefore, familiarity with the clinical and imaging features of various ovarian tumors is important in determining the likelihood of a tumor being benign or malignant. The aim of the present study was to describe the clinical and CT characteristics of Brenner tumors.

Brenner tumors are uncommon epithelial-stromal tumors that originate from the surface of the ovaries (4) and were first described in detail by Fritz Brenner in 1907 (5). Histologically, Brenner tumors exhibit epithelial cell nests growing in a fibrous stroma (6). These epithelial cells have the similar appearance to urothelial cells, and therefore Brenner tumors were classified as a transitional cell tumor type by the World Health Organization. Brenner tumors are divided into three primary types, including benign, borderline and invasive. Benign Brenner tumors may be found in women of any age, but are predominantly seen in women aged ~50 years old, while women with borderline or malignant Brenner tumors are typically 10 years older (7). The mean age of patients who participated in the present study was 57.2 years, and all cases were benign Brenner tumors. Almost all Brenner tumors are asymptomatic and discovered by accident. In ~30% of cases, a second tumor is identified in the same ovary, most often a serous or mucinous cystadenoma, or occasionally a teratoma (5,7,8). In the present study, 3 cases were identified with coexisting uterine leiomyomas, 2 with abdominal swelling pain and 2 with postmenopausal irregular bleeding. The post-operative histological examination confirmed 2 cases with differing degrees of endometrium thickening, 3 with uterine leiomyomas, 1 with uterine leiomyomas and endometrium thickening, 2 with ovarian cystadenoma, and 1 with ovary cysts. To the best of our knowledge, no literature has reported an association between ovarian Brenner tumors and coexisting comorbidities. Patients that also present with pleural fluid and ascites are said to have Meigs' syndrome (9). Ovarian Brenner tumors combined with Meigs's syndrome are e common tumor types. Here, no case was identified with pleural fluid and ascites, which was attributed to the limited number of cases investigated.

The CT manifestations of Brenner tumor include adnexal solid or solid-cystic, or cystic masses with a clear border, amorphous in appearance, with round, oval, irregular or lobulated morphologies. Benign Brenner tumors are typically small solid tumors with small cysts (10). Furthermore, grossly visible amorphous calcifications in the solid mass with mild or moderate enhancement are typical CT manifestations. Histologically, minimal cystic changes, necrosis and varying degrees of calcium crystal are often identified in the solid mass (11). In the current study, 5 cases had solid masses, of which 2 were with multiple scattered calcification and 1 with dot-like calcification. In addition, 3 cases were with evident enhancement and 1 with mild enhancement. According to the aforementioned description, borderline or invasive Brenner tumors are characterized by the presence of solid-cystic or cystic structures with papillary structures, and mild or moderate enhancement in the solid mass. In a study by Wang et al (12), ovarian Brenner tumors were described are typically unilateral and often accompanied by other tumor components, such as solid tissue in ovarian cancer. When a tumor is of uniform component, the CT imaging often reveals a homogeneous solid tumor with homogeneous or heterogeneous density.

When a tumor is accompanied by other tumor components, it may be solid-cystic or cystic and has partial calcification. Following enhancement, a benign Brenner tumor is slightly enhanced, while the borderline of the Brenner tumor is moderately/highly enhanced. In the present study, following the pre-surgery histology, 3 cases were confirmed as cystic-solid mass, of which 2 were borderline tumors and 1 was an invasive tumor. According to the post-surgery histology, epithelial nest with an intact border had no heterogeneous cell or mesenchyme infiltration. Furthermore, a cystic mass with a papillary nodular shadow (1/9 cases) was revealed on the CT scan. We hypothesized that the residue cystic or necrosis tumor resulted in an artifact shadow, thereby generating a papillary nodular shadow. We concluded that CT scans combined with a histology should be performed when diagnosing Brenner tumors. The observations of the current study are consistent with the previous description by Wang et al (12). Generally, malignant tumors should be considered when a cystic mass of the ovary with enhanced solid nodules is present (13); however, Brenner tumors do not adhere to this rule. Furthermore, the possibility of a Brenner tumor should be considered when there is the presence of combined dot-like calcifications in a cystic-solid lesion. The main CT manifestations of Brenner tumors were as follows: A solid mass in a small sized tumor; cystic-solid mass with multiple cystic changes during tumor growth; and single cystic lesions with a solid border when the presence of increasing degree of cystic changes, necrosis and septa destroy.

The differential diagnosis of Brenner tumor includes fibroma, cystoadenocarcinoma, Krukenberg tumor and leiomyomas. Fibroma presents as a homogeneous solid mass with a smooth border and minimal enhancement, but occasionally involves cystic changes and necrosis calcification (3). Cystoadenocarcinoma presents as an adnexal cystic-solid mass with an unclear border and cystic-solid borderline, an unequally thickened cystic wall with multiple nodules and septa, and is usually accompanied by peritoneal lumen implantation metastasis (14). Krukenberg tumors are usually bilateral with additional findings of primary malignancy, and frequently accompanied by abdominal and pelvic effusion or metastasis (15). Leiomyomas present with a limited amount of dot- or eggshell-like calcification with a similar degree of enhancement as Brenner tumors (16). Furthermore, the difference of blood supplies may be detected between Brenner tumor and leiomyomas (17).

In conclusion, the present study demonstrated that the CT features of benign ovarian Brenner tumors include multiple, scattered calcification focused in the solid component, and we propose that histological examination is necessary. These results aid in improving the understanding of the clinical manifestation, histological and CT features of ovarian Brenner tumors, which may aid in its diagnosis.