Ovarian Brenner tumor represents an uncommon type of ovarian epithelial neoplasm, predominantly benign, with malignant variants constituting approximately 1–5% of cases (1). These tumors typically arise from metaplasia of the ovarian surface epithelium and may occur at any age, although they are more prevalent in perimenopausal women aged 50 to 70 years (1, 2). Most patients are asymptomatic, and the tumors are often incidentally discovered following ovarian surgery performed for unrelated conditions. However, larger tumors may elicit symptoms such as abdominal distension or pain. Due to their rarity and subtle or atypical clinical presentations, Brenner tumors are commonly identified during routine physical examinations or surgical interventions for other pelvic disorders (1).

This report describes a rare case of ovarian MBT in a postmenopausal patient who presented with abnormal uterine bleeding without associated endometrial pathology. The imaging, histomorphological, and immunohistochemical features are discussed to enhance understanding of this entity.

A 53-year-old woman presented with a six-month history of intermittent vaginal bleeding occurring two years after natural menopause. An initial transvaginal ultrasound performed at a local hospital revealed a 4.48 × 3.40 cm hypoechoic mass in the left adnexal region, exhibiting a heterogeneous internal echotexture and punctate hyperechoic foci. The patient subsequently consulted an external medicine department, where surgical intervention was recommended but declined. On September 28, 2025, she experienced recurrent minor vaginal bleeding and was evaluated at the gynecology outpatient clinic of the Third Hospital of Hebei Medical University (Figure 1).

A follow-up transvaginal ultrasound demonstrated a 7.8 × 5.6 cm mass of mixed echogenicity adjacent to the left uterus. The cystic component contained multiple linear hyperechoic structures, while the local echoes in the solid area appeared uneven, with scattered punctate hyperechoic foci. The capsule was incomplete, with relatively clear boundaries. Doppler ultrasound indicated point-like linear blood flow signals within the solid region (Figure 2).

Upon admission, the physical examination revealed mild uterine enlargement. A non-tender, moderately mobile mass greater than 7 cm in diameter was palpated in the left adnexal region; the right adnexa was unremarkable. Laboratory investigations, including tumor markers (AFP, CEA, CA19-9, CA-125, CA15-3), reproductive hormones (LH, FSH, PRL, progesterone, estradiol, testosterone), complete blood count, and hepatic and renal function profiles, revealed no significant abnormalities. The patient was admitted for further diagnostic evaluation and management.

The World Health Organization (WHO) classifies Brenner tumors into three categories: benign, borderline, and malignant. MBT of the ovary is a rare malignancy, representing a distinct subtype of ovarian cancer that resembles invasive urothelial carcinoma. It is commonly associated with benign or borderline Brenner tumors. Clinical manifestations commonly include abdominal mass or pain, and some patients report abnormal vaginal bleeding. Tumors are predominantly unilateral (>80%), although bilateral cases have been reported (1, 2).

In the present case, the patient’s chief complaint was postmenopausal abnormal uterine bleeding lasting six months. Previous studies indicate that approximately 4–14% of Brenner tumors are associated with endometrial hyperplasia, often linked to stromal luteinization and estrogen secretion, which may explain abnormal bleeding in some patients (3–5). Nevertheless, transvaginal ultrasound showed an endometrial thickness of 0.5 cm, and preoperative curettage revealed only blood clots with minimal endometrial tissue. Postoperative pathology confirmed an atrophic endometrium, and serum estrogen and progesterone levels were within normal ranges. These findings do not support estrogen-driven endometrial hyperplasia as the cause of bleeding. Further consideration of alternative mechanisms suggests a potential role for tumor-related factors. Vascular endothelial growth factor (VEGF) is known to promote angiogenesis, invasion, metastasis, and chemotherapy resistance in ovarian cancer (6), and elevated VEGF can increase vascular permeability. It is therefore conceivable that MBT may produce low levels of bioactive substances or growth factors that are not detectable by routine hormone assays. Such substances could be locally converted to active estrogen within endometrial tissue or induce bleeding by altering vascular permeability. Routine hormonal testing may not capture these tumor-derived mediators. Furthermore, atrophic endometrium itself is a well-recognized cause of postmenopausal bleeding (7). In postmenopausal women, marked estrogen deficiency leads to a thin endometrium with fragile vessels prone to rupture. Thus, abnormal bleeding may occur even when circulating hormone levels are within normal limits. This mechanism offers an additional plausible explanation for the bleeding observed in this patient.

Preoperative diagnosis of Brenner tumor remains challenging because its histological features closely resemble transitional epithelium. Morphologically, benign Brenner tumor cells show mild cytological atypia, with lightly stained cytoplasm, oval nuclei, and well-defined boundaries between cell nests and surrounding stroma. In contrast, MBT demonstrates indistinct nest boundaries with stromal invasion. Residual benign or borderline Brenner components are often present, which is consistent with the characteristic histopathological features of MBT.

Angel Yordanov et al. reviewed the literature and identified 10 reported cases of MBT, including ovarian and extra-ovarian types. Their results indicated that urothelial differentiation markers, such as GATA3, p63, and CK7, were predominantly positive, while Pax-8 was typically negative (8). The immunohistochemical results in our patient are consistent with their findings. Positive staining was observed for GATA3, CK7, and p63, with a Ki-67 labeling index exceeding 40% in hotspot areas. Postoperative pathological examination demonstrated metastatic lesions involving bilateral fallopian tubes and uterine serosa, further supporting the diagnosis of ovarian MBT. Histopathological and immunohistochemical analyses confirmed the diagnosis of MBT, and the differential diagnosis included the following entities:

MBT is a rare tumor morphologically resembling urothelial carcinoma. This report described a patient with atypical symptoms preoperatively, and no significant abnormalities detected through tumor markers or conventional hormone assays. Although abnormal uterine bleeding is occasionally observed in Brenner tumor patients, the underlying causes remain uncertain. The present case proposes two possible mechanisms for such bleeding, both requiring further investigation.

In summary, this case highlights the necessity of further research into MBTs, particularly regarding pathogenesis, clinical features, and optimal therapeutic regimens. We emphasize the importance for clinicians to maintain a high index of suspicion for MBT, recommend regular health evaluations to patients, and ensure early diagnosis and management to enhance long-term patient survival.