Introduction/Background Accurately diagnosing abnormal ovarian mass is crucial to determine the scope of surgical intervention and adjuvant therapy. The proportion of metastatic ovarian tumours ranges from 5% to 30%. Gastrointestinal (GI) tract followed by breast and female reproductive organs are the most common origin. João Lobo et al. have reported 13% of metastases to the ovary originating from breast cancer. Most cases are known to be asymptomatic in contrast to the case presented in our centre.

Methodology We present the case of a 60-year-old woman complaining of abdominal distention, mild pain and discomfort. The initial ultrasound examination revealed bilaterally abnormal ovaries with ascites and left-sided hydrothorax. Consequent diagnostic work-up identified extensive malignant process of unknown primary origin: diffuse peritoneal carcinomatosis, ascites (>500cc), multiple foci of metastatic bone lesions, small (0,5–0,7 cm) contrast-enhancing lesions in both breasts, increased CA125 (328 U/mL), and normal levels of CA19–9 and CEA. Synchronous ovarian and breast cancer, or metastatic breast cancer was less suspected due to mammogram findings indicating dex-BI-RADS - 2 sin-BI-RADS - 3. Primary GI tract cancer was ruled out by esophago-gastro-duodenoscopy and colonoscopy. Metastatic ovarian/fallopian/peritoneal tumour, mesothelioma and tuberculosis were suggested as possible diagnosis by multidisciplinary team.

Results Diagnostic laparoscopy was performed with frozen section analysis, consistent with poorly differentiated carcinoma of unknown primary. Biopsy specimens sent for bacterioscopic examination was negative for tuberculosis. Peritoneal fluid cytology was negative for atypical cells. The immunohistochemistry report was notable for positive GCDFP, GATA, CK7, negative CA125, PAX8, WT1.

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Conclusion Definitive diagnosis of metastatic breast cancer invasive lobular carcinoma ER (80%) PR (10%) HER2-neu (1+) KI67 (12%) grade 3 cT0N1M1 was made and appropriate systemic therapy initiated. The case highlights the challenge of diagnosing occult breast cancer mimicking advanced stage ovarian cancer, emphasizing the importance of accurate diagnosis to ensure proper treatment and care for the patient.

Disclosures The authors have no conflicts of interest or financial disclosures to report.