Primary Peritoneal High-grade Serous Carcinoma Misinterpreted as Metastatic Breast Carcinoma: A Rare Encounter in Peritoneal Fluid Cytology.

BACKGROUND/AIM: Peritoneal fluid (PF) cytology is critical for distinguishing high-grade serous carcinoma (HGSC) from metastatic disease in patients with breast carcinoma who present with peritoneal carcinomatosis (PC).

CASE REPORT

A 50-year-old woman underwent surgery and adjuvant therapy for pT1N0 grade 2/2 luminal A breast carcinoma. Sixteen months postoperatively, palliative chemotherapy was administered following a pleural biopsy and diagnosis of metastatic carcinoma. The patient developed PC despite chemotherapy. PF cytology specimens suggested metastatic carcinoma. However, we observed a papillary cellular arrangement during the review of cytology slides. HGSC was confirmed by immunocytochemistry showing positive paired box 8 (PAX8) and Wilms' tumor 1 (WT1) expression and negative GATA-binding protein 3 expression.

CONCLUSION

In patients with breast carcinoma history, an awareness of characteristic cytomorphology of HGSC, including a papillary pattern with positive PAX8 and WT1 immunoreactivity, is essential to prevent the misdiagnosis of such cases and in ensuring accurate treatment and management.