The Importance of Metastatic Papillary Serous Carcinoma in Cases of High-grade Encapsulated Papillary Carcinoma of the Breast: An Extremely Rare Case and a Possible Pitfall

Encapsulated papillary carcinoma (EPC) of the breast has a favorable prognosis. High-grade EPCs, triple-negative or HER-2-positive, are dealt with as invasive carcinomas. Breast metastasis associated with serous carcinoma is a late-stage event. The discrimination between the two diagnoses can be very challenging.

A 79-year-old woman with a history of well controlled high-grade serous papillary carcinoma of the peritoneum went through a total left mastectomy and sentinel lymph node biopsy (SLNB) because of an invasive carcinoma in her left breast. In the lab, a peripheral nodular mass of 4cm was found. Microscopically, large intracystic papillary stalks, with high nuclear grade, surrounded by collagenous tissue were identified compatible with invasive encapsulated papillary carcinoma with positive estrogen receptor. A few months later, the patient was diagnosed with a supraclavicular cervical mass, which on fine needle biopsy (FNB) was indicative of metastatic serous papillary carcinoma. Immunohistochemical stains were similar in breast and previously treated peritoneal tumor showed ER+, PAX8+, p53+ (wild type) and high Ki-67 (80%). WT1 was positive only in peritoneal serous carcinoma. GATA-3 was weakly, scarcely expressed in both specimens. The findings pointed to metastatic serous papillary carcinoma (SPC) in the breast, mimicking primary carcinoma of the EPC type.

Pathology of breast metastases and distinction from primary breast cancers is done by a combination of morphological and IHC features. In our case, the lack of clinical history, the type of surgical approach (mastectomy and SLNB), the solitary lesion, the EPC pattern of growth and the diffuse ER+ staining, were indicative of primary breast lesion. Various morphologic growth patterns of metastatic PSC have been described, among which EPC-like needs to be considered.