Metaplastic carcinoma of the breast mimicking breast implant-associated squamous cell carcinoma: a challenging differential diagnosis.

Metaplastic carcinoma of the breast (MBC) is an uncommon disease that accounts for 0.2-1% of all invasive breast carcinomas, comprising a heterogeneous group of diseases characterized by differentiation of the neoplastic epithelium to squamous cells and/or mesenchymal-looking elements. Breast implant-associated squamous cell carcinoma (BIA-SCC) is a rare complication of breast implantation, with 22 cases reported in the literature. Due to the histological overlap between MBC and BIA-SCC, the differential diagnosis may be challenging, especially in patients with an advanced cancer-bearing breast implant, in which assessing the tumor's primary site may be difficult. The limited amount of scientific data on BIA-SCC implies the absence of a standardized diagnostic method and of a specific staging system to guide patients' clinical management. Of the 22 BIA-SCC cases reported in the literature, 14 (64%) had squamous metaplasia of the inner surface of the capsule, whereas in 10 (45%), there was a histologically proven spread to the extracapsular tissues without a detailed description of the capsule infiltration. Herein, we describe the case of a peri-implant tumor mass with squamous histology in a patient treated with mastectomy and implant-based breast reconstruction for a microinvasive breast carcinoma, in which the absence of squamous metaplasia of the capsule and of its neoplastic infiltration favored a diagnosis of MBC likely originating from the peri-implant tissue. This case suggests that in patients with peri-implant cancers with squamous differentiation, the extension of the tumor throughout the capsule thickness and the presence of squamous metaplasia of the capsule are critical factors that should be considered in the differential diagnosis between BIA-SCC and MBC. In addition, even in cases with capsule-confined tumors, the depth of the capsular involvement can be used to stage the disease, similar to what is currently recommended for BIA-ALCL.