New variant of breast-invasive lobular carcinoma with solid and encapsulated papillary carcinoma growth pattern

To date, five cases of invasive lobular carcinoma with solid and encapsulated papillary carcinoma (SPC and EPC) growth pattern were reported. In this article, we describe such a case that might represent a diagnostic pitfall. A 61-year-old woman had a mass on the left breast that was characterized by multiple expansile nodules with a fibrous capsule. Tumor cells were arranged in a solid pattern with inconspicuous delicate fibrovascular cores, some were irregular glandular tubes and papillary, similar to the growth pattern of SPC and EPC. The appearance of the tumor surrounding tissue suggested a special type of lobular carcinoma. The diagnosis of invasive lobular carcinoma was confirmed by immunohistochemistry, which revealed negative E-cadherin, positive cytoplasmic P120, and deleted myoepithelium. Next-generation sequencing revealed CDH1 mutations that further proved the diagnosis of invasive lobular carcinoma. The main differential diagnoses for this tumor are SPC, EPC, neuroendocrine carcinoma, and secretory carcinoma, for which immunohistochemical analysis is an essential diagnostic tool. The growth pattern of invasive lobular carcinoma with EPC and SPC is a variant of the invasive lobular carcinoma newly discovered. The understanding of this variant expands the morphological spectrum of invasive lobular carcinoma and will help prevent misdiagnosis.