Invasive Lobular Carcinoma Mimicking Papillary Carcinoma: A Report of Three Cases

Encapsulated and solid papillary carcinomas (EPCs and SPCs) are considered historically as a special form of ductal carcinoma in situ. Invasive lobular carcinoma (ILC) is characterised by a discohesive growth pattern. There are several variants of ILC, but, as yet, no papillary subtype has been identified. Here we report 3 cases of ILC presenting as papillary carcinoma (PC) with a typical solid papillary growth pattern. One case was reported on core biopsy as EPC (B5a). The 3 ensuing resection specimens showed features typical of SPC with a circumscribed malignant epithelial proliferation containing fibrovascular cores and surrounded, at least focally, by a thick fibrous capsule. The lobular nature of these tumours was confirmed on the resection specimens by the absence of E-cadherin and β-catenin membrane expression. The invasive nature was confirmed by the presence of entrapped fat cells, the absence of myoepithelial cells and focal merging of the solid papillary areas with classic ILC at the periphery. Of note, 1 case was a recurrent carcinoma without an in situ component.