Breast Carcinoma With Features of Both Solid Papillary Carcinoma and Invasive Lobular Carcinoma: Diagnostic Challenges and Considerations.

Solid papillary carcinomas (SPCs) are uncommon breast tumors, and they are listed as a separate entity of papillary neoplasm, with no clear etiology and pathophysiology recognized so far. SPC without invasion is treated and managed as a form of ductal carcinoma in situ; when invasion is present, both ductal and lobular carcinoma can be present as the invasive component. Here we present a breast tumor showing features of both SPC and invasive lobular carcinoma (ILC). For either diagnosis, this tumor has unusual morphologic and immunophenotypic characteristics that have not been reported in literature. Differential diagnosis between these two entities is reviewed and discussed. As more cases with SPC-like morphology showing lobular differentiation are reported, whether they should be categorized as SPCs or ILCs deserves additional discussions and investigations. We hope to inspire further studies and explorations on whether solid papillary morphology alone is sufficient for diagnosing SPCs and recommend routinely performing immunohistochemical stains when lobular differentiation is suspected to avoid misdiagnosis and downstaging of the tumor.