Pleomorphic lobular carcinoma in pleural fluid: diagnostic pitfall for atypical mesothelial cells

Pleomorphic lobular carcinoma (PLC) is a subtype of infiltrating lobular carcinoma because of its dyscohesiveness, linear infiltration pattern, and lack of membranous E-cadherin staining. However, it differs from classic lobular carcinoma because of its high-grade cytology and more aggressive clinical behavior. In breast fine-needle aspiration biopsies, PLC can be confused with invasive ductal carcinoma, particularly the apocrine variant. In this report, we illustrate how metastatic PLC in body fluid specimens shows many of the same cytomorphologic changes that occur in reactive/atypical mesothelial cells. Fortunately, the immunohistochemical staining pattern of PLC can help to distinguish it from other possible diagnoses in the differential, such as reactive/atypical mesothelial cells and other metastatic neoplasms. However, the frequent apocrine features seen in this variant of breast carcinoma can cause nonspecific immunohistochemical positivity that may make the interpretation difficult. This is the first report illustrating the cytopathology and immunohistochemical findings of pleomorphic lobular carcinoma in body cavity fluid cytology. Our case highlights the important issues and pitfalls to be aware of when making this diagnosis.