Mass-forming Acellular Keratin Masquerading as "Residual Tumor" in a Metaplastic Breast Carcinoma with Pathologic Complete Response: A Case Report.

Metaplastic carcinoma is an uncommon subtype of breast carcinoma which often shows a pathologic incomplete response to neoadjuvant chemotherapy. We report a metaplastic breast carcinoma with squamous differentiation showing pathologic complete response following neoadjuvant chemotherapy. The tumor bed showed keratinous debris without any viable residual tumor. Post-treatment, the acellular keratin persisted as a mass on physical exam and ultrasonography, suggesting an incomplete clinical response and resulting in clinical and pathologic discordance. This report expands the spectrum of histological findings in breast carcinoma with clinical and pathologic discordance in the neoadjuvant setting.