Clonal profiling of mixed lobular and ductal carcinoma revealed by multiplex ligation-dependent probe amplification and fluorescence in situ hybridization.

A needle biopsy of a mass in the right breast of a 36-year-old woman revealed invasive ductal carcinoma (IDC), and approximately 20% of cancer cells showed unequivocal membranous staining with the HercepTest. After systemic therapy with trastuzumab and paclitaxel followed by FEC (fluorouracil + epirubicin + cyclophosphamide), a right mastectomy was performed. By histological and immunohistochemical examinations, the resected tumor consisted mainly of E-cadherin-negative invasive lobular carcinoma (ILC), and the rest was ERBB2-positive IDC; thus, the diagnosis was mixed ductal and lobular carcinoma. Multiplex ligation-dependent probe amplification and fluorescence in situ hybridization (FISH) analyses revealed that ILC and IDC shared high-level amplification of CCND1 in homogeneously staining regions (HSR) and that IDC had an additional HSR-type amplicon of ERBB2. These findings strongly indicate that IDC and ILC had a common precursor cell with CCND1 amplification. Review of the biopsy specimen with FISH showed IDC with gene amplifications of CCND1 and ERBB2 as a minor component, IDC without amplification of CCND1 or ERBB2 as a major component, and a minute portion of ILC with CCND1 amplification. We speculate that chemotherapy and trastuzumab caused a marked reduction in IDC; however, ILC with CCND1 amplification was resistant to chemotherapy and grew.