Invasive carcinomas arising within fibroepithelial tumors represent an uncommon manifestation of breast cancer. We report the case of a 70-year-old woman who underwent mastectomy for a malignant phyllodes tumor measuring 6 cm. Histological workup of the specimen revealed a high-grade invasive ductal carcinoma 2.5 cm in diameter within the phyllodes tumor. DNA was isolated from microdissected epithelial and stromal components of the phyllodes tumor as well as from the invasive ductal carcinoma cells. Using multiplex polymerase chain reaction, a comparative allelotyping was performed with a panel of 11 microsatellite markers. The malignant stroma of the phyllodes tumor showed loss of heterozygosity at chromosome 16q23, 17q12, 17q25, and 22q13; the epithelial tumor component shared the loss of 16q23; whereas the invasive carcinoma had lost divergent alleles at 16q23, 17q12, and 17q25, indicating a lack of clonality between phyllodes tumor and invasive ductal carcinoma. Although our data are compatible with a previously postulated common origin of epithelial and stromal components of phyllodes tumors, the coexisting invasive ductal carcinoma appears to represent a true collision tumor.
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