Molecular subgroups of small (pT1) breast carcinomas belonging exclusively to the ductal infiltrating variety.

BACKGROUND The use of microarray technology has resulted in a new classification of breast cancer according to gene expression profiles. None of the reports published so far using this new classification has stratified the studied tumors by histology or size. MATERIALS AND METHODS This study was restricted to the ductal infiltrating variety only, and to pT1 size using the immunohistochemical markers estrogen receptor (ER), progesterone receptor (PR), HER2 and cytokeratin 5/6. ER+ and/or PR+, HER2- tumors were termed "luminal A"; ER+ and/or PR+, HER2+ "luminal B"; triple-negative, CK 5/6+ and/or HER1+ "basal-like"; with an additional category for ER-, PR-, HER2+ tumors termed HER2, and a final group of unclassified ones, negative for all five markers. RESULTS Out of 346 tumors, 251 (72.5%) were luminal A, 45 (13%) were "triple-negative" ("basal"-like), 20 (5.8%) were luminal B, and 30 (8.7%) were HER2. Luminal A, "triple-negative" ("basal"-like), and HER2-expressing tumors (luminal B + HER2) showed significantly different associations with histological and nuclear grade, mutant p53 expression and Ki67 labelling index. CONCLUSION Studies of the other, less frequent histological varieties of breast cancer, stratifying by tumor size, are mandatory to disclose which precise gene-expression pattern defines similar subgroups.