1 Clinicopathological Characteristics of Basal Type Breast Cancer in Triple-Negative Breast Cancer

Background: Triple-negative breast cancer (TNBC) is defined by the absence of estrogen receptor (ER), progesterone receptor (PgR) and human epidermal growth factor 2 (HER2) expression. Patients with TNBC derive no benefit from molecularly targeted treatments, such as endocrine therapy or trastuzumab, as they lack the appropriate targets for these drugs. TNBC is characterized by its biological aggressiveness and poor prognosis, and consists of two subtypes, basal and nonbasal. The purpose of our study is to differentiate the clinicopathological characteristics of the two subtypes. Methods: 367 patients with primary breast cancer were recruited from April 2004 to December 2010 at 1st Department of Surgery, Sapporo Medical University. ER, PgR, and HER2 status were evaluated in all cases. Moreover, we classi-fied TNBC into basal, nonbasal subtypes on the basis of immunohistochemical staining of epidermal growth factor re-ceptor (EGFR), cytokeratin (CK) 5/6. Basal type was defined as CK5/6-positive and/or EGFR-positive, and nonbasal type was defined as no expression of these two markers. Results: Breast cancer subtypes by molecular classification were Hormone receptor (HR)-positive/HER2-negative (61%), HR-positive/HER2-positive (10%), HR-negative/HER2-positive (14%), and HR-negative/HER2-negative (15%). There was no difference between the basal type and the non-basal type in clinicopathological factors. But, the basal type was significantly associated with Ki67 labeling index (p = 0.0002), p53 expression (p = 0.047), and BRCA1 expression (p = 0.03). Further, patients with the basal type TNB