Aim We evaluated pathological features of the ductal carcinoma in situ component of 241 triple negative invasive breast cancers.

Results We found that 151 (62.6%) in situ lesions were of high nuclear grade, and 236 (97.9%) were triple negative (oestrogen receptor, progesterone receptor, cerbB2 negative). Immunohistochemistry for cytokeratin (CK)5/6, CK14, CK17, epidermal growth factor receptor (EGFR), CD117, 34βE12, p63 and smooth muscle actin (SMA) revealed positive staining in 5 (2.1%), 60 (24.9%), 69 (28.6%), 37 (15.4%), 69 (28.6%), 137 (56.8%), 3 (1.2%) and 22 (9.1%) in situ ductal components respectively, with fair to substantial agreement of staining results (positive versus negative) between in situ and corresponding invasive elements for CK5/6, CK14, CK17, EGFR, CD117 and 34βE12; but none to fair agreement for p63 and SMA respectively. When the tri-panel of CK14, EGFR and 34βE12 was used to define the basal phenotype, 68% revealed basal-like expression of both in situ and invasive components of the same case.

Conclusions Our data support the notion that triple negative ductal carcinoma in situ is the precursor of the corresponding invasive counterpart, and that basal-like expression is maintained in the majority of invasive cancers associated with basal-like in situ disease. Future studies that prospectively evaluate morphological and biological characteristics of invasive cancers that develop from triple negative and basal-like ductal carcinoma in situ lesions will assist in validating these findings.