A new study has reported that invasive ductal carcinoma (ductal breast cancer) that co-exists with ductal carcinoma in situ (DCIS) tends to be less aggressive than size-matched pure ductal breast cancer, as long as the ductal breast cancer is not HER2 overexpressing (HER2+) or triple negative (ER-/PR-/HER2-). Ductal breast cancer coexisting with DCIS (ductal+DCIS) is more often estrogen receptor positive (ER+), progesterone receptor positive (PR+) and/or HER2+. To conduct the study, the authors analyzed tumor data from 1,355 consecutive women who underwent surgery for primary invasive ductal breast cancer. Patients with pure DCIS were not included.

Researchers are increasingly categorizing breast cancer according to whether it is luminal, basal (also known as basal-like) or HER2 positive. These are molecular phenotypes, as determined by gene expression profiling. Luminal tumors can further be subdivided into luminal A and luminal B. In practice, these breast cancer types often are defined using hormone receptor and HER2 status, as well as Ki-67 (a marker of proliferation), grouped in ways considered to be reasonable approximations of the molecular phenotypes. As such, there is not complete agreement concerning characteristics of these types. The authors divided the tumor samples into four phenotypes as follows: (1) luminal A (ER+ and/or PR+, HER2-, Ki-67 ≤ 12); (2) luminal B (ER+ and/or PR+, HER2-, Ki-67 > 12); (3) HER2+; and (4) basal-like (triple-negative). There were 396 patients with luminal A, 265 with luminal B, 258 with HER2+, and 117 with triple negative breast cancer.

Proliferation antigen Ki-67 expression was found to be lower in ductal+DCIS than in size-adjusted pure ductal luminal A or luminal B tumors. On the other hand, no significant differences were found between pure ductal breast cancer and ductal+DCIS in HER2+ or triple negative tumors. The authors conclude that the presence of coexisting DCIS in ductal breast cancer predicts lower biological aggressiveness in luminal cancers but not in the conventionally more aggressive HER2+ and triple negative subtypes.