Distinct clinical and prognostic features of infiltrating lobular carcinoma of the breast: Combined results of 15 International Breast Cancer Study Group clinical trials

Purpose: To determine how patients with infiltrating lobular carcinoma (ILC) differ from patients with the more common infiltrating ductal carcinoma (IDC) with regard to patient and tumor factors, local treatment, and patterns of recurrence. Patients and Methods: Twelve thousand two hundred six breast cancer patients entered onto 15 International Breast Cancer Study Group trials between 1978 and 2002 were categorized as having ILC, IDC, or other/mixed types. Results: Seven hundred sixty-seven tumors (6.2%) were classified as ILC, 8,607 (70.5%) were classified as IDC, and 2,832 (23.2%) were classified as other. The analysis is limited to the 9,374 patients categorized as either pure IDC or ILC. The median follow-up time was 13 years. Compared with IDC, ILC was associated with older age; larger, better differentiated, and estrogen receptor (ER)-positive tumors; and less vessel invasion. Mastectomy was used more frequently for ILC (P < .01). There was a significant (P < .01) early advantage in disease-free survival and overall survival for the ILC cohort followed by a significant (P < .01) late advantage for the IDC cohort after 6 and 10 years, respectively. Similar patterns were observed in cohorts defined by ER status. ILC was associated with an increased incidence of bone events but a decrease in regional and lung events (all P < .01). Conclusion: ILC is more than a histologic variant of breast cancer. The diagnosis of ILC carries distinct prognostic and biologic implications.