Breast-feeding is a known protective factor against breast cancer. Breast-feeding duration is influenced by hormone levels, milk production, and lifestyle factors. The aims were to investigate how breast-feeding duration and milk production affected tumor characteristics and risk for early breast cancer events in primary breast cancer patients. Between 2002 and 2008, 634 breast cancer patients in Lund, Sweden, took part in an ongoing prospective cohort study. Data were extracted from questionnaires, pathology reports, and patients’ charts from 592 patients without preoperative treatment. Breast-feeding duration ≤12 months of the first child was associated with higher frequency of ER+/PgR+ tumors (P=0.02). Median follow-up time was 4.9 years. Higher risk for early events was observed for breast-feeding duration of first child >12 months (LogRank P=0.001), total breast-feeding duration >12 months (LogRank P=0.008), as well as ‘excessive milk production’ during breast-feeding of the first child (LogRank P=0.001). Patients with ‘almost no milk production’ had no events. In a multivariable model including both ‘excessive milk production’ and breast-feeding duration of the first child >12 months, both were associated with a two-fold risk for early events, adjusted HRs 2.33 (95% CI: 1.25-4.36) and 2.39 (0.97-5.85), respectively, while total breast-feeding duration was not. ‘Excessive milk production’ was associated with a two-fold risk of early distant metastases, adjusted HR 2.59 (1.13-5.94), but not duration. In conclusion, ‘excessive milk production’ during breast-feeding was associated with higher risk for early events independent of tumor characteristics, stressing the need to consider host factors in the evaluation of prognostic markers.Electronic supplementary materialThe online version of this article (doi:10.1186/2193-1801-2-298) contains supplementary material, which is available to authorized users.

Background: Breast feeding is a known protective factor against breast cancer. Breast feeding duration is influenced by hormone levels, milk production, and lifestyle factors. We aimed to investigate how breast feeding duration and milk production affect tumor characteristics and risk for early breast cancer events in women who subsequently developed breast cancer. Methods: Between 2002 and 2008, 634 primary breast cancer patients in Lund, Sweden, took part in an ongoing prospective cohort study. Data were extracted from preoperative questionnaires, pathology reports, patients' charts and Swedish registries from 592 patients without preoperative treatment. Results: Breast feeding duration ≥12 months of the first child was associated with higher frequency of ER+/PgR+ tumors (P=0.02). Median follow-up time was 4.9 years. Higher risk for early events was observed for breast feeding duration of first child >12 months (LogRank P=0.001), total breast feeding duration >12 months (LogRank P=0.004), as well as ‘excessive milk production’ during breast feeding of the first child (LogRank P=0.003). Patients with ‘almost no milk production’ had no events. In a multivariable model including both ‘excessive milk production’ and breast feeding duration of the first child >12 months, both were significantly associated with a two-fold risk for early events, adjusted HRs 2.19 (95% CI: 1.17-4.11) and 2.61 (1.06-6.38), respectively, adjusted for age, invasive tumor size, axillary lymph node involvement, ER and PgR status, age at first birth, time since last birth, smoking status, and BMI, while total breast feeding duration was not. ‘Excessive milk production’ was associated with a two-fold risk of early distant metastases, adjusted HR 2.41 (1.09-5.32), but not breast feeding duration. Conclusion: ‘Excessive milk production’ during breast feeding was associated with higher risk for early events independent of tumor characteristics, while patients who reported ‘almost no milk production’ had no events. This finding stresses the need to consider host factors in the evaluation of prognostic markers. Citation Format: Emma Gustbée, Charlotte Anesten, Andrea Markkula, Maria Simonsson, Carsten Rose, Christian Ingvar, Helena Jernstrom. Excessive milk production during breast feeding prior to breast cancer diagnosis is associated with increased risk for early events. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Breast Cancer Research: Genetics, Biology, and Clinical Applications; Oct 3-6, 2013; San Diego, CA. Philadelphia (PA): AACR; Mol Cancer Res 2013;11(10 Suppl):Abstract nr B115.