Abstract P6-09-32: The association of breast cancer subtype and breast cancer survival with parity and time since last birth
Authors: De Mulder H , Laenen A , Wildiers H , Punie K , Poppe A , Remmerie C , Nevelsteen I , Smeets A , Van Nieuwenhuysen E , Van Limbergen E , Floris G , Vergote I , Neven P 
Published: March 2017

Abstract
Background: Pregnancy affects breast cancer risk but it's influence on breast cancer subtype and prognosis remains controversial. We studied the effect of parity and time since last birth on breast cancer subtype and outcome in women aged ≤50 years at diagnosis.
Patients and Methods: A retrospective multivariate cohort study including all premenopausal women aged ≤50 years (N=1306) at diagnosis and primarily treated with surgery (N=1176) or neo-adjuvant chemotherapy (N=130) at University Hospitals Leuven (Jan. 2000 – Dec. 2009); local and systemic therapies were consistent with guidelines when treated. Tumor subtypes were defined by tumor grade and receptor expression for estrogen receptor (ER), progesterone receptor (PR) and human epidermal growth factor receptor 2 (HER-2) amplification; ER+PR+/-HER-2- cases were Luminal A- like if grade 1-2 and Luminal B like if grade 3; HER-2+ cases were Luminal HER-2 if ER+ and HER-2 like if ER-; triple negative breast cancer (TNBC) were ER-PR-HER-2-. Outcome endpoints were breast cancer subtype, disease free (DFS) and distant disease free survival (DDFS) by parity and in parous women comparing short (&lt;5 years) versus long (≥5 years) time since last birth. Statistics used were Cox proportional hazard model. Results were corrected for age at diagnosis, tumor size, lymph node status and tumor subtype.
Results: Breast cancer subtypes didn't differ between nulliparous and parous women but subtypes differed significantly in parous women by time interval since last birth (p&lt;0.001). Breast cancers within 5 years of last birth were proportionally more likely TNBC and HER-2 like compared to Luminal A (p=0.026 and p=0.003 respectively) than breast cancers ≥5 years after last birth even when corrected for age at diagnosis. After a mean follow-up period of 10 years, parous women had a better DFS compared to nulliparous women (DFS: HR 0.754; CI 0.593-0.959; p=0.021) but after correction for known prognostic factors, only a trend remained (HR 0.783; CI 0.611-1.004; p=0.054). In parous women, those with a longer time interval since last birth had a better DFS than women with a recent pregnancy (HR 0.965; CI 0.948-0.982; p&lt;0.001). However, after correction for known prognostic factors, this association was completely attenuated (HR 0.997; CI 0.972-1.023; p=0.828). Comparable results were seen for DDFS.
Conclusion: After correction for age at diagnosis, parity does not but recent birth does affect breast cancer subtype. Such tumors are proportionally more likely ER-negative namely TNBC and HER-2 like. We observed a trend for better DFS for parous women. The prognostic value of time since last birth is mostly due to tumor characteristics and age at time of diagnosis.
Citation Format: De Mulder H, Laenen A, Wildiers H, Punie K, Poppe A, Remmerie C, Nevelsteen I, Smeets A, Van Nieuwenhuysen E, Van Limbergen E, Floris G, Vergote I, Neven P. The association of breast cancer subtype and breast cancer survival with parity and time since last birth [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P6-09-32.