Abstract P4-03-20: Association between reproductive factors (parity and age first full term birth) and the frequency of estrogen receptor negative breast cancer according to age at diagnosis
Authors: Vangoitsenhoven Maja , Theys Evert , Van Baelen Karen , Laenen Annouschka , Desmedt Christine , Floris Giuseppe , Wildiers Hans , Punie Kevin , Han Sileny , Baert Thaïs , Amant Frédéric , Smeets Ann , Nevelsteen Ines , Vanhoudt Rani , Neven Patrick 
Published: March 2023

Abstract
Background: The frequency of early estrogen receptor (ER)-negative breast cancers (BC) decreases with increasing age at diagnosis (Partridge P, JCO 2016). Giving birth is generally considered a protective factor for the occurrence of BC in a woman’s lifetime. However, for up to more than 20 years following childbirth, BC risk may be higher. This appears to be due to an increase in hormone receptor-positive BC, especially in women older at the time of 1st childbirth (Nichols HB, Ann Int Med 2019). This means that the ER-negative BC are relatively less frequent in these 2 decades after delivery. In this study, we aimed at retrospectively investigating the impact of parity and age at first full term pregnancy (FFTP) on the frequency of ER-negative BC according to the age at diagnosis in a large institutional cohort of patients diagnosed with BC.
Patients and Methods: We considered all patients diagnosed and/or treated with early BC in UZ Leuven between January 2000 and November 2020. ER-negativity was defined as &lt; 1% positive cells. Age at diagnosis was subdivided in categories of 5 years. Parous women could be having a low (1 or 2 children) or high parity (&gt; 2 children) and age of FFTP was arbitrarily divided into &lt; or ≥ 27 years of age. BMI was considered a possible confounder and was corrected for. A logistic regression model was used for data analysis with ER-negative status as binary outcome and FFTP class and multiparity as explanatory variables. To test whether the difference in ER-negative proportions between FFTP classes depends on multiparity, we modelled the interaction between multiparity and FFTP class.
Results: We included 9955 consecutive female patients after excluding missing values. 8358 out of 9955 women had at least 1 child (84%). In our study population, parity as such was not an independent variable for BC subtype. Women with a FFTP ≥ 27y as compared to those with a FFTP &lt; 27y were less likely to have an ER-negative BC. The p-value for the interaction term between high parity (&gt;2 children) and FFTP class equals p= 0.0044. Hence, there is statistical evidence to suggest that the differences between FFTP categories with regards to ER-negative BC may depend on multiparity. Table 1 shows a lower incidence of ER-negative BC in the FFTP ≥ 27y group compared to the FFTP &lt; 27y group in case of high parity. Table 2 shows the absolute proportion. These results seem to be independent of BMI.
Conclusion: Women with a FFTP at 27 years of age or older and more than 2 children have proportionally less ER-negative type breast cancers as compared to women with a FFTP before 27 years of age. This difference between FFTP classes is not observed in absence of high parity.
Table 1: effect of FFTP class by multiparity
Table 2: proportion of ER-negative breast cancer subtype
Citation Format: Maja Vangoitsenhoven, Evert Theys, Karen Van Baelen, Annouschka Laenen, Christine Desmedt, Giuseppe Floris, Hans Wildiers, Kevin Punie, Sileny Han, Thaïs Baert, Frédéric Amant, Ann Smeets, Ines Nevelsteen, Rani Vanhoudt, Patrick Neven. Association between reproductive factors (parity and age first full term birth) and the frequency of estrogen receptor negative breast cancer according to age at diagnosis [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P4-03-20.