The paradigm shift, the defi nition of pregnancy-associated breast cancer is outdated

Background. Pregnancy-associated breast cancer (PABC) is diagnosed during pregnancy or within one year postpartum and is characterized by a more aggressive clinical course and poorer prognosis compared to breast cancer in non-pregnant patients. This is attributed to delayed diagnosis, distinct tumor biological features (such as HER2-positivity or triple-negative status), as well as differences in the tumor microenvironment during pregnancy and lactation. Objective: to determine the prognosis of breast cancer depending on the time to diagnosis (during pregnancy or in the early postpartum period). Material and Methods. The study included 56 patients diagnosed with pregnancy-associated breast cancer (PABC) who received treatment at N.N. Petrov National Medical Research Center of Oncology and V.A. Almazov National Medical Research Center (Russian Ministry of Health) between January 2016 and August 2024. Among them, 35 cases of breast cancer were diagnosed during pregnancy (PrBC), 21 patients were diagnosed in the postpartum period (PPBC). These patients were matched with two control groups, comprising 70 and 42 patients, respectively, based on age at diagnosis (±3 years), year of diagnosis (±2 years), clinical stage, and immunohistochemical (IHC) subtype. Results. The mean patient age was 34.9 years (range: 27–45 years). Most pregnant women sought medical attention at breast cancer stages II–III (28/35, 80 %), with regional lymph node (LN) involvement (20/35, 57.1 %). Among lactating patients, 71.4 % (15/21) presented at stages II–III, with LN metastasis observed in 71.4 % (15/21). In the PPBC group, 66.7 % (14/21) of patients had hormone receptor-positive tumors. The majority of PrBC cases (21/35, 60 %) were ERand PR-negative, with half of the patients exhibiting triple-negative breast cancer subtype (16/35, 45.7 %). The median follow-up duration was 50 months (range: 6 months to 8 years). Disease progression occurred in 22.9 % (8/35) of pregnancy-associated cases and 38.1 % (8/21) of lactationassociated cases. Three-year disease-free survival (DFS) rates were 73.1 % in the PrBC group versus 85.3 % in controls (p=0.014), and 51.1 % in the PPBC group versus 81.9 % in controls (p=0.032). Conclusion. The timing of breast cancer diagnosis (during pregnancy or in the early postpartum period) may serve as an independent prognostic factor for adverse outcomes. Our study demonstrated a statistically significant reduction in DFS rates in both PrBC (13.8 % decrease) and PPBC (30.8 % decrease) groups compared to the matched controls (p<0.05). Only separating pregnancy-associated breast cancer into PrBC and PPBC can improve our understanding of the tumor’s biological behavior during pregnancy, lactation, and involution.