Postpartum breast cancer: Analysis of factors related to pregnancy and identification of differential prognostic markers.

557 Background: Postpartum breast cancer (PPBC) has an increasing incidence. It is associated with a worse prognosis compared to other breast cancers diagnosed outside of this stage. Objectives: To study clinical and histopathological differences between PPBC (diagnosed within 10 years after delivery) and non-postpartum breast cancer (NPBC, diagnosed more than 10 years after delivery or in nulliparous). Analyze whether reproductive factors such as parity or lactation influence these characteristics. Methods: An observational, analytical and retrospective case-control study is carried out. For the first part, we consider those women with PPBC as case; healthy mothers as control. For the second part, we consider PPBC women as case; NPBC women as control. The data have been analyzed with SPSS v.22. Results: 2698 women under 45 years were recruited. 264 (9.8%) have breast cancer and 2434 (90.2%) are healthy women. The percentage of ex-smokers is higher in PPBC compared to healthy mothers (30.6% vs. 19.7%) (X2=9.632; p=0.022; SS). The proportion of women with previous gynecological pathology is higher in PPBC group than in the group of healthy mothers (48.9% vs. 25.7%) (X2=33.767; p<0.001; SS). The same occurs with previous breast pathology (51.9% vs. 21.7%) (X2=62.75; p<0.001; SS). Women with early PPBC (diagnosed within 5 years after delivery) have higher percentage of previous mastitis compared to the rest of groups that develop tumor beyond 5 years postpartum (20.3% vs. 3.6%) (X2=17.76; p=0.023; SS). The percentage of female pregnancies is more frequent in early PPBC (58.6%) compared to non-postpartum or late PPBC (42.4%) (X2=4.039; p=0.044; SS). The percentage of women who abruptly discontinued breastfeeding is higher in those with PPBC with 27.5% compared to healthy mothers (12.6%) (X2=18.448; p<0.001; SS) (Table). PPBC has higher percentage of stages II (63.9%), the other group of NPBC is mostly diagnosed in stage I (64.9%) (X2=9.808; p=0.002; SS). We found higher elevated expression of Ki67 in early PPBC (90.1%), compared to non-postpartum cancer or late PPBC (77%) (X2=6.252; p=0.012; SS). We observe higher proportion of negative estrogen receptors in early PPBC (36.6%) compared to late PPBC or non-postpartum cancer (13.6%) (X2=9.880; p=0.002; SS). Conclusions: Postpartum breast cancer presents different clinical and histopathological characteristics, highlighting differences in early postpartum period. PPBC has worse prognosis, it constitutes a different entity. Breastfeeding abrupt interruption is associated with development of PPBC. [Table: see text]