Clinical subtypes and prognosis of pregnancy-associated breast cancer: results from the Korean Breast Cancer Society Registry database.

Purpose: We analyzed the clinicopathologic characteristics and prognosis of pregnancy-associated breast cancer (PABC) according to clinical subtypes to better understand the characteristics of PABC.
Methods: A total of 83,792 female patients between the ages of 20 and 49 were enrolled in the Korean Breast Cancer Society Registry database from January 1, 1996 to December 31, 2015. 'PABC' is defined as breast cancer diagnosed during pregnancy or within 1 year after delivery. Other patients were defined as 'non-PABC' patients.
Results: In non-PABC patients, luminal A subtype was the most common (50.2%). In PABC patients, TNBC was the most common (40.4%) subtype, while luminal A comprised 21.2% and HER2 subtype comprised 17.3%. There was a significant difference in overall survival (OS). In non-PABC patients, TNBC had the highest HR (HR 2.3, 95% CI 2.1-2.6). In PABC patients, the luminal B subtype (HR+ HER2-high Ki67) had the highest HR at 7.0 (95% CI 1.7-29.1). In multivariate analysis of OS by subtypes, PABC patients had significantly higher HR than non-PABC patients in the HER2 subtype (HR 2.0, 95% CI 1.1-3.7) and luminal B subtype (HR+ HER2-high Ki67) (HR 4.4, 95% CI 1.6-12.3).
Conclusion: PABC showed different biologic features than non-PABC. PABC had a particularly poor prognosis in the luminal B (HR+ HER2-highKi67) and HER2 subtypes. To improve the prognosis of PABC, treatment should be considered according to subtype. Development of drugs that can be used during pregnancy is needed.