Oestrogen receptor‐β1 but not oestrogen receptor‐βcx is of prognostic value in apocrine carcinoma of the breast

Apocrine carcinoma of the breast, which frequently expresses oestrogen receptor‐β (ER‐β) in the absence of ER‐α and only infrequently is treated endocrinologically, gives an opportunity to investigate the clinicopathological role of ER‐β in breast cancer independent of ER‐α expression or tamoxifen treatment. Several isotypes of ER‐β, ER‐β1–5 etc., have been identified thus far; however, the clinicopathological importance of each ER‐β isotype in breast cancer is still uncertain. Here we aimed to clarify the clinicopathological importance of ER‐β1 and ER‐βcx (ER‐β2) in apocrine carcinomas, immunohistochemically examining expressions of ER‐β1 and ER‐βcx in 47 apocrine carcinomas. Positivity for ER‐β1 and ER‐βcx was observed in 41 (87%) and 18 (38%) of 47 cases, respectively. ER‐β1 positivity was related to smaller tumor size (P=0.0359), lower histological grade (P=0.0322), and higher disease‐free survival (P<0.0001), whereas ER‐βcx status was related to none of these parameters. ER‐β1 positivity was also associated with favorable clinical outcome in 24 so‐called triple‐negative (ER‐α‐negative/PR‐negative/HER2‐negative) apocrine carcinomas. ER‐β1 itself, independent of ER‐α expression and tamoxifen treatment, seems to have a tumor‐suppressive effect, at least in apocrine carcinomas. Further study of ER‐β1 is desired to optimize breast cancer treatment.