High expression of progesterone receptor may be an adverse prognostic factor in oestrogen receptor-negative/progesterone receptor-positive breast cancer: results of comprehensive re-evaluation of multi-institutional case series

Oestrogen receptor (ER)-negative (–) progesterone receptor (PgR)-positive (+) is the least common combination of steroid receptor expression observed in breast cancer. There are many controversies regarding the actual existence of ER–/PgR+ phenotype. In the current study, we aimed to perform comprehensive immunohistochemical re-evaluation of ER–/PgR+ breast cancers from multiple institutions. A total of 135 cases of ER–/PgR+ breast cancer were collected from 11 institutions from the period 2006–2020 and subsequently stained with three clinically validated anti-ER antibody clones: SP1 (Roche), 1D5 (Dako), and EP1 (Dako), and two anti-PgR antibody clones: 636 (Dako), and 1E2 (Roche). Clinicop…