The existence of progesterone receptor (PgR) expression in oestrogen receptor (ER)-negative breast carcinoma is controversial. Here, we re-evaluated ER-negative/PgR-positive (ER-/PgR+) carcinoma cases by immunohistochemical staining (IHC).

We selected patients who underwent surgery for primary breast carcinoma from our databases at Dokkyo Medical University Hospital and Kameda General Hospital. Among the 9844 patients, the largest series in Japan, 27 (0.3%) were initially diagnosed as ER-/PgR+ breast carcinomas and we re-evaluated by IHC.

The re-evaluated IHC showed that of the 27 patients with the initial results of ER-/PgR+, 12 were ER+/PgR+, 8 were ER-/PgR-, and 7 were ER-/PgR+. ER was negative in 12 of 27 patients (44.4%), and PgR was positive in 8 of 27 patients (29.6%). In our seven re-evaluated and confirmed as ER-/PgR+ cases, the staining proportions of tumor cells were 0% in ER and 1-69% (average 15.8%) in PgR. The average staining proportion of PgR in the re-evaluated ER-/PgR+ phenotype was lower than the initial diagnosis. Histological grading was as follows: grade I, one case; grade II, two cases; grade III, four cases. There were two lymph-node-positive cases.

The ER-/PgR+ phenotype was confirmed after re-evaluation of ER and PgR assessment by a different pathologist. We recommend that pathologists discuss with clinicians, or re-test and re-evaluate ER/PgR expression, particularly in low-grade carcinoma and with a high staining proportion of PgR in the ER-/PgR+ phenotype.