Estrogen receptor α-negative and progesterone receptor-positive breast cancer: Lab error or real entity?

A retrospective study comparing the estrogen receptor (ER) α subtype and progesterone receptor (PR) profile of breast carcinomas amongst 1625 cases over 2.5 years was carried out. Strictly speaking it is generally believed that breast carcinomas can biochemically express PR only if they are ER-positive. However, a few ERα-PR+ cases do exist paradoxically. This class of tumors was the focus of our study in which we looked at the possible reasons for such an immunophenotype and compared it with a group of ERα+PR+ breast carcinomas. An internationally recognized immunohistochemical method employing monoclonal antibodies against estrogen and progesterone receptors was used. Correlations with established risk factors i.e. menopausal status, grade, tumor size and lymph node status were analyzed for our study group (ERα-PR+) and compared with a control (ERα+PR+). Out of the total 1625 cases, 29.91% (486) were ERα+PR+, 5.11% (83) were ERα+PR-, 56.86% (924) were ERα-PR- and 8.12% (132) were ERα-PR+. Patients' age was significantly lower in the ERα-PR+ group (P=0.002). Statistical analysis of the grading between the two study groups revealed no significant difference (P=0.091), although the ERα-PR+ group contained significantly more poorly differentiated tumors than the ERα+PR+ one (P=0.032). Tumor size was also significantly larger in the ERα-PR+ than in the ERα+PR+ group (P=0.046). The frequency of lymph node metastases was independent of receptor profile. In conclusion, our study group does exhibit characteristics which are suggestive of a distinct breast cancer phenotype (ERα-PR+) with a different etiology and prognosis.