Biochemically estrogen receptor-negative, progesterone receptor-positive breast carcinoma. Immunocytochemical hormone receptors and prognostic factors.

The significance of breast carcinomas biochemically negative (-) for estrogen receptor (ER), but positive (+) for progesterone receptor (PR), is poorly understood. It has been proposed that these tumors, more common in younger women, contain ER whose presence is masked in a biochemical binding assay by endogenous estrogen. Such tumors should be positive for ER by immunocytochemical assay. In a series of 319 tumors, 18 (5.6%) were biochemically ER-PR+. Patient age in this group (mean age, 59 years; range, 36 to 89 years) was slightly, but not significantly, lower than in patient groups with other receptor profiles. Only two (11%) of the 18 tumors were ER+ by immunocytochemical assay, not higher than the immunocytochemical ER positivity rate of seven (13%) of 62 biochemically ER-PR- tumors in the same series. Progesterone receptor was positive by immunocytochemistry in nine (50%) ER-PR+ tumors, lower than the rate in biochemically ER+PR+ tumors (158 [87%] of 181). The tumor size, histologic tumor grade, and S-phase fraction showed trends to be higher in biochemically ER-PR+ tumors than in ER+PR+ tumors. Biochemically ER-PR+ breast carcinomas are biologically different from ER+PR+ tumors.