Progesterone receptor determination in human breast tumors by immunocytochemical and biochemical techniques

Progesterone receptors were determined on frozen sections from 74 primary human breast tumors by an immunocytochemical assay using an indirect avidin-biotin peroxidase method. In the same tumors, cytosol estrogen (ERc) and progesterone receptors (PgRc) were determined by ligand binding assay, and nuclear estrogen (ERn) and progesterone receptors (PgRn) were determined by an immunoassay. Immunocytochemical staining was seen in 36% of tumors. It was predominantly nuclear and there was extensive cell to cell heterogeneity. When the immunocytochemical results were compared to PgRc the agreement rate was 63%, but it was 77% when compared to PgRn. About one third (38%) of PgRc positive tumors were immunocytochemically defined as negative. Thus a significant discordance exists between this immunocytochemical assay for PgR and both the conventional radioligand assay (used for PgRc) and the relatively new enzyme immunoassay (used for PgRn). However discordance rates were critically influenced by the arbitrary cutoff levels that were used to define receptor positivity in the biochemical assays. Our studies support the addition to, rather than the substitution of, immunocytochemical methods, to the conventional biochemical assays for PgR, until long-term follow-up studies of patients with PgRn and immunocytochemical PgR determinations become available.