Low incidence of estrogen receptor in breast carcinomas with rapid rates of cellular replication

The results of assay for estrogen receptor (ER) in the tumor cytosol and thymidine labeling indices (TLI) of 63 primary invasive carcinomas of the breast were analyzed. The ER assay was performed by using dextran‐charcoal to adsorb unbound tritiated estradiol‐17/β (E2) in the cytosol. The TLI was measured as the number per hundred of neoplastic cell nuclei labeled by tritiated thymidine. A significant association between low TLI and presence of ER was found. Whereas all 19 tumors with TLI < 2.5 contained ER in the primary lesion or in axillary metastases and ER was found in 25 of 30 tumors with TLI between 2.5 and 10, only 4 of 14 tumors with TLI > 10 contained ER (p < 0.001). TLI and saturable binding of E2 were significantly negatively correlated (r = ‐0.436, p < 0.001). It is concluded that dedifferentiation in breast carcinomas is associated with both high TLI and absence of ER, and suggested that the carcinomas with the most rapid proliferative rates will include the highest proportion of tumors unresponsive to hormonal therapy.