Receptor potency estimate: a more reliable indicator of prognosis than estrogen receptor levels

The receptor potency estimate (RPE), a calculated parameter which is sensitive to both the number of receptors present as well as their affinity for steroid ligand, was developed and used to evaluate response to endocrine therapy in breast cancer patients. The clinical course of 19 estrogen receptor positive, stage III breast cancer patients was examined as a function of their RPE. Of those tumors which recurred (nine), the mean disease free interval was 18.1 months. Within this group, the mean disease free interval for RPE negative tumors (six) was 12.8 months while for RPE positive tumors (three) it was 28.7 months (p = 0.05). The overall three year survival of the 19 patients was 81 percent. However, when viewed as a function of RPE, the three year survival was 69 percent for RPE negative tumors and 100 percent for RPE positive tumors. The data suggest that the receptor potency estimate is a much better predictor of clinical response to estrogen therapy than estrogen receptor content alone.