Prediction of response to endocrine therapy in breast cancer using immunocytochemical assays for pS2, oestrogen receptor and progesterone receptor

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Abstract
Histological sections obtained from 70 patients with breast cancer, all of whom had received endocrine therapy for meta‐static or locally advanced disease, were assessed for specific immunocytochemical staining of oestrogen receptor and the oestrogen‐induced protein pS2. There was also sufficient material from 25 patients for an assessment of progesterone receptor by immunocytochemistry. We found that, when using a „cut‐off”︁ point of 50% for ER and PR, and of 25% for pS2, ER was positive in 22/29 responders and in 12/41 non‐responders, and thus was significantly associated with response to endocrine therapy. Similarly, in those subjects in whom PR was measured, PR was positive in 5/14 responders and negative in all II non‐responders, again being significantly correlated with response. However, pS2 did not relate to response, being only positive in 10/29 responders and negative in 26/41 non‐responders. The different response categories varied in their „percentage of positivity”︁ as determined by the 2 tests. Thus, for ER and pS2 we observed: complete response—71 % for ER compared with 14% for pS2; partial response—77% compared with 41%; stable disease—36% compared with 64%; and progressive disease—27% compared with 27%. We conclude that at the present time ER appears to be the most reliable indicator for predicting response to endocrine therapy in patients with breast cancer.