• Anna Nordenskjöld, Helena Fohlin, Tommy Fornander, Britta Löfdahl, Lambert Skoog, and Olle Stål.
• Department of Oncology, Institute of Clinical Sciences, Sahlgrenska Academy, Gothenburg, Sweden.
• Breast Cancer Res. Treat. 2016 Nov 1; 160 (2): 313-322.

PurposeThe independent predictive information from progesterone receptor (PgR) positivity for breast cancer treated with tamoxifen has been questioned after an overview by the Early Breast Cancer Trialists' Collaborative Group (EBCTCG). However, the studies in the overview were to a large content performed before modern PgR immunohistochemistry (IHC) was developed. We therefore investigated the predictive value of PgR determined with IHC in estrogen receptor (ER)-positive tumors from patients participating in the Stockholm trial of adjuvant tamoxifen therapy.MethodsThe Stockholm Breast Cancer Study Group conducted a randomized trial during 1976 through 1990 comparing adjuvant tamoxifen versus control. The patients were stratified according to tumor size and lymph node status in high-risk and low-risk groups. In this study, we evaluated 618 patients with ER-positive "low-risk" breast cancer (size ≤ 30 mm, lymph node-negative) for whom PgR was determined by IHC at one pathology laboratory. The median time of follow-up was 21 years.ResultsPatients with ER-positive tumors that were also PgR-positive by IHC did benefit from tamoxifen, while we could not show any long-term benefit for those with tumors positive for ER only (recurrence rate ratio 0.43, 95 % CI 0.29-0.62 and 0.87, 95 % CI 0.52-1.46, respectively). We further investigated the influence of different levels of PgR positivity on recurrence risk. The results show that at all receptor levels with ≥10 % stained PgR-positive cells, the patients did benefit from tamoxifen. There was no clear linear trend in benefit with increasing proportion of stained cells.ConclusionsPgR positivity determined by IHC is a marker indicating long-term benefit from adjuvant tamoxifen in patients with ER-positive tumors.

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