Randomized trial of two versus five years of adjuvant tamoxifen for postmenopausal early stage breast cancer. Swedish Breast Cancer Cooperative Group.

Abstract

Postsurgical treatment with tamoxifen has been shown to improve overall survival among patients with early stage breast cancer. However, the optimal duration of tamoxifen treatment remains controversial. A multicenter, randomized trial was initiated in Sweden in the early 1980s to compare 2 years with 5 years of adjuvant tamoxifen in the treatment of postmenopausal women younger than 75 years of age who had early stage, axillary lymph node-negative or -positive, invasive disease. The trial was planned and organized by the Swedish Breast Cancer Group, and it involved five regional breast cancer study organizations (South Sweden, South-East Sweden, Stockholm, Uppsala-Orebro, and North Sweden). During the period from 1983 through 1991, a total of 3887 patients were entered in the trial; 3545 (91%) women remained alive and recurrence free at 2 years and could thus contribute meaningful information to the 2-year (n = 1801) versus 5-year (n = 1744) comparison. Primary surgery consisted of either modified radical mastectomy or breast-conserving surgery. Radiation therapy was indicated for patients with lymph node-positive disease and was generally offered to all women who were treated with breast-conserving surgery. Only 89 (2.5%) of the 3545 women who were recurrence free at 2 years received adjuvant chemotherapy concurrently with tamoxifen. Twenty-milligram daily doses of tamoxifen were used at two centers, and 40-mg daily doses were used at the remaining three centers. Estrogen receptor status of the tumor was known for 2987 women (77% of the entered patients). Primary end points in the trial were event-free survival (local-regional recurrence, distant metastasis, contralateral breast cancer, or death) and overall survival. Survival curves were estimated by use of the life-table method. The Cox proportional hazards model was used to make comparisons between the 2- and 5-year treatment groups. Patients assigned to receive 5 years of tamoxifen, compared with 2 years of tamoxifen, experienced statistically significant improvements in event-free survival (relative hazard = 0.82; 95% confidence interval [CI] = 0.71-0.96) and overall survival (relative hazard = 0.82; 95% CI = 0.69-0.99). These findings translate into an 18% relative reduction in both first events (95% CI = 4%-29%) and mortality (95% CI = 1%-31%) with the longer treatment. Overall survival at 10 years was estimated to be 80% among patients in the 5-year tamoxifen group who were alive and recurrence free at 2 years, compared with 74% among corresponding patients in the 2-year treatment group. The benefit associated with the longer treatment extended to women with lymph node-positive as well as lymph node-negative disease, but it appeared to be restricted to women whose tumors were classified as estrogen receptor positive. Five years of adjuvant tamoxifen is more beneficial than 2 years in the treatment of postmenopausal women with estrogen receptor-positive, early stage, invasive breast cancer.