RADIATION THERAPY ALONE VERSUS RADIATION THERAPY WITH HORMONAL THERAPY FOR WOMEN WITH EARLY STAGE BREAST CANCER: A POPULATION-BASED STUDY

Purpose: There is interest in de-intensifying adjuvant treatment after breast conserving surgery (BCS) for women with low-risk breast cancer to avoid exposure to unnecessary side effects. Previous studies examined the benefit of radiation therapy (RT) when added to hormonal therapy (HT) for women with low-risk hormone receptor positive (HR+) breast cancer. However, there have been few studies which compared RT alone versus RT+HT. Our aim is to compare outcomes of women with early-stage breast cancer treated in British Columbia who have received RT alone versus RT+HT, with a primary endpoint of breast cancer-specific survival (BCSS). Materials and Methods: This was a retrospective study of all women aged 50 and over, who were diagnosed from January 1, 2005 to December 31, 2014 with T1-2 N0, HR+, HER-2/neu negative breast cancer, and treated with BCS and no chemotherapy. Baseline demographic, tumour, treatment details, and clinical outcomes were collected. Luminal A was defined as those with estrogen receptor positive (ER+) and progesterone receptor positive (PR+) and HER-2/neu negative and tumour Grades 1-2. All other cases were Luminal B. Two groups were defined: women who received RT alone and women who received RT+HT. Kaplan-Meier analyses were used for local control and survival endpoints. The Fine-Gray method was used with non-breast cancer death as a competing risk for BCSS. Multivariable analysis was performed to determine predictors of BCSS. Results: A total of 4,728 women satisfied the inclusion criteria. There were 784 women (16.6%) who received RT alone compared to 3,944 (83.4%) who received RT+HT. Median age was 65, and those who were treated with RT alone were significantly older than those treated with RT+HT (68 versus 64, p<0.001). Majority of patients were ER+ (99.8%) and a minority (0.2%) were PR+ only. Most patients were Luminal A (68%) versus Luminal B (32%) subtype. Local relapse-free survival at five and 10 years for RT alone were 98.9% and 97.0% and for RT+HT were 99.1% and 98.0% (p=0.54). The BCSS at five and 10 years for RT alone were 98.9% and 97.6% and for RT+HT were 98.9% and 97.0% (p=0.77). In multivariable competing-risk analysis, higher grade and LVI positivity negatively predicted for decreased BCSS, but there was no difference in BCSS risk between those who received RT+ HT versus RT alone, with HR 0.95 (95% CI 0.57 – 1.58, p=0.83). Conclusions: For women with low-risk breast cancer, adjuvant RT alone or RT+HT resulted in similar breast cancer-specific survival. A randomized trial is needed to determine whether HT may be omitted for some low-risk patients.