Purpose — Randomized trials studying endocrine therapy (ET) with and without radiation therapy (RT) following breast-conserving surgery (BCS) have detected differences in local recurrence (LR) but not survival among elderly women with hormone receptor positive stage I breast cancer (BC). We assembled a population-based cohort of such women to examine the use and outcomes associated with or without the administration of adjuvant radiotherapy (RT) or ET.
Methods — Women aged ≥ 65 years with stage I BC treated with BCS in Ontario between 2010 and 2016, their treatments and outcomes were ascertained using deterministic linkages of administrative databases. Multivariable Cox regression models were used to evaluate risks of ipsilateral LR and of any first in-breast event, categorizing women by their treatment.
Results — 5076 women were treated with BCS followed by RT + ET (n = 1964), RT alone (n = 1325), ET alone (n = 719), or no adjuvant treatment (n = 1068). Median follow-up was 5 years. LR occurred in 0.9% after adjuvant RT + ET, 1.4% after RT alone, 3.1% after ET alone, and 9.4% after BCS alone (p < 0.001). The adjusted risk of LR was increased in those who received no adjuvant therapy (HR = 13.43, CI: 7.89, 22.85), or ET alone (HR = 4.03, CI: 2.14, 7.59). The adjusted risk of any first in-breast event was greatest among those without any adjuvant therapy (HR = 7.61, 95%CI: 5.21, 11.11, p < 0.0001). Absolute and adjusted risks of any first in-breast event were comparable between those with ET alone (HR = 2.09, 95%CI: 1.27, 3.43, p = 0.0038) and those with RT alone (HR = 1.91, 95% CI: 1.25, 2.91, p = 0.0028).
Conclusions — Older women with stage I BC who receive no adjuvant therapy have a significant absolute risk of LR and any first in-breast event, whereas the absolute risk of these events among those with either RT alone or ET alone is only slightly higher than among those treated with both.