The days are short, but the years are long.
Top Line: The landmark trials omitting breast radiation in low-risk older women left many wondering: where’s the trial omitting 5 to 10+ years of endocrine therapy?
The Study: After all, fewer than half of these women complete the recommended duration of endocrine therapy due to side effects while the proportion of women who cannot complete 5-15 radiation treatments approaches zero. (Side note: none of these women are likely to see more benefit than trouble from a boost—see below for one of many reasons why.) This pragmatic SEER analysis tells us what many rad oncs would have guessed. It includes over 16,000 women 66+ years of age post-breast conserving surgery in 2007-2012 for stage I estrogen receptor-positive breast cancer classified into four adjuvant treatment groups:  endocrine therapy plus radiation (ET+RT, 44%),  endocrine therapy alone (ET, 7%),  radiation alone (RT, 41%), or  neither (9%). Of note here is that throughout this 6-year window, radiation use plummeted from 49% to 30%, perhaps as a result of maturation of said landmark trials above. Relative to ET+RT, both no therapy (HR 3.7) and ET (2.2) were associated with higher incidence of a second breast cancer event but RT (HR 1.2) was not. The absolute rates were:  2.2% with ET+RT,  3.2% with ET,  3% with RT, and  7% with neither. Clinical factors associated with higher occurrence of a second breast cancer event were, unsurprisingly, HER2+ and pT1c disease.
TBL: As most nomograms suggest, radiation monotherapy likely gives an ever so slight disease control advantage over endocrine monotherapy in older women with low-risk breast cancer, while its obvious benefit remains the intrinsic advantage of 5 days versus 5 years of therapy. | Gerber, Int J Radiat Oncol Biol Phys 2021