Top Line: Quick, what are the long term risks of breast radiation? We’re willing to bet hypothyroidism wasn’t top of mind.
The Study: Here is a look at thyroid function outcomes among 4073 women receiving adjuvant breast radiation during the decade 2007-2016. They were split into three cohorts depending on field arrangement: (1) whole breast alone (n=2468), regional node irradiation (RNI) with cranial border at subclavian artery arch per ESTRO (n=215), and (3) RNI with cranial border at cricoid cartilage per RTOG (n=1390). Why the ESTRO / RTOG split among those receiving RNI? First, note that the subclavian artery arch is considerably lower than the cricoid, making the RTOG volumes more liberal. Second, team ESTRO has estimated that <4% of post-radiation locoregional recurrences (i.e. <4% of an already low % of women receiving radiation) occur outside of the ESTRO field borders. The primary outcome of rate of hypothyroidism at 3 years was indeed significantly higher for the RNI-RTOG cohort (2.2%) than for the RNI-ESTRO (0.9%) or whole breast (0.8%) cohorts, and subgroup analyses pointed to even higher rates among those <60 years of age. An exploratory analysis of 200 cases from each cohort calculated significant variations in mean thyroid doses (EQD2) of 0.23 Gy after whole breast, 1.93 Gy after RNI-ESTRO, and 7.89 Gy after RNI-RTOG.
TBL: Regional nodal breast irradiation doesn’t have to be all or nothing, and a more conservative superior supraclavicular field border for less advanced cases is likely to spare toxicity without sparing microscopic tumor control. | Choi, In J Radiat Oncol Biol Phys 2021