Thyroid dose guidance in elective supraclavicular breast radiotherapy: a national survey of practice patterns in Switzerland.

Abstract

Background: Supraclavicular fossa (SCF) irradiation is integral to adjuvant radiotherapy (RT) for node-positive breast cancer. Due to anatomical proximity, the thyroid gland is frequently exposed, leading to hypothyroidism in approximately 20-40% of long-term survivors. Despite robust dose-response evidence, thyroid management remains inconsistent and breast cancer-specific dose guidance is lacking. This nationwide survey evaluated current practice patterns regarding thyroid delineation, SCF contouring, and thyroid dose guidance during elective SCF irradiation in Switzerland.

Methods: A structured electronic questionnaire was distributed between May and November 2025 to all 30 Swiss radiation oncology centers treating breast cancer. Institutional characteristics, thyroid contouring practices, SCF target delineation, and applied thyroid dose guidance were assessed. Analyses were descriptive only.

Results: Thirty-three responses from 25 of 30 centers (83%), including all university hospitals, were analyzed. Modern RT techniques were widely implemented (VMAT 79%, IMRT 15%). Routine thyroid contouring was reported by 91% of respondents, of which 94% used AI-assisted tools. However, only 55% applied explicit thyroid dose guidance. Reported mean thyroid dose (D_mean) thresholds ranged from 12 to 35 Gy and clustered at 13.5-15 Gy, 20-21 Gy, and 30-32 Gy. Excluding ALARA ("as low as reasonably achievable") responses, the median D_mean guidance was 21 Gy (IQR 16-30 Gy). SCF delineation varied substantially, particularly regarding superior and medial margins, and 64% reported CTV/PTV cropping to reduce thyroid dose.

Conclusions: Despite strong dose-response evidence and widespread availability of advanced RT techniques, thyroid protection during elective SCF irradiation remains inconsistently implemented. These findings reveal a gap between evidence and practice and support the development of harmonized, breast cancer-specific recommendations, including routine thyroid delineation, anatomy-based SCF contouring, and explicit, oncologically safe dose guidance-particularly D_mean targets of ≤ 13.5 Gy, with ≤20-21 Gy as a pragmatic upper limit-to reduce radiation-induced hypothyroidism without compromising cancer control.