Quality assessment of 2705 treatment plans in the randomised Danish Breast Cancer Group Skagen trial 1

Based on prospectively uploaded treatment plans, a comprehensive analysis of the radiation therapy (RT) in the DBCG Skagen trial 1 is presented.

From 2015 to 2021, 2963 breast cancer patients referred to adjuvant locoregional breast cancer RT after breast conserving surgery (BCS) or mastectomy were randomised to 50 Gy/25 fractions versus 40 Gy/15 fractions. Tumour-bed boost was simultaneous integrated boost (SIB). Delineated structure volumes were per ESTRO guidelines. Dosimetric data were obtained from the individual treatment plans.

RT treatment plans from 2705 patients treated at 14 centres in 4 countries were analysed corresponding to 94 % of the per protocol patients. Overall, 1407 had BCS, 1298 had mastectomy, and 437 had SIB. The randomisation was balanced: 1329 (50 Gy) versus 1376 (40 Gy). All plans had target volume delineations per ESTRO guidelines. No significant differences were observed regarding delineated volumes nor regarding inter-centre variations per randomisation arm. Compliance to protocol constraints was generally high, however, the "hot-spot" dose compliance was higher in 50 Gy plans compared to 40 Gy plans due to stricter high dose constraints for the 40 Gy arm. The minimum coverage of target volumes was similar per randomisation arms. Respiratory gating technique was used in 83 % of the patients. When divided into subgroups per laterality and breathing technique, significantly higher lung and heart doses were detected in non-gated compared to gated plans.

A high degree of protocol compliance was documented in the DBCG Skagen trial 1. No differences were observed among the treatment plans in the two randomisation arms.