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• Detailed evaluation of received radiotherapy in the randomized SENOMAC trial. • Inclusion of level I in the target more common after sentinel node biopsy only. • Significant dose in axillary level I regardless of inclusion in target volume. • No dose difference to axillary target volumes between randomization groups. • Inclusion of axillary level I raised dose to the humeral head. Recommendations for regional radiotherapy (RT) of sentinel lymph node (SLN)-positive breast cancer are debated. We here report a RT quality assessment of the SENOMAC trial. The SENOMAC trial randomized clinically node-negative breast cancer patients with 1–2 SLN macrometastases to completion axillary lymph node dissection (cALND) or SLN biopsy only between 2015–2021. Adjuvant RT followed national guidelines. RT plans for patients included in Sweden and Denmark until June 2019 were collected (N = 1176) and compared to case report forms (CRF). Dose to level I (N = 270) and the humeral head (N = 321) was analyzed in detail. CRF-data and RT plans agreed in 99.3 % (breast/chest wall) and in 96.6 % of patients (regional RT). Congruence for whether level I was an intended RT target was lower (78 %). In accordance with Danish national guidelines, level I was more often an intended target in the SLN biopsy only arm (N = 334/611, 55 %,) than in the cALND arm (N = 174/565, 31 %,). When an intended target, level I received prescribed dose to 100 % (IQR 98–100 %) of the volume. However, even when not an intended target, full dose was delivered to > 80 % of level I (IQR 75–90 %). The intentional inclusion of level I in the target volume more than doubled the dose received by ≥ 50 % of the humeral head. Congruence between CRF data and RT plans was excellent. Level I received a high dose coverage even when not intentionally included in the target. Including level I in target significantly increased dose to the humeral head. [ABSTRACT FROM AUTHOR]