Benefit of respiratory gating in the Danish Breast Cancer Group partial breast irradiation trial

Background and purpose: Partial breast irradiation (PBI) has been the Danish Breast Cancer Group (DBCG) standard for selected breast cancer patients since 2016 based on early results from the DBCG PBI trial. During trial accrual, respiratory-gated radiotherapy was introduced in Denmark. This study aims to investigate the effect of respiratory-gating on mean heart dose (MHD). Patients and methods: From 2009 to 2016 the DBCG PBI trial included 230 patients with left-sided breast cancer receiving external beam PBI, 40 Gy/15 fractions/3 weeks. Localization of the tumor bed on the planning CT scan, the use of respiratory-gating, coverage of the clinical target volume (CTV), and doses to organs at risk were collected. Results: Respiratory-gating was used in 123 patients (53 %). In 176 patients (77 %) the tumor bed was in the upper and in 54 patients (23 %) in the lower breast quadrants. The median MHD was 0.37 Gy (interquartile range 0.26–0.57 Gy), 0.33 Gy (0.23–0.49 Gy) for respiratory-gating, and 0.49 Gy (0.31–0.70 Gy) for free breathing, p < 0.0001. MHD was < 1 Gy in 206 patients (90 %) and < 2 Gy in 221 patients (96 %). Respiratory-gating led to significantly lower MHD for upper-located, but not for lower-located tumor beds, however, all MHD were low irrespective of respiratory-gating. Respiratory-gating did not improve CTV coverage or lower lung doses. Conclusions: PBI ensured a low MHD for most patients. Adding respiratory-gating further reduced MHD for upper-located but not for lower-located tumor beds but did not influence target coverage or lung doses. Respiratory-gating is no longer DBCG standard for left-sided PBI.