With the Halcyon version 1.0, all imaging setup fields are taken using digital megavoltage imaging panels. When designing a Halcyon plan, four different setup fields can be selected. For the 10 patients, we designed four VMAT plans with different setup fields on the Halcyon: high‐quality MV CBCT (the gantry rotates clockwise from 260° to 100°, delivering 10 MUs, simply called CBCT-H); low‐dose MV CBCT (delivering 5 MUs in a clockwise gantry rotation from 260° to 100°, simply called CBCT-L); high‐quality orthogonal MV radiograph pair (images acquired with 0° and 90° and delivering 2 MUs for each field, simply called MV-H) and low‐dose orthogonal MV radiograph pair (images acquired with 0° and 90° and delivering 1 MU for each field, simply called MV-L). For the four plans, two anticlockwise 160°–200°and two clockwise 200°–160° rotation arcs were used.

On the Trilogy, whole and partial arc plans, referred to as T-4arc and T-8arc, respectively, were generated. The whole-arc plan consisted of two anticlockwise 160°–200° and two clockwise 200°–160° rotation arcs. The partial-arc plan consisted of total 8 partial arcs. For unilateral breast, four 100° arcs like a bowknot were generated. In the two plans, the medial x-jaw was set to the minimum site (− 2 cm) to minimize the irradiated volume of the lungs and heart. A 6 MV X-ray was used, and the dose rate was set to 600 MU/min. Two whole and partial arc plans, referred to as H-4arc and H-8arc, were designed on the Halcyon with the same arc angle as T-4arc and T-8arc mentioned above. In the Halcyon plans, a 6 MV FFF X-ray was used at the maximum dose rate of 800 MU/min. Low-dose MV CBCT was selected for image guidance as Flores-Martinez et al. [6] suggested.

The prescription dose was 50 Gy in 2-Gy fractions. All plans were designed with the Eclipse version 15.5 treatment planning system (Varian Medical Systems, Palo Alto, CA, USA) using an analytic anisotropic algorithm (AAA). For all VMAT plans, the PTV was extended to 5 mm outside the skin, named PTVop, a 10-mm bolus was used on the skin outside the PTV, and the dose of PTVop was optimized. In the final dose calculation, the bolus was deleted. The dose normalization was 95% volume of the PTV received 100% prescription dose. All plans used the same optimization parameter settings, with the goal of minimizing the doses to the lungs, heart and LAD while ensuring PTV dose coverage. No dose constraints were applied to the skin and LV during the optimization.