VMAT plans

The VMAT plans contained 5 coplanar arcs overall, including four 60° arcs rotated in clockwise/counterclockwise fashion within the range of [θ_c−10°, θ_c+50°], [θ_c+50°, θ_c−10°], [θ_i+10°, θ_i−50°], and [θ_i−50°, θ_i+10°], and one 50° arc rotated in clockwise fashion from 0° and 50°. The angles of the collimators of the four 60° arcs were designed at 30° or 330°, and the corresponding angle to the 50° arc was set to 0°. The jaws of the 50° arc were adjusted to irradiate only the PTV in SCV regions. All the VMAT plans were optimized and calculated with a maximum dose rate of 600 MUs/min.

The prescribed dose was set as 2 Gy per fraction and a total dose of 50 Gy. For 3D-CRT plans, the prescribed 95% isodose covered at least 95% of the PTV, and the percentage of the PTV receiving a radiation dose greater than 115% of the prescription was less than 2%. For IMRT and VMAT plans, the prescribed 100% isodose covered at least 95% of the PTV, and the percentage of the PTV receiving a radiation dose greater than 110% of the prescription dose was less than 2%. For the 3D-CRT, IMRT, and VMAT plans, the ipsilateral lung V₂₀ (percentage of the volume receiving ≥20 Gy) was limited to 35% while the V₅ (percentage of the volume receiving ≥5 Gy) was limited to 75%. The maximum dose of spinal cord was less than 40 Gy. The contralateral breast V₅ was limited to 10%.