MR-Linac Radiotherapy - The Beam Angle Selection Problem.

23 patients treated at a Unity MR-linac were included. BAO_x plans (x=7-12 beams) were generated for all patients. Analyses of BAO₁₂ plans resulted in CS_x class solutions. BAO_x plans, CS_x plans, and plans with equi-angular setups (EQUI_x, x=9-56) were mutually compared.

Computer-generated beam angle CS could replace individualized BAO without loss in plan quality, while reducing planning complexity and calculation times, and resulting in a simpler clinical workflow. CS and BAO largely outperformed equi-angular treatment. With the developed CS, time consuming beam angle re-optimization in daily adaptive MR-linac treatment could be avoided. Further systematic research on computerized development of beam angle class solutions for MR-linac treatment planning is warranted.

For x>7, plan quality for CS_x and BAO_x was highly similar, while both were superior to EQUI_x. E.g. with CS₉, bowel/bladder D_mean reduced by 22% [11%, 38%] compared to EQUI₉ (p<0.001). For equal plan quality, the number of EQUI beams had to be doubled compared to BAO and CS.

With the large-scale introduction of volumetric modulated arc therapy (VMAT), selection of optimal beam angles for coplanar static-beam IMRT has increasingly become obsolete. Due to unavailability of VMAT in current MR-linacs, the problem has re-gained importance. An application for automated IMRT treatment planning with integrated, patient-specific computer-optimization of beam angles (BAO) was used to systematically investigate computer-aided generation of beam angle class solutions (CS) for replacement of computationally expensive patient-specific BAO. Rectal cancer was used as a model case.