MR-Linac Radiotherapy – The Beam Angle Selection Problem

BACKGROUND: 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. MATERIALS AND METHODS: 23 patients treated at a Unity MR-linac were included. BAO(x) plans (x=7-12 beams) were generated for all patients. Analyses of BAO(12) 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. RESULTS: For x>7, plan quality for CS(x) and BAO(x) was highly similar, while both were superior to EQUI(x). E.g. with CS(9), bowel/bladder D(mean) reduced by 22% [11%, 38%] compared to EQUI(9) (p<0.001). For equal plan quality, the number of EQUI beams had to be doubled compared to BAO and CS. CONCLUSIONS: 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.