Impact of field number and beam angle on ERE for lung stereotactic body radiotherapy with 1.5 T MR-Linac

Abstract Purpose To investigate the impact of field number and beam angle on ERE during IMRT planning with 1.5 T MR-Linac for lung stereotactic body radiotherapy (SBRT). Material and methods IMRT plans for three representative lung cases who treated with 1.5 T MR-Linac were generated using TPS Monaco 5.40.01. In each case, five types of plans were generated: (1) the “original plan” generated using 5-fields and beam angles manually optimized in ipsilateral tumor side, with no transverse magnetic field (TMF); (2) the plans generated by reoptimizing and recalculating the original plan, with a 1.5 T TMF; (3) the plans generated with 5-fields, with beam angles manually optimized in anterior-posterior (AP) direction; (4) the plans generated with equidistant 5-fields; and (5) the plans generated with equidistant 9-fields. These plans were compared using a variety of dose-volume-parameters (DVPs). Results The DVPs under TMF showed the presence of 1.5 T TMF can slightly change for all lung cases studied. When the plans generated with beam angles manually optimized in anterior-posterior direction, results showed that the plans improved conformity and homogeneity for PTV and reduced the dose to tissue surface. Furthermore, the results showed that the equidistant/non-equidistant comparison of the previous target anatomy result in a more favourable dose distribution for the equidistant beam setting. At last, the results showed the dose distributions for PTV with 9 and 5 equidistant beams were not significantly different. Conclusion The plan quality can be significantly changed by the presence of a 1.5 T TMF for lung SBRT with MR-Linac. The selection of optimal fields number and beam angle can substantially reduce or even eliminate these changes, without deteriorating overall plan quality.