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

Abstract

PurposeTo investigate the impact of field number and beam angle on ERE during IMRT planning with 1.5T MR-Linac for lung stereotactic body radiotherapy (SBRT). Material and methodsIMRT plans for three representative lung cases who treated with 1.5T 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 1.5T 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). ResultsThe DVPs under TMF showed the presence of 1.5T 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. ConclusionThe plan quality can be significantly changed by the presence of a 1.5T 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.