Impact of Field Number and Beam Angle on ERE for Lung Cancers Radiotherapy Planning in 1.5 T MR-Linac

Abstract

The MR-Linac offers great potential for high-precision delivery of radiation therapy and allows for direct monitoring of intrafraction motion without being invasive, needing surrogates, or exposing the patient to additional radiation. However, the electron deflection resulting from presence of a transverse magnetic field (TMF) can affect the dose distribution, particularly the electron return effect (ERE) at tissue interface. The purpose of the study is to investigate the dose effects of ERE at lung tissue interface and reduce the ERE by increasing the field number and optimizing the beam angle during intensity-modulated radiation therapy (IMRT) planning for lung patents. IMRT plans for four representative lung cases were generated following commonly used clinical dose volume criteria and using TPS Monaco 5.40. In each case, three types of plans were generated: (1) the plans with 5 fields and beam angle in tumor side generated with or without a TMF (5Fside); (2) the plans with 9 fields and beam angle in tumor side generated with or without a TMF (9Fside); (3) the plans with 9 fields and beam angle in the opposite direction generated with or without a TMF (9Fopposite). These plans were compared using a variety of dose-volume-parameters (DVPs), including V100%, Dmean, Heterogeneity Index (HI), Conformity Index (CI), Dmax, and D1cc in OARs (organ at risk) and tissue interface. All the optimizations and calculations in this work were performed on static data. Results showed that no significantly differences for 5 and 9 fields plans generated without the presence of the 1.5 T TMF. The 5Fside and 9Fside plans with TMF had slightly worsened homogeneity and conformity of PTV than plans without TMF, with the mean HI and CI difference being more than 50% and 16% respectively. Similarly, the 5Fside and 9Fside TMF plans results in considerable increase in Dmax and Dmean on PTV, mostly between 10% and 25%. And the Dmax and D1cc on tissue interface also increased by 10% to 25%. However, the 9Fopposite TMF plans lead to much smaller dose effects from ERE compared 5Fside and 9Fside TMF plans. Additionally, there was no significantly difference in DVPs of PTV and tissue interface in 9Fopposite TMF plans in comparison to the plans without TMF, with the difference being less than 1%. The doses on PTV and tissue interface can be significantly changed by the presence of a 1.5 T TMF for lung patient in MR-Linac. The selection of optimal fields number and beam angle can substantially reduce or even eliminated these changes, without deteriorating overall plan quality.