Room: AAPM ePoster Library
Purpose: electron deflection resulting from presence of a transverse magnetic field (TMF) can affect the dose distribution in 1.5T MR-Lianc.This study aims to investigate the dose effects of electron return effect (ERE) at lung tissue interface and reduce the ERE by increasing the field number and optimizing the beam angle for lung IMRT planning.
Methods: plans for four representative lung cases were generated following commonly used clinical dose volume criteria and using TPS Monaco 5.40.Six plans were generated for each case:(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 DVH parameters (DVPs), including Heterogeneity Index (HI),Conformity Index (CI),Dmean,Dmax,and D1cc in organ at risk and tissue interface.
Results: showed that no significantly differences for 5 and 9 fields plans generated without the TMF. The 5Fside and 9Fside plans with TMF had slightly worsened homogeneity and conformity of PTV than plans without TMF. Additionally these plans results in considerable increase in Dmax and Dmean on PTV and Dmax and D1cc on tissue interface. Nevertheless the 9Fopposite TMF plans lead to much smaller dose effects from ERE compared with 5Fside and 9Fside TMF plans. There was no significantly difference in DVPs of PTV and tissue interface in 9Fopposite TMF plans compared to the plans without TMF, with the difference less than 1%.
Conclusion: doses distribution can be significantly changed by the precense of a 1.5T TMF for lung patients in MR-Linac. The selection of optimal fields number and beam angle can substantially reduced or even eliminated these changes, without deteriorating overall plan quality.
Funding Support, Disclosures, and Conflict of Interest: National Natural Science Foundation of China (No. 11805292); Natural Science Foundation of Guangdong, China (No. 2018A0303100020)