Dose Effect of Transverse Magnetic Field on IMRT Plans Delivered in an MR-Linac

Abstract

The dose effect of magnetic field in radiation therapy (RT) beams, including the electron return effect (ERE) that occurs near a tissue interface, needs to be addressed for MR-Linac, an emerging RT technology. The effect is, so far, mostly studied in simple phantom geometries. In this work, we investigate this dose effect on realistic IMRT plans encountered in MR-Linac. IMRT plans were generated using planning CTs for representative cases of prostate and pancreas cancers using a research Monaco system that employs a Monte Carlo dose engine including a uniform transverse magnetic (B) field of 1.5T. The plans were generated using clinical dose volume (DV) constraints with and without the presence of the B field. Plans without optimization (i.e., reconstruction of dose distributions for the plans generated with the B field), were also obtained. The qualities of these plans were compared using commonly used DV parameters, including the minimum and maximum dose (Dmin and Dmax), inhomogeneity coefficient [IC = (Dmax-Dmin)/Dmin], volume receiving 100% of the prescription dose (pd) (V100), and dose delivered to 95% of target volume (D95) for PTV, and Dmax, mean dose (Dmean), volume irradiated by > 70 Gy (V70), and V45 for organs at risk (OAR) (e.g., rectum, rectal wall, bladder, bladder wall, duodenum, stomach, and small bowel). The statistical uncertainty on dose was < 1%. The dose effect of the B field on the DV parameters of the IMRT plans was generally less than 4%, with a few exceptions. A summary of the maximal changes of the reconstructed plans with the B field (but no optimization) from the plans without the B field for the selected DV parameters are listed in the Table, where DLT denotes the maximal absolute difference of the percentage data between the plans with and without the B field, and DLT is the maximal relative dose difference (i.e., the maximal ratio of absolute dose difference and the reference point dose from the plans without the B field). The difference for a DV parameter can be reduced to less than 2% by the plan optimization if it is included as a DV constraint.Oral Scientific Abstract 210; TableStructureDlt (Dmin)Dlt (Dmax)DLT (IC)DLT (V100)Dlt (D95)Dlt (Dmean)DLT (V70)DLT (V45)Prostate PTV1.9%1.1%3.6%1.4%0.9%Bladder1.7%1.4%0.3%Rectum3.0%3.3%0.8%Bladder wall1.7%3.5%0.3%Rectal wall3.0%1.5%0.5%Pancreas PTV3.4%0.8%5.9%3.3%0.8%Duodenum1.7%2.0%2.0%Small bowel1.0%1.4%0.3%Stomach1.2%1.1%0.5% Open table in a new tab The dose effect of a 1.5T transverse magnetic field on IMRT plans of the prostate and pancreatic cancer cases is generally small (< 4%), and is due mostly to the small segments in the IMRT plans and/or the ERE. The effect can be largely reduced by the plan optimization process for the cases studied.