Influence and optimization strategy of the magnetic field in 1.5 T MR-linac liver stereotactic radiotherapy

Objective To compare intensity reduction plans for liver cancer with or without a magnetic field and optimize field and subfield numbers in the intensity-modulated radiotherapy (IMRT) plans designed for liver masses in different regions. Methods This retrospective study included 62 patients who received radiotherapy for liver cancer at Shandong Cancer Hospital. Based on each patient's original individualized intensity-modulated plan (plan.sub.1.5 T), a magnetic field-free plan (plan.sub.0 T) and static intensity-modulated plan with four different optimization schemes were redesigned for each patient. The differences in dosimetric parameters among plans were compared. Results In the absence of a magnetic field in the first quadrant, PTV D.sub.min increased (97.75 [+ or -] 17.55 vs. 100.96 [+ or -] 22.78)%, D.sub.max decreased (121.48 [+ or -] 29.68 vs. 119.06 [+ or -] 28.52)%, D.sub.98 increased (101.35 [+ or -] 7.42 vs. 109.35 [+ or -] 26.52)% and HI decreased (1.14 [+ or -] 0.14 vs. 1.05 [+ or -] 0.01). In the absence of a magnetic field in the second quadrant, PTV D.sub.min increased (84.33 [+ or -] 19.74 vs. 89.96 [+ or -] 21.23)%, D.sub.max decreased (105 [+ or -] 25.08 vs. 104.05 [+ or -] 24.86)%, and HI decreased (1.04 [+ or -] 0.25 vs. 0.99 [+ or -] 0.24). In the absence of a magnetic field in the third quadrant, PTV D.sub.max decreased (110.21 [+ or -] 2.22 vs. 102.31 [+ or -] 26)%, L-P V.sub.30 decreased (10.66 [+ or -] 9.19 vs. 5.81 [+ or -] 3.22)%, HI decreased (1.09 [+ or -] 0.02 vs. 0.98 [+ or -] 0.25), and PTV D.sub.min decreased (92.12 [+ or -] 4.92 vs. 89.1 [+ or -] 22.35)%. In the absence of a magnetic field in the fourth quadrant, PTV D.sub.min increased (89.78 [+ or -] 6.72 vs. 93.04 [+ or -] 4.86)%, HI decreased (1.09 [+ or -] 0.01 vs. 1.05 [+ or -] 0.01) and D.sub.98 increased (99.82 [+ or -] 0.82 vs. 100.54 [+ or -] 0.84)%. These were all significant differences. In designing plans for tumors in each liver region, a total number of subfields in the first area of 60, total subfields in the second zone of 80, and total subfields in the third and fourth zones of 60 or 80 can achieve the dose effect without a magnetic field. Conclusion In patients with liver cancer, the effect of a magnetic field on the target dose is more significant than that on doses to organs at risk. By controlling the max total number of subfields in different quadrants, the effect of the magnetic field can be greatly reduced or even eliminated. Keywords: Liver canc