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

ObjectiveTo 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.MethodsThis 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 (plan1.5 T), a magnetic field-free plan (plan0 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.ResultsIn the absence of a magnetic field in the first quadrant, PTV Dmin increased (97.75 +/- 17.55 vs. 100.96 +/- 22.78)%, Dmax decreased (121.48 +/- 29.68 vs. 119.06 +/- 28.52)%, D98 increased (101.35 +/- 7.42 vs. 109.35 +/- 26.52)% and HI decreased (1.14 +/- 0.14 vs. 1.05 +/- 0.01). In the absence of a magnetic field in the second quadrant, PTV Dmin increased (84.33 +/- 19.74 vs. 89.96 +/- 21.23)%, Dmax decreased (105 +/- 25.08 vs. 104.05 +/- 24.86)%, and HI decreased (1.04 +/- 0.25 vs. 0.99 +/- 0.24). In the absence of a magnetic field in the third quadrant, PTV Dmax decreased (110.21 +/- 2.22 vs. 102.31 +/- 26)%, L-P V30 decreased (10.66 +/- 9.19 vs. 5.81 +/- 3.22)%, HI decreased (1.09 +/- 0.02 vs. 0.98 +/- 0.25), and PTV Dmin decreased (92.12 +/- 4.92 vs. 89.1 +/- 22.35)%. In the absence of a magnetic field in the fourth quadrant, PTV Dmin increased (89.78 +/- 6.72 vs. 93.04 +/- 4.86)%, HI decreased (1.09 +/- 0.01 vs. 1.05 +/- 0.01) and D98 increased (99.82 +/- 0.82 vs. 100.54 +/- 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.ConclusionIn 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.