Fifty-seventh annual meeting of the American association of physicists in medicine
SU-E-T-351: Effects of Isocenter Location, MLC Aperture Angle and Gantry Angle On Dose Distribution
Single-isocenter-multiple-target arc plans will have some of the targets displaced from the isocenter. This is a study of the effects of target size, the distance between the isocenter and a target, how many degrees of gantry angle per multileaf collimator (MLC) apertures, and how many degrees of gantry angle per calculation.
Dynamic arc plans are generated with the isocenter placed at 0, 5 and 10 cm from the center of the targets with sizes of 1, 2 and 3 cm in Head/Neck phantom CT images respectively. Each arc plan has a 60 degree gantry rotation, calculated with gantry angle resolution of 1, 2 and 5 degrees, and MLC aperture resolution of 1, 2, 5 and 6 degrees. Some Static-field plans are generated with static fields having gantry angle changes every 1 or 2 degrees and MLC aperture changing every 2 or 6 degrees to simulate actual LINAC delivery situation of arc plans of same gantry and MLC aperture resolutions.
When the isocenter is placed inside the target, all plans are identical regardless of target size, gantry resolution and MLC aperture resolution. When the isocenter is placed a certain distance outside the target center, the aforementioned parameters will affect the plan results. Decreasing target size and increasing distance from the isocenter will increase the effects. For a 1 cm the target with isocenter 10 cm away, the differences between the arc plan and the static-field plan are: maximal dose 12.8%, minimal dose 24% and MU 14.9%.
Arc plan calculation interpolates MLC apertures. This interpolation will cause dose discrepancy between the arc plan and actual delivery when the target size is small and is away from the isocenter. Single-isocenter-multiple-target arc plan should be calculated with finest MLC aperture resolutions and finest gantry angle resolutions to optimize the plan.