Dosimetric evaluation of the gantry sag effect in clinical SRS plans

The gantry sag introduces a largely reproducible variation of the radiation field center around the radiation isocenter. The purpose of this work is to assess the change of the dose distribution caused by the gantry sag in clinical stereotactic plans.

Brain stereotactic radio surgery treatment plans were evaluated and grouped according to radiation therapy planning technique. Group 1 was planned with volumetric arc therapy technique using coplanar arcs while Group 2-non-coplanar arcs. To simulate the gantry sag effect in the treatment planning system, the original plan segments were divided into four groups according to corresponding gantry angles: upper, lower, left and right quadrants. Then, isocenter of the upper quadrant was shifted towards "Gun", isocenter of the lower quadrant was shifted towards "Target" and isocenter of the left and right quadrants was left at its original positions. The magnitude of the shift was 0.5, 1 and 1.5 mm in each direction, corresponding to 1, 2 and 3 mm of gantry isocenter diameter. To estimate the changes in dose distribution between the original and modified plans, the following dose-volume metrics were tracked: planning target volume (PTV) coverage (V_99;PTV), hotspot dose in PTV (D_PTV;0.015cc)), coldspot doses in PTV (D_{PTV;(V-0.015cc)}), conformity and gradient indexes, maximum point doses in organs at risk (OAR, D_OAR;0.015cc) and outside PTV (D_{outsidePTV;0,015cc}). For the second group of patients volume of brain receiving 12 Gy (V_12Gy) was analyzed.

The mean relative change of all metrics was within -2%/+2.5% range for both techniques for isocenter diameter up to 2 mm. Isocenter diameter of 3 mm causes significant changes in V_99;PTV, conformity and gradient indexes for coplanar, and additionally in D_{PTV;(V-0.015cc)} for non-coplanar plans. The largest increase of maximum point dose in OAR was 1.1, 2.1 and 3.2% for ±0.5, ±1 and ±1.5 mm shift, respectively.

The results demonstrate dosimetric effect of gantry sag depending on its value. By itself, the gantry sag effect does not produce clinically perceptible dose changes neither for PTV nor for OARs for shift ranges up to ±1 mm, both for coplanar and non-coplanar delivery techniques. For the larger gantry sag magnitude dosimetric changes can become significant, especially for non-coplanar plans. It indicates that 2 mm diameter tolerance of gantry isocenter postulated in TG-142 is reasonable, as variations in excess of this value start to affect the overall dosimetric and spatial uncertainty.

Dosimetric evaluation of the gantry sag effect in clinical stereotactic radio surgery plans is presented for the first time.