Room: Exhibit Hall
Purpose: Determination of skull geometry in GammaPlan is based on one of two sources: CT imageset segmentation or measurement of 24 discrete measurements of the skull. The majority of GammaKnife centers perform skull measurements to characterize the patientâ€™s anatomy for dose calculations. Variability of these measurements and their influence on dosimetry was examined in this study.
Methods: Changes in the necessary treatment time were evaluated based on sequentially altering all or some of the 24 discrete skull measurements. Time to deliver 100Gy to the UCP was determined in GammaPlan for radii ranging from 40-140mm. The 2nd phase changed one of the 24 skull measurements from 80mm to 100mm, repeating for all 24 discrete locations. The 3rd phase repeated the prior step for off-axis points +/-50mm in each of the orthogonal directions.
Results: The centrally-positioned target required 0.6%/mm more treatment time with uniformly increasing concentric skull size. For the 2nd phase, increasing the skull radius at each individual location resulted in 0.05%/mm and 0.06%/mm more treatment time for the "B" and "C" rows, with no significant changes for the "A" and "D" rows. For the 3rd phase, sensitivity to off-axis positioning in the Z direction resulted in no significant change for Z=150, but the top skull measurement perturbed the treatment time by 0.25%/mm. For the equatorial shifts in the X- and Y-directions, variations in skull measurements increased treatment times by 0.20%/mm only for perturbation of skull measurements adjacent to the off-axis points. These corresponded to C1 and C5 for +/-X and C3 and C7 for +/-Y, respectively. Variations in skull measurements at other locations did not substantially alter treatment times. Findings were independent of shot size.
Conclusion: This work indicates that skull measurement variability should be considered for enhancing dose calculation accuracy and patient treatments.