Room: Exhibit Hall
Purpose: The purpose of this study is to investigate the dose calculation grid (DCG) size for gamma analysis of three-dimensional dose distribution of volumetric-modulated arc therapy (VMAT) by using a rotating 2D detector array of finite size of the detector element.
Methods: A VMAT plan of TrueBeam for a head-and-neck (H&N) case was created on RANDO phantom using AutoPlanning of Pinnacle treatment planning system. The verification plan was created on the OCTAVIUS 4D phantom. The size of DCG was varied from 2 to 5 mm. The gamma passing rate (GPR) for these data were analyzed with and without spatial shifts to LR, AP, and SI directions artificially given on the gamma analysis. The sensitivity of GPR to the spatial shift was defined as the mean reduction of GPR. The sensitivity was compared among different DCGs for 3mm/3% and 2mm/2% criteria. These analyses were also performed for another plan with the interpolation of the gantry angle from 4 degrees to 2 degrees.
Results: GPR for 5-mm DCG was greater than the one for 2-mm DCG by 3.5% and 4.3% for 4- and 2-degree plans, respectively. This result is understood by considering the detector size of OCTAVIUS 729 (5-mm cubic). Since the measurement data is averaged within the detector volume, the larger DCG works better for this case using OCTAVIUS 4D. On the other hand, the sensitivity for didn't show significant difference among DCGs investigated. This result is also supported by the matching of the effective spatial resolution of the calculation and the measurement.
Conclusion: We investigated DCG for the H&N VMAT plan in terms of GPR and its sensitivity to the relative spatial displacement of the dose distribution. For the case investigated, the larger DCG showed better matching with the measurement device.