Room: Exhibit Hall
Purpose: A secondary dose calculation is performed for all treatment plans at our institution, including 3D conformal, electron, IMRT, and VMAT plans. Common passing criteria is <5% or <3MUs, whichever is less. A commercial software is used to calculate the dose of a point within a sampling radius and compare the dose to that in the clinical plan. The purpose of this study is to investigate the impact of sampling radius and to determine the optimal sampling radius for clinical application.
Methods: We reviewed clinical beams checked at our institution during 2017. Each beam was initially calculated in the treatment planning system using a dose voxel of 4mm or less. Any beam checked using a sampling radius â‰¥7mm, which is approximately the diagonal distance of a 4mm dose voxel, was re-analyzed using a smaller sampling radius.
Results: Of 8718 beams reviewed, 3% (263) were originally checked using a sampling radius â‰¥7mm. Upon further investigations of applying heterogeneity correction using equivalent depth with field size scaling, using accurate thickness of bolus, adding blocks for flash, or choosing a new point of interest, 98% of the 263 beams passed the secondary check with a sampling radius <7mm. The remaining 2% beams that did not pass using sampling radius <7 mm were lung SBRT plans with small targets and high modulations. Furthermore, with a more stringent sampling radius of 5 mm, 92% of the 263 beams passed the secondary check with the same criteria.
Conclusion: The determination of sampling radius should account for the size of dose voxel in clinical plans. Passing criteria of <5% or <3MUs using a 5 mm sampling radius is met for the majority of clinical plans calculated with a 4mm dose voxel, except for the ones of small field size and with high modulation.