(Sunday, 7/14/2019)
Room: ePoster Forums
Purpose: Pre-treatment dose verification of IMRT plans has been performed over 20 years. However, the acceptance criteria are still in dispute. The purpose of our study is to elucidate the information provided by these criteria using analysis of the 2D planar dose distributions from standard fields under controlled condition.
Methods: Uncertainties for a point dose at 10cm depth were characterized for MapCheck2 diode array, Pinnacle treatment planning software, and delivery using an Elekta Versa-HD linac. Spatial uncertainty was determined by the quadrature sum of setup and mechanical errors on imaging, delivery, and measurement systems. Dose calculation uncertainties were determined separately for the steep and shallow dose gradient regions. This information was used to establish logical tolerance limits for the dosimetric verification of IMRT plans.
Results: Accuracy of dose calculation is sensitive to the MLC penumbral model and the resolution of calculation grids in steep dose-gradients, and insensitive in shallow dose-gradients. A spatial tolerance of 3mm was defined for point dose comparison for all regions. Dosimetric tolerance varied between 2-10% depending on the dose gradient, calculation grid resolution, interpolation, and field size. Based on the dose grid, the difference of interpolated, measured and calculated, distributions can be increased by factors 2.8x and 2.0x in steep and shallow dose-gradients, respectively. Using our tolerance criteria, dosimetric errors such as 1 mm MLC shift can be detected in the steep dose gradient. These errors would have been missed by the typical acceptance criteria of 3%/3mm or 2%/2mm at 95% passing rate.
Conclusion: A single tolerance limit cannot be meaningfully applied for plan verification. Interpolation of dose distributions substantially increases the difference in their comparison, hence the analysis methods that require interpolation (e.g., gamma-index) are inaccurate. We recommend that less ambiguous results can be provided by dose verification at well-defined positions to warrant further actions.
Keywords
Quality Assurance, Intensity Modulation
Taxonomy
Not Applicable / None Entered.
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