(Sunday, 7/14/2019)
Room: ePoster Forums
Purpose: Head and neck(H&N) plans using VMAT technique are widely used since it requires fewer MUs, faster delivery, and highly conformal dose-distributions. Due to higher dose-gradient around target-volumes and OARs of IMRT and VMAT plans, there is concern of using optimal grid-size for dose calculations.This study compares DVH parameters using various calculation grid-sizes and angular-resolutions.
Methods: Five H&N patients with simultaneous-integrated-boost(SIB) were planned with VMAT using Eclipse AAA-algorithm.Doses were computed using grid-sizes of 1,2,3,4,5mm and angular-resolutions of 1,2 and 5°. 45 plans were computed, 9 plans for each patient with varying grid-size and angular-resolution.Doses of 70Gy,60Gy & 54Gy in 33 fractions prescribed to cover 95% of high-risk, intermediate and low-risk PTVs, respectively.Monitor-Units(MUs),Max, Min, Mean doses and ICRU-83 dose-parameters D98%, D50%, and D2% for PTVs, Max, Mean and 1cc-volume doses for OARs: spinal-cord, brainstem, parotids, mandible and cochlea were compared.
Results: Plan compared with 1mm/1° resolution, as grid-size increases MUs increases upto 3% and stayed same for 2mm-grid and all angular-resolution plans for same grid-sizes.PTV Max-dose were within 1% for all plans.For PTVs min-doses were lower upto 7%, mean-doses higher upto 2%, D2% and D50% were higher upto 3% and 2% respectively for 5mm-grid.Max-dose to spinal-cord and D1cc varied upto 3%. Brainstem max and D1cc doses were higher upto 7% and 3% respectively as grid-size increases to 5mm.Madible max-dose and D1cc varied ±2% and ±3%. Parotids mean-dose and D50% were higher upto 4% and 8% respectively. Cochlea mean-dose higher 3-31% for varying grid-size 2-5mm. Impact of angular increment for same grid-size was insignificant(<1%) for PTVs & critical-structures.
Conclusion: Varying angular-resolution did not affect dose to either PTV or OARs significantly. However, MUs and doses to PTVs and OARs changes with varying the calculation grid-size. Therefore caution should be observed for selecting optimal grid-size for accurate dose calculations for H&N VMAT plans.
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