(Sunday, 7/14/2019)
Room: ePoster Forums
Purpose: To assess the dosimetric impact of gEUD(generalized Equivalent Uniform Dose) optimization on nasopharyngeal carcinoma(NPC) volumetric modulated arc therapy(VMAT) cases and to quantify the referred optimization parameters.
Methods: A total of 9 NPC patients treated with VMAT were randomly selected from our institution. They are initially planned with traditional dose-volume optimization, more specifically with a mean objective to parotids. For each case, new VMAT plans were generated with the parameter α ranging from 1 to 20 replacing mean objective to parotids, while keeping the other optimization constraints for targets and other organs at risk(OARs) unchanged. The α values were set to 1,3,5,10,15and 20. The priority of gEUD objective was 50. Three dynamic arcs were used in all VMAT plans. For PTV/PGTV, conformity index(CI) were calculated. Mean parotids dose, maximum dose to lens, spinal cord, stem, optic nerves and optic chiasm were used as OAR dosimetry. Repeated measurement analysis of variance and paired t-tests were used for statistical analysis.
Results: Dosimetric comparisons show superiority for gEUD optimization over traditional dose-volume optimization without sacrifice on other dosimetric endpoints. For the traditional dose-volume optimization and the gEUD optimization with α equals 1,3,5,10,15 and 20, the mean dose of parotids were 2719.7±250, 2696.2±234.5, 2533.6±246.6, 2470.5±252.4, 2428.9±231.5, 2423.5±227.2, 2431.8±243.9 cGy, respectively(p=0.006), CI for PGTV 0.706±0.44, 0.709±0.45, 0.701±0.50, 0.703±0.42, 0.691±0.37, 0.681±0.39, 0.672±0.39, respectively (p<0.001) and CI for PTV 0.803±0.21, 0.803±0.203, 0.802±0.2, 0.80±0.21, 0.794±0.22, 0.793±0.22, 0.792±0.19,respectively (p=0.006). The mean parotids dose initially decreased as α increased and did not further improve while the α more than 10. The CI for PTV/PGTV decreased as αincreased.
Conclusion: gEUD based optimization prevails over dose-volume optimizations to reduce the mean parotids dose. Balancing the trade-off between parotid sparing and CI for PTV/PGTV ,a recommended optimization parameter would be 5-10.
Funding Support, Disclosures, and Conflict of Interest: Science and Technology Department of Jilin Province (grant no:201603040YY) Development and Reform Commission of Jilin Province (grant no: 2016C053-1)
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TH- External beam- photons: treatment planning/virtual clinical studies
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