The Impact of Grid Size and Statistical Uncertainty on the Accuracy of VMAT Plans Based on Dosimetric Parameters for Head and Neck Cancers

Aim Grid size and Statistical Uncertainty (SU) settings have an impact on the efficiency and accuracy of the treatment plans. Hence, it is very important, to find an optimum combination of the two-dose calculation parameters for efficient treatment planning. This study aims to study the impact of Grid Size and SU on Head and Neck cancer plans which were planned using the Volumetric-Modulated Arc Therapy (VMAT) technique. Materials and Methods 15 patients were selected for this retrospective study who had undergone radiation therapy at KMC, Manipal for various Head and Neck cancers. VMAT plans were generated for each case in Monaco 5.11 Treatment Planning System (TPS) using the Monte Carlo algorithm. For each patient, nine plans were generated in which the Grid size values were taken as 2 mm, 3 mm, and 5 mm and for each Grid size, SU settings were varied as 1%, 2%, and 5%. The plans were analysed and evaluated using outcome measures such as the Homogeneity Index (HI) and the Conformity Index (CI). Results The plans were evaluated as per the ICRU guidelines. In all the plans, PTV coverage achieved was above 95% with the maximum coverage (97.009) achieved with the 3 mm/1% combination. Overall, the effect of varying Grid Size and SU was insignificant in the plans generated, but there were few variations in the dose calculation parameters. For 2 mm/1%, 2 mm/2%, 2 mm/5%, 3 mm/1%, 3 mm/2%, 3 mm/5%, 5 mm/1%, 5 mm/2%, 5 mm/5%, the mean values of HI were 1.073, 1.078,1.078, 1.075, 1.0805, 1.0805, 1.068, 1.079, 1.089, and the mean values of CI were 0.968, 0.967, 0.967, 0.968, 0.958, 0.96, 0.954, 0.946, 0.948, respectively. Among all the combinations, the results for 5 mm/1% yielded more homogeneity, whereas 2 mm/1% and 3 mm/1% combinations resulted in better conformity. The 3 mm/1% combination gave maximum target coverage and quite fewer Monitor Units (MU) as well. Conclusion 3 mm Grid Size and 1% SU is suggested for the Head and Neck carcinomas planned using the VMAT technique.