Aim: This study evaluates the impacts of using different evaluation criteria on gamma pass rates in two commercially available QA methods employed for the verification of VMAT plans using different hypothetical planning target volumes (PTVs) and anatomical regions. Introduction: Volumetric modulated arc therapy (VMAT) is a widely accepted technique to deliver highly conformal treatment in a very efficient manner. As their level of complexity is high in comparison to intensity-modulated radiotherapy (IMRT), the implementation of stringent quality assurance (QA) before treatment delivery is of paramount importance. Material and Methods: Two sets of VMAT plans were generated using Eclipse planning systems, one with five different complex hypothetical three-dimensional PTVs and one including three anatomical regions. The verification of these plans was performed using a MatriXX ionization chamber array embedded inside a MultiCube phantom and a Varian EPID dosimetric system attached to a Clinac iX. The plans were evaluated based on the 3%/3 mm, 2%/2 mm, and 1%/1 mm global gamma criteria and with three low-dose threshold values (0%, 10%, and 20%). Results: The gamma pass rates were above 95% in all VMAT plans, when the 3%/3mm gamma criterion was used and no threshold was applied. In both systems, the pass rates decreased as the criteria become stricter. Higher pass rates were observed when no threshold was applied and they tended to decrease for 10% and 20% thresholds. Conclusion: The results confirm the suitability of the equipments used and the validity of the plans. The study also confirmed that the threshold settings greatly affect the gamma pass rates, especially for lower gamma criteria.

AimTo evaluate the impact of couch translational shifts on dose–volume histogram (DVH) and radiobiological parameters [tumour control probability (TCP), equivalent uniform dose (EUD) and normal tissue complication probability (NTCP)] of volumetric modulated arc therapy (VMAT) plans and to develop a simple and swift method to predict the same online, on a daily basis.MethodsIn total, ten prostate patients treated with VMAT technology were selected for this study. The plans were generated using Eclipse TPS and delivered using Clinac ix LINAC equipped with a Millennium 120 multileaf collimator. In order to find the effect of systematic translational couch shifts on the DVH and radiobiological parameters, errors were introduced in the clinically accepted base plan with an increment of 1 mm and up to 5 mm from the iso-centre in both positive and negative directions of each of the three axis, x [right–left (R-L)], y [superior–inferior (S-I)] and z [anterior–posterior (A-P)]. The percentages of difference in these parameters (∆D, ∆TCP, ∆EUD and ∆NTCP) were analyzed between the base plan and the error introduced plans. DVHs of the base plan and the error plans were imported into the MATLAB software (R2013a) and an in-house MATLAB code was generated to find the best curve fitted polynomial functions for each point on the DVH, there by generating predicted DVH for planning target volume (PTV), clinical target volume (CTV) and organs at risks (OARs). Functions f(x, vj), f(y, vj) and f(z, vj) were found to represent the variation in the dose when there are couch translation shifts in R-L, S-I and A-P directions, respectively. The validation of this method was done by introducing daily couch shifts and comparing the treatment planning system (TPS) generated DVHs and radiobiological parameters with MATLAB code predicted parameters.ResultsIt was noted that the variations in the dose to the CTV, due to both systematic and random shifts, were very small. For CTV and PTV, the maximum variations in both DVH and radiobiological parameters were observed in the S-I direction than in the A-P or R-L directions. ∆V70 Gy and ∆V60 Gy of the bladder varied more due to S-I shift whereas, ∆V40 Gy, ∆EUD and ∆NTCP varied due to A-P shifts. All the parameters in rectum were most affected by the A-P shifts than the shifts in other two directions. The maximum percentage of deviation between the TPS calculated and MATLAB predicted DVHs of plans were calculated for targets and OARs and were found to be less than 0·5%.ConclusionThe variations in the parameters depend upon the direction and magnitude of the shift. The DVH curves generated by the TPS and predicted by the MATLAB showed good correlation.