Pre-treatment 2D and 3D dosimetric verification of volumetric arc therapy. A correlation study between gamma index passing rate and clinical dose volume histogram

OBJECTIVES: To evaluate methods for the pre-treatment verification of volumetric modulated arc therapy (VMAT) based on the percentage gamma passing rate (%GP) and its correlation and sensitivity with percentage dosimetric errors (%DE). METHODS: A total of 25 patients with prostate cancer and 15 with endometrial cancer were analysed. The %GP values of 2D and 3D verifications with different acceptance criteria (1%/1 mm, 2%/2 mm, and 3%/3 mm) were obtained using OmniPro and Compass. The %DE was calculated using a planned dose volume histogram (DVH) created in Monaco’s treatment planning system (TPS), which relates radiation dose to tissue and the patient’s predicted dose volume histogram in Compass. Statistical correlation between %GP and %DE was verified using Pearson’s correlation coefficient. Sensitivity was calculated based on the receiver operating characteristics (ROC) curve. Plans were calculated using Collapsed Cone Convolution and the Monte Carlo algorithm. RESULTS: The t-test results of the planned and estimated DVH showed that the mean values were comparable (P > 0.05). For the 3%/3 mm criterion, the average %GP was acceptable for the prostate and endometrial cancer groups, with average rates of 99.68 ± 0.49% and 99.03 ± 0.59% for 2D and 99.86 ± 0.39% and 99.53 ± 0.44% for 3D, respectively. The number of correlations was poor for all analysed data. The mean Pearson’s R-values for prostate and endometrial cancer were < 0.45 and < 0.43, respectively. The area under the ROC curve for the prostate and endometrial cancer groups, was lower than 0.667. CONCLUSIONS: Analysis of the %GP versus %DE values revealed only weak correlations between 2D and 3D verifications. DVH results obtained using the Compass system will be helpful in confirming that the analysed plans respect dosimetric constraints.