Optimal Selection of Plane and Alignment Based On Quantitative Evaluation Using Three-Dimensional (3D) Dose Data for IMRT and VMAT Dosimetry
H Tachibana1, 3*, R Takahashi2, T Umeda3, (1) UT Southwestern Medical Center, DALLAS, TX, USA (2) Cancer Institute Hospital of JFCR, KOTO, TOKYO, JAPAN (3) Kisatato University of Graduate School of Medical Sciences, SAGAMIHARA, KANAGAWA, JAPANSU-E-T-372 Sunday 3:00:00 PM - 6:00:00 PM Room: Exhibit Hall
Purpose: Gamma, DTA and dose difference (DD) evaluations could take into account only the two axes on the evaluation plane for IMRT and VMAT plan verification dosimetry. However, some steep dose gradient region on another axis with the errors of measurement setup and the laser alignment can decrease the agreement between the calculation and measurement on the plane. In this study, we developed an optimal selection method of finding optimal plane of the evaluation and adequate alignment of the measurement data using three-dimensional dose data.
Methods: After the isocenter alignment of the measurement data manually, the optimal selection engine reconstructs dose planes at certain locations on the axis perpendicular to the evaluation plane from the 3D data of dose calculation. Then, the calculation planes are compared with the measurement plane using gamma evaluation in order to choose the candidates of the best plane of the evaluation. Using the candidates with the combinations with rotations and shifts, DD evaluation is conducted to find out the best plane and the best combination of shift and rotation. To assess the effectiveness of the method, a head and neck IMRT plan measurement was performed and the path ratios of gamma evaluation (3%/3mm criteria) were measured with optimal selection and non-optimal selection method, respectively.
Results: Average pass ratio of gamma evaluation with the optimal selection method was 14.2%-better than with the non-optimal selection method. Additionally, the method found 0.5mm- and 1.5mm-deviations in the two-axis on the evaluation plane, respectively. 1.0mm-deviation was also measured on the axis perpendicular to the plane.
Conclusions: IMRT and VMAT verification measurement with the method could provide us not only the dosimetric check of the plan with better result but also the mechanical test of isocenter laser alignment.