3D VMAT Verification Based on Monte Carlo Log File Simulation with Experimental Feedback from Film Dosimetry

Abstract

A model based on a specific phantom, called QuAArC, has been designed for the evaluation of planning and verification systems of complex radiotherapy treatments, such as volumetric modulated arc therapy (VMAT). This model uses the high accuracy provided by the Monte Carlo (MC) simulation of log files and allows the experimental feedback from the high spatial resolution of films hosted in QuAArC. This cylindrical phantom was specifically designed to host films rolled at different radial distances able to take into account the entrance fluence and the 3D dose distribution. Ionization chamber measurements are also included in the feedback process for absolute dose considerations. In this way, automated MC simulation of treatment log files is implemented to calculate the actual delivery geometries, while the monitor units are experimentally adjusted to reconstruct the dose-volume histogram (DVH) on the patient CT. Prostate and head and neck clinical cases, previously planned with Monaco and Pinnacle treatment planning systems and verified with two different commercial systems (Delta4 and COMPASS), were selected in order to test operational feasibility of the proposed model. The proper operation of the feedback procedure was proved through the achieved high agreement between reconstructed dose distributions and the film measurements (global gamma passing rates > 90% for the 2%/2 mm criteria). The necessary discretization level of the log file for dose calculation and the potential mismatching between calculated control points and detection grid in the verification process were discussed. Besides the effect of dose calculation accuracy of the analytic algorithm implemented in treatment planning systems for a dynamic technique, it was discussed the importance of the detection density level and its location in VMAT specific phantom to obtain a more reliable DVH in the patient CT. The proposed model also showed enough robustness and efficiency to be considered as a pre-treatment VMAT verification system.

Fig 1. QuAArC phantom.

Different setups of QuAArC phantom prototype (top) and the final PMMA phantom (bottom). On the top, the big setup is depicted with radiochromic films rolled at two different radial distances. The independent inside part of the phantom is shown on the bottom, and constitutes the small setup.

Fig 2. Flowchart describing the proposed model.

Film measurements in QuAArC phantom are implemented in a feedback process in order to adjust the MUs from the log file for reweighting the full MC simulated CPs to obtain the final reconstructed DVH on patient CT.

Fig 3. Effects of considering a different discretization level from the log files.

Percent dose difference matrices of the inner film scroll versus MC Log (left) and versus the corresponding QuAArC solution (right) for coarse discretization (top) and fine discretization (bottom), corresponding to prostate VMAT plan B.

Fig 4. QuAArC system reports for fine and coarse discretization.

Comparison between original and experimentally adjusted solutions for both discretizations. Angular MU distributions from the original log file MC simulation and the experimentally adjusted with QuAArC (a) and the corresponding MU differences for coarse discretization (b). The same for fine discretization (d and e). DVHs comparison between Monaco TPS solution, MC log file simulation and QuAArC reconstructed solution, for coarse (c) and fine discretization (f). All for prostate VMAT plan B.

Fig 5. Proof of concept of the experimental feedback process for the prostate case from Monaco.

Film and QuAArC dose distributions, corresponding percent dose differences and gamma analysis are shown for outer and inner scrolls for VMAT Plan A (first and second rows, respectively), and the same for VMAT Plan B (third and fourth rows).

Fig 6. Proof of concept of the experimental feedback process for the H&N case from Monaco.

Film and QuAArC dose distributions, corresponding percent dose differences and gamma analysis are shown for outer and inner scrolls of VMAT Plan A (first and second rows, respectively), and the same for IMRT Plan B (third and fourth rows).

Fig 7. Proof of concept of the experimental feedback process for the prostate and H&N VMAT plans from Pinnacle.

Film and QuAArC dose distributions, corresponding percent dose differences and gamma analysis are shown for outer and inner scrolls.

Fig 8. DVHs comparison between Monaco TPS solution and QuAArC reconstructed solution.

VMAT plan A (a) and B (b), for the Prostate case and VMAT plan A (c) and IMRT plan B (d), for the H&N case.

Fig 9. DVHs comparison between Pinnacle TPS solution and QuAArC reconstructed solution.

Prostate VMAT plan (left) and H&N VMAT plan (right).