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 measure-
ments (global gamma passing rates > 90% for the 2%/2 mm criteria). The necessary discre-
tization 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.Ministerio de Ciencia y Tecnología SAF2011- 27116; IPT-2011-1480-900000