Imaging and radiation therapy: Gate Monte Carlo simulation of a MV-CBCT flat panel with specific application in head and neck cancer

Abstract

Purpose In radiotherapy treatment, it is necessary to position the patient before each session to ensure that the beams target the tumor while sparing the surrounding healthy tissue. This step is made possible via different imaging techniques like the ‘‘electronic portal imaging device” (EPID) which is the most common approach. In this configuration, the imaging source is the same as the treatment source, while a flat panel detector is automatically placed under the patient via an articulated arm. In this context, our study was focused on the Monte Carlo modeling of such an EPID in order to simulate both planar and 3D CBCT images. Methods The work presented here follows a preliminary study in which the treatment part of a clinical LINAC (Siemens Oncor) was modeled in GATE v6.2. The objective was to calculate the dose delivered by complex treatments like IMRT. The objective of the study is to extend this model by incorporating the imaging part of the LINAC, which consists of a flat panel detector (Perkin Elmer XRD1640). For this purpose, geometrical and physical data provided by the manufacturer were first accurately modeled in GATE. Assessments of portal imaging were performed by performing dose calculations in a CQ IMRT phantom with GATE for two measurement points with protocols of 8, 15 and 60 UM and comparing the doses with clinical measurements. And with performing 2D CT projections over a 200° arc, 1° increments, and 41 cm × 41 cm field of view for homogeneous and heterogeneous structures (introduction of a patient head and neck CT patient). Results For the calculation of the deposit dose at the isocentre of the CQ IMRT phantom, respectively GATE gave doses of 5.734 ± 0.169, 11.008 ± 0.777 and 46.006 ± 0.711 cGy compared with measurements are: 6.821, 12,469 and 49.600 cGy protocols for 8, 15 and 60 UM. From the obtained series of 2D projections a 3D reconstruction was performed in each case, and the dimensions of the reconstructed structures were compared with actual objects. Conclusion We have successfully developed within the GATE platform a complete and detailed model of a 160 MLC LINAC with its MV-CBCT flat panel. Future work will consist in simulating epidbased transit dosimetry.