Implementation of a radiotherapy treatment verification tool based on Epid and Monte Carlo simulations

Due to the complexity of modern radiotherapy delivery techniques, patient-specific pre-treatment verification tools are needed. The use electronic portal imaging devices EPID for this purpose has the potential to reduce the amount of human and equipment resources needed for this purpose. The aim of this study is to develop a pre-treatment dose verification tool for the ELEKTA AXESSE^TM linac at UC-CHRISTUS Cancer Center, using the EPID attached to the accelerator and the Monte Carlo method, for the verification of intensity-modulated radiotherapy treatments (IMRT). The implementation was carried out in two stages. First, the energy fluence of the plan to be verified is obtained using MATLAB®. The second part corresponds to the dose reconstruction by the EGSnrc system. This part requires the generation of a phase space file, derived from the previously obtained fluence, to be used as input for the dose calculation. To obtain the energy fluence several steps are followed. The portal images of the plan to be verified are acquired, normalized and corrected for different effects: panel detector sensitivity and horns profile, and transmission in the multileaf collimators (MLC). The corrected image is then calibrated to portal dose. After that, energy fluence is obtained by means of a deconvolution with a kernel. This kernel is composed of a linear combination of exponentials. Once the fluence distribution was available, the space and energy spectrum were sampled with 2 × 107 particles to generate a phase space file. The latter was the used as an EGSnrc input for the Monte Carlo dose reconstruction. The dose reconstruction was performed using the DOSXYZnrc routine, with the phase space file previously generated with MATLAB®, the dose reconstruction is done with 2 × 108 histories. The reconstruction was performed for different conditions, initially, we xix work with square fields on analytical cubic phantoms. Then, we tested the tool with an IMRT plan of seven overlapping fields on phantoms created from CT data from the PTW octavius detector and a patient. The reconstructed relative dose was verified for the square fields and the IMRT plan on the CT of the octavius phantom, using dose difference and the gamma index with individual criteria of 3 mm/3%. A gamma index passing rate of 96.9% was obtained for the 2D evaluation of the IMRT plan. Aditionally, a graphical interface was implemented to obtain the energy fluence in a phase space file and perform the gamma evaluation. The pre-treatment dose verification tool was succesfully implemented and proved to be useful for the evaluation of 2D relative dose distributions in homogeneous and heterogeneous media. Further measurements and developments will be required to calibrated and fine tune the reconstructions, as well as to incorporate new evaluation features