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Dosimetric Verification of Radiation Therapy Treatments using Monte Carlo Method

par Alhamada, Husein
Président du jury Labeau, Pierre-Etienne
Promoteur Pauly, Nicolas
Co-Promoteur Simon, Stéphane
Publication Non publié, 2022-10-27

Thèse de doctorat

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Résumé :

Radiotherapy is one of cancer treatment modalities, using ionization radiation to kill the cancer cells. The precise calculation of the delivered dose to the tumor tissue and healthy tissues is a vital step for clinical outcome of the radiotherapy. Monte Carlo (MC) is versatile and accurate algorithm used for dose calculation in the radiotherapy field. The objective of this dissertation is to compare the dose measurement and dose calculation with MC for different linear accelerators.This dissertation consists of six chapters, it starts with a general introduction which include basics of radiation, dose calculation algorithms, introduction of external radiotherapy, and intraoperative radiation therapy. The dosimetric impact of the shielding disk used during intraoperative electron radiation therapy(IOERT) is investigated. Different designs are currently used in clinical practice, the study includes four shielding disk types: Aluminum (Al)/Lead (Pb), PMMA/Copper (Cu)/PMMA, Aluminum (Al)/Copper (Cu) and Aluminum (Al)/Steel with their specific thicknesses. The theoretical study was conducted using the EGSnrc Monte Carlo (MC) code. On the other hand, the measurements were carried out with Gafchromic films for the four shielding disks for the same setup inside the water phantom. Finally, a comparison of the simulated and measured PDD curves was performed. This part of the dissertation concludes that the dose values above and under the shielding disks were acceptable for the four studied shielding types. It demonstrates that it is possible to use any of them clinically, while the best shielding disk was the Al/Pb since it has minimum thickness and a small backscatter enhancement. The misplacement positions of the shielding disk have dosimetric impacts in the treatment volume and a small translation could have a significant influence on healthy tissues. The chapter three includes a study of the dosimetric characteristics of four possible clinical positioning scenarios of the shielding disk. A new alignment mechanism of the shielding disk with the applicator was used.Part of this dissertation is illustrated the feasibility of using Monte Carlo (MC) for treatment planning of IOERT procedure to get 3D dosimetry by using patient’s CT images. It is a step forward for IOERT dosimetry and allows to obtain accurate dosimetry of treated volumes. The accuracy of treatment plans calculations of stereotactic body radiation therapy (SBRT) and the feasibility of using MC as quality control on daily basis was studied. The conclusions of chapter five were that SBRT treatment plans of MonacoTM TPS agreed with MC results, and that it was possible to use MC for 40 treatment plans verifications as independent QC tool.Finally, a study is done to illustrate the difference between the dose to water (Dw) and the dose to medium (Dm), the TPSs calculation is compared with the results of MC. The final part of dissertation confirms that Dw is overestimated in the bone by XVMC, and Acuros XB algorithms. In other media such as lungs, air or water, there is no differences between Dw and Dm. There is no a precise conversion factor which could be used to get Dw from Dm. It is more precise to calculate Dw using MC with various water densities.