Dosimetric evaluation of density override methods in target volumes for VMAT technique in lung SBRT plans

Objective: This study aimed to investigate the impact of different density override methods applied to target volumes on the quality and dosimetric accuracy of lung SBRT plans using the VMAT technique, through experiments with a moving phantom. Method: At the Department of Radiation Oncology, Akdeniz University, a custom-moving phantom was constructed using tissue-equivalent materials mimicking lung and tumor densities, capable of simulating respiratory motion. Both three-dimensional and four-dimensional computed tomography scans of the phantom were acquired. Target volumes were contoured on these images, and various density override strategies were applied. Based on both standard and density overridden images, VMAT plans were generated. Dosimetric measurements were performed using the moving phantom in conjunction with radiochromic films, and film-based results were compared with planning system calculations for each method. Results: The PTVP method resulted in the lowest maximum dose within the target, minimizing hot spots and ensuring a more homogeneous dose distribution compared to other methods. Regarding plan efficiency, the PTVP method was notable for yielding the lowest monitor units and shortest beam on times. Gamma analysis of fixed film measurements indicated that all plans achieved a passing rate exceeding 94.4% under the 3 mm/3% dose difference criteria, suggesting high agreement between measurement and calculation. Dose profiles obtained from moving film measurements demonstrated that, for both the AveCT and all electron override methods (ITVP, HP, PTVP), the measured dose to the GTV was consistently higher than predicted in the treatment planning system. Conclusion: The results obtained from this thesis study have revealed that, in lung SBRT plans performed using the VMAT technique, especially the PTVP density override method, can ensure the GTV receives sufficient dose while providing a reduction in healthy lung irradiation, increasing the quality of treatment plans, and shortening the treatment time.