Dosimetric impact of organ motion with 4D-CT based treatment planning in lung stereotactic ablative radiotherapy
Creators: name: Tajaldeen, Abdulrahman
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 320222
subject: Radiology and organ imaging
classificationCode: 321110
subject: Radiation therapy

description: Stereotactic ablative radiotherapy (SABR) plays a major role in the treatment of lung cancer. Advances in external beam radiotherapy, such as three-dimensional conformal radiotherapy (3D CRT), intensity modulated radiation Therapy (IMRT) and volumetric modulated arc therapy (VMAT) tightly conform dose to the target volume, in turn reducing dose to the surrounding critical structures. Organ motion and setup error are two important parameters that have significant effects on the final treatment outcome in lung SABR. The effect of organ motion has a greater effect on the dose to the tumour volume that is prone to movement due to respiration. Lung cancer is one such site where the position of the tumour volume is significantly affected with respiration.<br>Several methods have been proposed to combat tumour movement in lung cancer radiotherapy. The most common and widely followed method is to define a Maximum Intensity Projection (MIP) based tumour volume obtained from a series of CT images scanned at regular respiratory phases. The MIP based target volume encompasses the movement of tumour volume during four dimensional computed tomography (4D-CT) imaging and a treatment plan is generated based on this volume. One of drawbacks with this methodology is the inclusion of normal tissues as part of the target volume. The volume of normal tissue included as part of the MIP volume increases with increase in tumour volume movement. The MIP based volume includes low density areas but it is treated as an Internal Target Volume (ITV) and the calculation is based on Average Intensity Projection (AIP) images.<br><br>Besides the organ motion challenge in treating the lung SABR, dose calculation is compromised due to the presence of low-density lung tissues surrounding the thoracic tumours [1, 2]. Lower lung densities give rise to higher doses inside the lung, and hence there is a possibility of under-dosage in the periphery of the tumour when using small fields and high-energy beams [1, 2]. The use of small-beam field sizes in the SABR technique with the presence of low density in the lung tissue can lead to exacerbating the charged particle disequilibrium (CPD) condition, where the electrons increase significantly [1, 2].<br><br>The three main aims of this thesis are as follows:<br>The first aim:<br>Quantify the dosimetric impact of organ motion during the treatment of lung SABR utilising an in-house custom-designed thorax dynamic phantom with the PRESAGE 3D dosimeters. This thesis also explores the dosimetric differences between the MIP based planning (conventional method) and the phase specific planning methods.<br><br>The second aim:<br>Investigate the accuracy of different dose calculation algorithms in SABR of lung using phantoms and retrospective clinical lung cases. This thesis includes a study on the dosimetric variation among three SABR techniques: 3D CRT, IMRT and VMAT using most common dose calculation algorithms anisotropic analytic algorithms (AAA) and Acuros XB.<br><br>The third aim:<br>Introduce a method for patient-specific quality assurance (QA) for SABR treatment plans as a means to replace the traditional film dosimetry.<br><br>These studies will be of great values to the radiation therapy centres by improving the simulation/imaging, treatment planning, delivery planning and patient specific QA in SABR of lung.