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UBC Theses and Dissertations

Adaptive radiotherapy treatment corrections to account for patient-specific systematic soft tissue deformations: prostate, lung, and head and neck cancer Sonier, Marcus


Soft tissue deformations during radiotherapy impact patient dosimetry and treatment accuracy. Systematic or patient-specific deformations may occur in any body region. This dissertation focuses on clinically–relevant deformations over the course of treatment that have not been extensively addressed: 1) effect of the Transrectal Ultrasound (TRUS) probe deformation on the prostate during brachytherapy, 2) motion induced CT reconstruction error for small targets with a synthetic lung model for patient-specific breathing in lung external beam radiotherapy, and 3) weight loss and tumor debulking during treatment in head & neck radiotherapy with clinical solutions. Influence of the TRUS probe on the prostate results in displacement of implanted radioactive seeds post-implant with a change in target and organ doses. Shifts in seed positions showed that motion of prostate post-implant occurred in 3-dimensions, was non-uniform, and dependent on prostate region, tending in the superior and posterior directions. Comparisons between the in-vivo measured microMOSFET doses with the Variseed calculated doses yielded a mean and standard deviation of -5.0±25.2%. When considering the TRUS probe’s effect on urethral dose, stratifying patients into those with and without periurethral seeds on the pre-plan resulted in mean and standard deviations of 12.8±10.0% and -13.8±12.9%, an increase and decrease in urethral dose respectively (p=0.0006). The geometric accuracy of lung radiotherapy is affected by 4DCT image quality and requires quantification of the effect of AP motion reconstruction artifacts on object shape/volume for various breathing patterns and to provide treatment planning recommendations for target sizes below a minimum threshold. Target reconstruction is most accurately represented on the exhale phase and was found to be the most reliable for target contouring when the range of motion exceeded 3x the tumor diameter. A 3D-printed patient-specific breathing phantom will aid in visualizing errors in small SBRT targets to enable highly accurate treatment. Contour changes in head and neck radiotherapy results in bolus gaps affecting dosimetric coverage of superficial targets. Determining the acceptable gap before replanning a patient's treatment is subjective and patient-specific. Dosimetric VMAT measurements illustrate replanning should be considered at gap widths >6mm to maintain treatment integrity. Alternatively, 3D-printing missing tissue is one solution.

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