Permanent Prostate Brachytherapy Dosimetry: Critical Assessments and Advancements

• Author / Creator

  Liu, Derek MC

• The dissertation explores some of the limitations and assumptions in the current approach to permanent prostate brachytherapy (PPB) dosimetry. Where possible, novel insights and practical improvements are proposed, which aim to enhance the accuracy of radiation dose delivery during treatment. The thesis assesses prostate delineation accuracy during treatment planning, seeks to improve dose calculation by accounting for edema, and investigates previously unstudied effects due to probe-induced prostate deformation. Prostate delineation during treatment planning directly impacts dose delivery accuracy and conformity. Prostate contouring, commonly performed on transrectal ultrasound (TRUS) images, is compared to that on magnetic resonance (MR) images, which provide superior soft tissue contrast. Patients are imaged on both modalities and the delineated prostates are compared in terms of volume, shape, and observer contouring variability. The prostate volume and inter- and intra- observer variability, quantified by the volume and overlap, were similar between the imaging modalities. MR offered the potential for improved delineation at the prostate base and apex, regions known to be difficult to contour on ultrasound. However, more delineation experience on MR is likely necessary before the benefits can be realized. Systematic prostate volume differences attributable to the choice of reconstruction algorithm could be mistaken for contouring bias between the imaging modalities. The accuracy of the commonly used planimetry and frustum algorithms is evaluated in the context of TRUS image acquisition, using simulated contours representing geometrical objects with known volumes. For TRUS imaging characterized by a large inter-slice gap and non-random positioning of the first imaged slice, the planimetry algorithm slightly overestimated the prostate volume (by roughly 4 %) while the frustum algorithm underestimated the volume (by 3 %). Prostatic edema gives rise to a dynamic, correlated movement of the prostate and implanted seeds, resulting in deviations in the doses calculated at a single time point. The edema-related effects on planning and post-implant dosimetry are investigated using a clinically-informed edema model previously developed. For patient-average edema parameters, incorporating edema resulted in roughly 2 % lower dose and a small (3 Gy) reduction in prostate D90. The dosimetric differences were similar between patients and between planning and post-implant distributions. The effect of prostatic edema was therefore largely determined by the patient-specific edema response. The general solution to the problem of dose calculation incorporating edema is computationally intensive. A fast calculation method was previously reported based on dose kernel convolution using the Fourier transform. However, limitations on the seed placement resulted in unacceptable dose calculation errors. A novel method is proposed using Fourier-compatible kernel interpolation, expanding upon the original method and enabling unrestricted seed placement. The method substantially improved the clinically relevant dose accuracy with negligible additional computation cost. Prostate deformation due to the TRUS probe is generally not accounted for during treatment. Although the deformation is small, the expected impact on dosimetry has not been investigated and is usually assumed to be negligible. A novel investigation is performed to characterize and quantify the probe-induced seed movement and its impact on dosimetry. Implant movement patterns are observed, providing insight into modeling the underlying prostate deformation. Although the observed movements were generally small (mostly < 2 mm), the overall contraction of the implant distribution resulted in a non-negligible prostate D90 average increase of 4 Gy (range 0 to 8 Gy). The movement (up to 5 mm) of extra-prostatic seeds in the lateral peripheral regions had potential consequence for local target coverage. The study demonstrated that probe-induced deformation of the prostate is not always negligible as commonly assumed. In summary, the dissertation assesses the current approach to PPB dosimetry and introduces improvements pertaining to target delineation, volume calculation, dose calculation, and implant delivery. With further research and development these refinements could be implemented in the clinic, where they have the potential to improve patient treatment outcomes.

• Subjects / Keywords

  • Prostate Cancer
  • Brachytherapy
  • Dosimetry

• Graduation date

  Fall 2014

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R37S7J38F

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.