Accurate Dosimetry for Ocular Brachytherapy: Measurement, Delivery Uncertainty, and Dose Calculation Studies

• Author / Creator

  Morrison, Hali A.E.K.

• Ocular brachytherapy has been found to be an excellent alternative for the treatment of ocular melanomas compared to the predominantly used treatment prior to the 1980s of enucleation. Tumour control rates are generally >90%, overall survival rates are equivalent to enucleation, and ocular brachytherapy offers the major benefit of globe preservation and for many patients, vision retention. There are however relatively high rates of long-term toxicity and radiation side-effects which can affect the visual outcomes for patients receiving this treatment. Current clinical guidelines for ocular brachytherapy given by the American Association of Physicists in Medicine (AAPM) Task Group (TG)-129 and American Brachytherapy Society Ophthalmic Oncology Task Force (ABS-OOTF) reports include recommendations for treatment dose rate, dose prescription, irradiation duration, plaque size selection, radionuclide selection, and treatment planning/dose calculation methods. There are however a number of limitations in the current dosimetry formalism. This work makes improvements to three areas of ocular brachytherapy dosimetry including experimental measurement methodology, treatment delivery, and dose calculation, all in a concerted effort to improve the accuracy and quality of clinical ocular brachytherapy. This in turn is expected to improve the comparison of different treatment protocols, provide more accurate experimental treatment verification, and lead to a better understanding of the current limitations and sources of uncertainty associated with this treatment. Experimental dosimetry methods using EBT3 Gafchromic film were developed and specifically optimized for low-dose-rate and low-energy brachytherapy sources, which reduced the uncertainty in low-energy brachytherapy dose measurements by more than a factor of two. This enabled high accuracy measurements to be performed with two different styles of eye plaques loaded with I-125 sources (16 mm COMS and Super9 Eye Physics plaques). Comparisons were made with the Plaque Simulator treatment planning system (TPS), which performs water-based dose calculations with corrections for the heterogeneous plaque materials, and with the Monte Carlo (MC) package MCNP6. The results of this work provided additional computational and experimental validation of Plaque Simulator within the central region of the plaque where agreements were very good. However, larger differences at off-axis positions and in some of the EBT3 film measurements indicated areas of uncertainty present in the treatment planning calculations and also in treatment delivery. These uncertainties as well as others present in the full treatment process were comprehensively analyzed to determine the total treatment uncertainty at the prescription point (tumour apex) for the two plaques studied. Total uncertainty at the tumour apex is on the order of 15% (k=2), indicating that the tumour could potentially be under-dosed by a corresponding amount. Some centres apply an apex margin to account for tumour height uncertainty, but the size of an appropriate dosimetric margin has not been previously determined. Our analysis provides an uncertainty-based rational approach to determining the margin size required to ensure adequate dose coverage of the tumour apex, which ranged from 1.2 to 1.8 mm. The application of this dosimetric margin can potentially reduce the risk of local failure due to insufficient radiation dose. The current dose calculation formalism for brachytherapy assumes an infinite homogeneous water medium, which has been shown to be inaccurate for many types of treatments, including ocular brachytherapy. With the goal of performing fully patient specific dose calculations accounting for both heterogeneous plaque and patient tissues, I-125 source data and COMS eye plaques were for the first time adapted for implementation in a newly available TPS utilizing a collapsed-cone superposition dose calculation algorithm. The accuracy of the algorithm (trade named Advanced Collapsed cone Engine [ACE]) was evaluated for single I-125 seeds, for differently sized COMS plaques in water, and for two scenarios incorporating heterogeneous patient tissues: a voxelized eye phantom, and a patient CT dataset. Overall, ACE was found to agree well with MC simulations within the tumour and along the plaque central axis, however larger differences were found near the edge of the plaque lip and at tissue interfaces. Doses calculated with ACE were consistently closer to MC doses than those determined by Plaque Simulator, which ignores the effect of heterogeneous patient tissues. This work establishes a foundation to perform model based dose calculations for ocular brachytherapy in clinical practice.

• Subjects / Keywords

  • Model-based dose calculations
  • Ocular brachytherapy
  • Uncertainty analysis
  • Monte Carlo
  • Dosimetry
  • Radiochromic film

• Graduation date

  Fall 2017

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3SB3XC4M

• 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.