Quality assurance for high dose rate brachytherapy treatment planning optimization: using a simple optimization to verify a complex optimization

Applying the column generation method to the intensity modulated high dose rate brachytherapy inverse planning problem

Objective.Intensity modulated high dose rate brachytherapy (IMBT) is a rapidly developing application of brachytherapy where anisotropic dose distributions can be produced at each source dwell position. This technique is made possible by placing rotating metallic shields inside brachytherapy needles or catheters. By dynamically directing the radiation towards the tumours and away from the healthy tissues, a more conformal dose distribution can be obtained. The resulting treatment planning involves optimizing dwell position and shield angle (DPSA). The aim of this study was to investigate the column generation method for IMBT treatment plan optimization.Approach.A column generation optimization algorithm was developed to optimize the dwell times and shield angles. A retrospective study was performed on 10 prostate cases using RapidBrachyMCTPS. At every iteration, the plan was optimized with the chosen DPSA which would best improve the cost function that was added to the plan. The optimization process was stopped when the remaining DPSAs would not add value to the plan to limit the plan complexity.Main results.The average number of DPSAs and voxels were 2270 and 7997, respectively. The column generation approach yielded near-optimal treatment plans by using only 11% of available DPSAs on average in ten prostate cases. The coverage and organs at risk constraints passed in all ten cases.Significance.The column generation method produced high-quality deliverable prostate IMBT plans. The treatment plan quality reached a plateau, where adding more DPSAs had a minimal effect on dose volume histogram parameters. The iterative nature of the column generation method allows early termination of the treatment plan creation process as soon as the dosimetric indices from dose volume histogram satisfy the clinical requirements or if their values stabilize.

Monte Carlo simulation and dosimetry measurements of an experimental approach for in vitro HDR brachytherapy irradiation

Irradiation of tumor cell lines is a useful way to investigate the effects of ionizing radiation on biological molecules. We designed an easy and reproducible approach for in vitro experimental high dose rate brachytherapy, which was simulated by a Monte Carlo code and dosimetrically characterized by experimental methods to evaluate the correspondence between planned doses and doses absorbed by the cells. This approach is an acrylic platform containing T25 tissue culture flasks and multiwell tissue culture plates. It allows nine parallel needles carrying an ¹⁹²Ir source to irradiate the adherent cells. The whole system composed of the acrylic platform, tissue culture flasks and ¹⁹²Ir source tracking was simulated by the Monte Carlo N-Particle transport code (MCNPX). Dosimetric measurements were taken by well ionization chamber and radiochromic films. There was a slight difference, averaging from 2% to 7%, between the MCNPX results and film dosimetry results regarding uniform radiation created by the source arrangement. The results showed different values for planned and measured doses in each cell culture plate, which was attributed to the non-equivalent water material used and to the lack of full scattering coming from the top of the platform. This last contribution was different for each tissue culture plate and an individual dose correction factor was calculated. The dose correction factor must be applied to match the planned dose and the actual doses absorbed by the cells. The designed approach is an efficient tool for in vitro brachytherapy experiments for most commercial cell culture plates.

A new optimization algorithm for HDR brachytherapy that improves DVH-based planning: Truncated Conditional Value-at-Risk (TCVaR)

Purpose:To introduce a new optimization algorithm that improves DVH results and is designed for the type of heterogeneous dose distributions that occur in brachytherapy.Methods:The new optimization algorithm is based on a prior mathematical approach that uses mean doses of the DVH metric tails. The prior mean dose approach is referred to as conditional value-at-risk (CVaR), and unfortunately produces noticeably worse DVH metric results than gradient-based approaches. We have improved upon the CVaR approach, using the so-called Truncated CVaR (TCVaR), by excluding the hottest or coldest voxels in the structure from the calculations of the mean dose of the tail. Our approach applies an iterative sequence of convex approximations to improve the selection of the excluded voxels. Data Envelopment Analysis was used to quantify the sensitivity of TCVaR results to parameter choice and to compare the quality of a library of 256 TCVaR plans created for each of prostate, breast, and cervix treatment sites with commercially-generated plans.Results:In terms of traditional DVH metrics, TCVaR outperformed CVaR and the improvements increased monotonically as more iterations were used to identify and exclude the hottest/coldest voxels from the optimization problem. TCVaR also outperformed the Eclipse-Brachyvision TPS, with an improvement in PTVD95% (for equivalent organ-at-risk doses) of up to 5% (prostate), 3% (breast), and 1% (cervix).Conclusions:A novel optimization algorithm for HDR treatment planning produced plans with superior DVH metrics compared with a prior convex optimization algorithm as well as Eclipse-Brachyvision. The algorithm is computationally efficient and has potential applications as a primary optimization algorithm or quality assurance for existing optimization approaches.