Session Item

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 is created at the expense of increasing complexity of the treatment planning process. In this study column generation method is investigated for IMBT treatment plan optimization.

A column generation optimization algorithm is developed to optimize the dwell times and shield angles. At every iteration, the plan is optimized with the chosen dwell position and shield angle (DPSA) combinations and the DPSA that would best improve the cost function at every iteration is added to the plan. The optimization process can be stopped when the clinical plan evaluation criteria have been met to limit the plan complexity. In this work the optimization was stopped when no more DPSAs is expected to add value to the current plan.

The column generation approach yielded a near-optimal treatment plan with 387 DPSAs from 2080 possible DPSAs for a prostate case. The final iteration contained a plan with a complexity five times smaller than the original one. Figure 1 shows that the cost function converges to the asymptotic cost, which is the cost when all DPSAs are included in the optimization. Figure 2 illustrates the dosimetric indices of interest at each iteration. After some fluctuations D1cc and D2cc of the organs at risk (OAR) reach their maxima during the first 50 iterations, then decrease to stabilize after 100 iterations. With the D2cc of both the bladder and the rectum being below 10 Gy, and the D1cc of the urethra below 15 Gy, the dosimetric requirement for the OAR are satisfied while still having V100 > 15 Gy in the planning target volume.

The column generation method produced a high-quality deliverable prostate IMBT plan. The treatment plan quality eventually 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 dose volume histogram parameters satisfy their clinical requirements or if they have stabilized.