AbstractID: 9401 Title: Seed- and needle- optimization for brachytherapy using greedy heuristic
and adjoint functions
We have developed an efficient brachytherapy treatment-planning algorithm based on region of interest (ROI) adjoint functions and a greedy
heuristic (GH). The goal of the treatment plan is to find a seed- and needle- configuration that delivers a desired dose to the target while sparing
critical structures. For this work, the adjoint function of an ROI is defined as the sensitivity of the dose in the ROI to a unit strength seed at any seed
position. This sensitivity attaches a physical significance to a seed position associated with the ROI, and distinguishes our method from other
optimization methods. We define the adjoint ratio as the sum of the critical structures’ adjoint functions divided by the target adjoint function. This
adjoint ratio provides an intuitive means of selecting seed positions for seed placement. This ratio is computed once for each seed position prior to
the optimization process and seed positions are ranked according to their ratio value.
The GH searches a defined search space for highly ranked positions at each step. Seeds within the search space defined by an isodose surface
constraint are selected up until the preset number of needles is reached. Thereafter the isodose surface constraint together with the needle
configuration restricts the search space for further seed selections. The GH results are compared to those from the branch-and-bound method. Both
methods produced similar quality treatment plans, however the GH required only 4 seconds whereas the BB required 1 hour and 40 minutes of
computation time.