Interactively exploring optimized treatment plans

A new paradigm for treatment planning is proposed that embodies the concept of interactively exploring the space of optimized plans. In this approach, treatment planning ignores the details of individual plans and instead presents the physician with clinical summaries of sets of solutions to well-defined clinical goals in which every solution has been optimized in advance by computer algorithms.

Before interactive planning, sets of optimized plans are created for a variety of treatment delivery options and critical structure dose-volume constraints. Then, the dose-volume parameters of the optimized plans are fit to linear functions. These linear functions are used to show in real time how the target dose-volume histogram (DVH) changes as the DVHs of the critical structures are changed interactively. A bitmap of the space of optimized plans is used to restrict the feasible solutions. The physician selects the critical structure dose-volume constraints that give the desired dose to the planning target volume (PTV) and then those constraints are used to create the corresponding optimized plan.

The method is demonstrated using prototype software, Treatment Plan Explorer (TPEx), and a clinical example of a patient with a tumor in the right lung. For this example, the delivery options included 4 open beams, 12 open beams, 4 wedged beams, and 12 wedged beams. Beam directions and relative weights were optimized for a range of critical structure dose-volume constraints for the lungs and esophagus. Cord dose was restricted to 45 Gy. Using the interactive interface, the physician explored how the tumor dose changed as critical structure dose-volume constraints were tightened or relaxed and selected the best compromise for each delivery option. The corresponding treatment plans were calculated and compared with the linear parameterization presented to the physician in TPEx. The linear fits were best for the maximum PTV dose and worst for the minimum PTV dose. Based on the root-mean-square error between the fit values and their corresponding data values, the linear fit appears to be adequate, although higher order polynomials could give better results. Some of the variance in fit is due to the stochastic nature of the simulated annealing optimization algorithm, which does not reproduce the exact same results in repetitions of the same calculation. Using a directed search algorithm for plan optimization should produce better parameter fits and, therefore, better predictions of plan characteristics by TPEx.

Using TPEx, the physician can easily select the optimum plan for a patient, with no imposed arbitrary definition of the "best" plan. More importantly, the physician can readily see what can be achieved for the patient with a given delivery technique. There is no more uncertainty about whether or not a better plan exists. By comparing the "best" plans for different delivery options (e.g., three-dimensional conformal radiotherapy versus intensity-modulated radiation therapy), the physician can gauge the clinical benefits of greater technical complexity. However, before the TPEx process can be clinical useful, faster computers and/or algorithms are needed and more studies are needed to better model the spaces of optimized solutions.