REAL-TIME 3D DOSE CALCULATION AND DISPLAY - A TOOL FOR PLAN OPTIMIZATION

Purpose: Both human and computer optimization of treatment plans have advantages; humans are much better at global pattern recognition, and computers are much better at detailed calculations. A major impediment to human optimization of treatment plans by manipulation of beam para meters is the long time required for feedback to the operator on the e ffectiveness of a change in beam parameters. Our goal was to create a real-time dose calculation and display system that provides the planne r with immediate (fraction of a second) feedback with displays of thre e-dimensional (3D) isodose surfaces, digitally reconstructed radiograp hs (DRRs), dose-volume histograms, and/or a figure of merit (FOM) (i.e ., a single value plan score function). This will allow the experience d treatment planner to optimize a plan by adjusting beam parameters ba sed on a direct indication of plan effectiveness, the FOM value, and t o use 3D display of target, critical organs, DRRs, and isodose contour s to guide changes aimed at improving the FOM value. Methods and Mater ials: We use computer platforms that contain easily utilized parallel processors and very tight coupling between calculation and display. We ported code running on a network of two workstations and an array of transputers to a single multiprocessor workstation. Our current high-p erformance graphics workstation contains four 150-MHz processors that can be readily used in a shared-memory multithreaded calculation. Resu lts: When a 10 x 10-cm beam is moved, using an 8-mm dose grid, the ful l 3D dose matrix is recalculated using a Bentley-Milan-type dose calcu lation algorithm, and the 3D dose surface display is then updated, all in < 0.1 s. A 64 x 64-pixel DRR calculation can be performed in < 0.1 s. Other features, such as automated aperture calculation, are still required to make real-time feedback practical for clinical use. Conclu sion: We demonstrate that real-time plan optimization using general pu rpose multiprocessor workstations is a practical goal. Parallel proces sing technology provides this capability for 3D planning systems, and when combined with objective plan ranking algorithms should prove effe ctive for optimizing 3D conformal radiation therapy. Compared to our e arlier transputer work, multiprocessor workstations are more easily pr ogrammed, making software development costs more reasonable compared w ith uniprocessor development costs. How the dose calculation is partit ioned into parallel tasks on a multiprocessor work station can make a significant difference in performance. Shared-memory multiprocessor wo rkstations are our first choice for future work, because they require minimum programming effort and continue to be driven to higher perform ance by competition in the workstation arena.