Room: Exhibit Hall | Forum 7
Purpose: VMAT collimator angle selection for radiation therapy treatments is largely based upon historical data. This algorithm looks to incorporate patient target structure contours to optimize collimator angles for each control point in a VMAT arc.
Methods: Contoured structures from a previously treated prostate patient were exported from Eclipse and analyzed in Matlab. A 3D point-cloud was extracted for the target structure and subsequently projected to create a 2D surface from the beamâ€™s eye view for a gantry angle of interest. The 2D surface was rotated to simulate collimator rotation and at each degree increment two Riemann sums were calculated: a fine trapezoidal sum and a rectangle sum with intervals that match the projected leaf width. The superior fit of the trapezoidal sum provided a best-case scenario: where the target coverage is unimpeded by the MLC leaves and the normal tissue is blocked. For each collimator angle, a score was calculated by taking the absolute difference between the trapezoidal and the rectangle sum.
Results: With scores calculated for each collimator angle at a given gantry angle, an optimal collimator angle can be provided for each control point within a VMAT arc. The global collimator angle that matched the most control points was 45-degrees, which matches the first arc of the original Eclipse plan for the patient. The optimization weight for 45-degrees was boosted by the fact that roughly 40% of control points only had 45 degrees as a potential solution; the other 60% of control points had multiple collimator angles that performed positively. Organs at risk will need to be considered in future iterations of the algorithm.
Conclusion: This study demonstrates initial viability of a geometric tool for collimator optimization and outlines required areas of future development. Additional patient contours need to be tested to produce results with improved scope.