SU-E-T-468: A Novel Complexity Index to Evaluate IMRT QA

Abstract

Purpose: To establish a quantitative complexity measure for IMRT plans. Methods: We propose a quantitative complexity index (CI) of IMRT plans that considers the dose variations within each IMRT field and compares to dose variations uniform rectangular fields of comparable size. The ratio of area to perimeter of a 1‐D dose profile is a measure of complexity, as the complexity of dose profile increases, this ratio would decrease as the perimeter would increase for a given area. This ratio would be greatest for an open field. By normalizing ratio of an IMRT field to that of uniform field of similar size, the variation of this ratio with field size would be eliminated. The complexity index of a plan would be weighted average of complexity indices of its individual fields. A graph of gamma versus CI would guide the clinical physicist in accepting an IMRT QA in a more rational manner. The calculated dose distributions were used to calculate the CI. We exposed EDR films in solid phantom to uniform and intensity modulated fields using 6 MV photons. The films were normal to CAX of beams. Results: The gamma analyses of these films were performed using 3% dose, 3mm DTA criteria with a 10% dose threshold. Some of the uniform and the modulated fields were measured and analyzed repeatedly to evaluate the random uncertainty. We noticed a gradual decrease in the percent of pixels with gamma values ≤1 with increasing complexity — from about 94% for large uniform fields to about 83% for modulated fields. The decrease was significant compared to the random uncertainty. Small open fields (4×4) had lower % pixel with gamma ≤1 than larger fields (10×10) (88%). Conclusion: A quantitative complexity analysis was established and would be a useful tool in evaluating gamma analyses of IMRT QA.