SU-E-T-212: Influence of the Modulation Index On Daily Quality Assurance in Rapid Arc Treatments

Purpose: At our Institute some measured parameters for daily quality assurance (DQA) of dynamic arc therapy plans showed an unexpected behavior, therefore an investigation of the influence of the magnitude of modulation was conducted. Methods: In our clinical practice all DQAs of dynamic arc therapy plans are measured and analyzed prior to treatments using commercial software. For this study these plans were additionally exported to our in-house software written in MATLAB.The developed software extracted the leaf position, gantry angle, cumulative meterset weight of each control point (CP) and the total number of Monitor Units (MU) of each arc. Based on this information we calculated the leaf travel distance, irradiated segment area, number of MUs and dose rate for each CP. These data allowed us to calculate the modulation indexes (MI) of the plans, applying five different definitions of MI. The results were then correlated to the results of our DQA measurements.To validate the software, additional plans of known MIs were created and analyzed. For confirmation, the calculated parameters were compared to the segmented treatment table (STT) coming from the treatment planning system. Results: All calculated CP-parameters matched the STT by 99% or better. For linac 1, the comparison of the MI evaluation and the DQA results showed a slight tendency: 91.3% failed DQA plans had a MI lower than the average value. For this definition we consider that the lower the MI the higher the modulation. The results of the linac 2 present no significant relevance due to the low sample sizes for each DQA software. Conclusion: Available data and given definitions of the modulation index do not bring conclusive results; one cannot find a clear and distinct correlation with the failure of the DQA. The ongoing analysis with an increased sample size might lead to another conclusion.