Comparison of calculation algorithms to predict the IQM detector response for various modulation degrees of VMAT treatment plans on linear accelerator equipped with the HD120 MLC

Purpose The calculation model for the integral quality monitor (IQM) system does not take into account the characteristics of the HD120 multileaf collimator (MLC), which some Varian accelerators are equipped with. Some treatment plans prepared with this collimator are characterized by a high level of modulation. The aim of the work was to prepare a model for that collimator and to determine the influence of modulation on the results of the verification carried out with the use of the IQM system. Methods The short- and long-stability of the IQM detector response were verified by measuring the signal for a 6 MV FFF beam with the static field of 10 × 10 cm2 size. The obtained results were compared with the measurements performed with the PTW Farmer chamber. Next, the signal for 35 static square fields 4 × 4 cm2 , covering the whole field 38 × 20 cm2 , were measured with the IQM. Based on the results of these measurements, the original calculation model has been changed, in order to achieve the smallest differences between calculations and measurements. While tuning the model, the characteristics of the HD120 MLC were included. Measurements were performed for 30 clinical plans (86 arcs) prepared with 6 MV FFF beams. Among those 30 plans, there were 5 multi-target plans with single isocenter. For each plan the modulation complexity score (MCS) was calculated. The measurement results were compared with the calculation results performed with the original and authors' calculation model. Results Very good stability of the short- and long-stability of the IQM detector response were obtained. Measurements performed for 35 static fields revealed that for the manufacturer's and for the authors' models the deviation exceeded 3% for 12 and 5 of the 35 static fields respectively. The differences for the manufacturer's and authors' algorithms were in the range of ±2% for the 15 and 26 of the fields respectively. For original and the authors' models the differences between measured and calculated signals (starting with the segment number 40) were within the range of ±3.5% for 87.6% and 96.7% of all arcs for the respective models. For both models the dependence of the compliance of measurements and calculations on the modulation complexity score was observed. For most of the very modulated arcs the measured signal was at least 3% lower than the calculated one. The largest differences between measurements and calculations were obtained for single isocenter multi-target plans. Conclusions The signal predicted by an algorithm taking into account the real geometry of the collimating system of the Edge accelerator (equipped with the HD120 MLC) made it possible to obtain greater consistency between the measurements and calculations. We characterized the dependence between the modulation complexity score of each arc and the compliance of the measurements and calculations. Much worse results were obtained for single isocenter multi-target plans. This article is protected by copyright. All rights reserved.