Quality assurance in VMAT technique: Software tool for real-time control of the linac

Purpose In order to evaluate the quality of treatment plan in VMAT, we have developed software to view and compare the expected and delivered parameters. The purpose is to evaluate the differences in the ‘‘fluence” delivered in three situations: impact of a new MLC calibration, the complexity of the treatment plan and performance comparison between linac. Materials and method The linac parameters (Synergy with MLCi2, RTD 7.01, Elekta) have been recorded in real time (4 Hz) and saved in a text file. Using this file and the RT PLAN data (Pinnacle version 9.0, Philips), Matlab software enables to visualize the variations of linac parameters (leaves positions, UM, dose rate, gantry angle, etc.) during treatment delivery. The product area.UM is defined to estimate the difference in fluence generated by the linac. The difference at each control point is normalized to the average value of these parameters. The irradiation sequences with several degrees of complexity have been defined: a cylindrical target, a concave target (AAPM TG119) and clinical cases (prostate, head and neck). The study was repeated for three linac Synergy/Elekta. Results With the concave geometry, increasing the dose gradient of 4.27 cGy/mm, 4.90 cGy/mm and 6.10 cGy/mm causes deviations of 0.03% (SD = 4.77%), –0.50% (SD = 5.59%) and –0.74% (SD = 5.56%), respectively. The analysis before and after MLC calibration shows deviations of –0.12% (SD = 4.01%) before calibration and –0.10% (SD = 4.02%) after calibration. Concerning comparative performance study of different linac, it shows that average deviation for the cylindrical geometry varies from –0.10% to –0.41% and standard deviation ranged from 4.02% to 6.62% for three linac. Conclusion These results show the potential of the software to evaluate the control parameters of the linac during treatment delivery. The differences observed indicate that the complexity of the plan is related to differences in the fluence, due to increased constraints on the linac parameters. This quality assurance tool allows physicists to check the stability and the accuracy of the linac according to the constraints for the implementation of the plan VMAT.