Quality control of linac in daily volumetric-modulated arc therapy

Abstract

Objective To verify the daily delivery accuracy of volumetric-modulated arc therapy(VMAT) and achieve the process quality control (PQC) of linac using statistical process control (SPC)technology. Methods The log files of all treatments were taken out from the linac system. An in-house software which created by Matlab 7.14 was used to analyze the daily parameters accuracy and the mechanism and dose delivery accuracy of the linac for each VMAT, and an daily evaluation report was automatically created for the linac. The stability of the linac and the deviation of the delivery accuracy for various cancer sites were also analyzed. To achieve the process quality control of linac using SPC technology, the control limit of each parameter was calculated by Johnson alternation and the single value control charts were drawn. Results 76 patients mainly with nasopharyngeal carcinoma, cervical carcinoma, rectal cancer and laryngeal cancer, got the treatment daily with VMAT technology, and a total of 2 446 arcs were delivered by the linac in continuous 16 days. The gantry angle error, y and x collimator position error, MLC leaf position error and dose delivery error were 0.49°, 0.09 mm, 0.38 mm, 0.31 mm, 0.05 MU, respectively. Compare to the nasopharynx, uteri, rectal and larynx cancer, it can be seen that the variation factors of dose delivery and gantry angle errors for various cancer sites were higher, which was 8.10%and 4.54%, respectively. Through the process quality control of the linac, it was found that all the parameters were in control. While some abnormal points arose (the error was greater than UCL) , and the ratio of the gantry angle which was out of control (0.45%) was the highest in all parameters. Conclusions linac delivery accuracy could be monitored and verified during the whole VMAT treatment for each patient, and the daily running condition of linac could be monitored. Through the introduction of SPC technology and the control charts, the process quality control of linac in VMAT could be achieved. It is a financial and valid method for the daily quality assurance of linac. Key words: Volumetric-modulated arc therapy; linac; Statistical process control; Process quality control; Position accuracy