Applications of statistical process control to advanced radiotherapy techniques

Abstract

The statistical process control (SPC) is the new novel tool in radiotherapy QA process. This research is one of the first studies that applied SPC concept to advanced radiotherapy techniques. For the first research part, the X control chart was performed to establish the reasonable control limits of % gamma pass of nasopharyngeal case in 278 IMRT QA plans and 159 VMAT QA plans, and to assess the efficiency of QA process by using process capability index. The result showed lower control limit of gamma pass of IMRT and VMAT QA of 85% and 90%, respectively. The VMAT QA process was more capable with higher capability index than IMRT. The second part employed the X and EWMA charts for 2 years of weekly output constancy check. The number of point used to calculate control limits was varied from 1 to 5 months of data for all energies, and the λ and L were two more parameters variation. It was found that 2-3 months of data should be employed to calculate control limit of X chart, and 1 month of data should be used to calculate EWMA control limit. At least 20 in-control data point should be achieved to find the process capability and process acceptability with 95% confidence interval. The last part of research work was the setup of local tolerance limits in different data distribution types. Our local tolerance limit for normal distribution data type of percent point dose difference between measurement and calculation of 631 prostate IMRT QA plans was 3.6%. The appropriate lower tolerance limit for left-skewed distribution data type of percent gamma pass of 157 head and neck VMAT QA plans was 88.22%, which was lower than universal action limit. The upper tolerance limit for right-skewed distribution data type of homogeneity index of 150 VMAT PTV plans for head and neck cases was 0.19. The SPC can assist in the QA work in radiotherapy with more efficient than using traditional QA concept.