A comprehensive analysis of the IMRT dose delivery process using statistical process control (SPC)

Abstract

The aim of this study is to introduce tools to improve the security of each IMRT patient treatment by determining action levels for the dose delivery process. To achieve this, the patient-specific quality control results performed with an ionization chamber-and which characterize the dose delivery process-have been retrospectively analyzed using a method borrowed from industry: Statistical process control ͑SPC͒. The latter consisted in fulfilling four principal well-structured steps. The authors first quantified the short term variability of ionization chamber measurements regarding the clinical tolerances used in the cancer center ͑Ϯ4% of deviation between the calculated and measured doses͒ by calculating a control process capability ͑C pc ͒ index. The C pc index was found superior to 4, which implies that the observed variability of the dose delivery process is not biased by the short term variability of the measurement. Then, the authors demonstrated using a normality test that the quality control results could be approximated by a normal distribution with two parameters ͑mean and standard deviation͒. Finally, the authors used two complementary tools-control charts and performance indices-to thoroughly analyze the IMRT dose delivery process. Control charts aim at monitoring the process over time using statistical control limits to distinguish random ͑natural͒ variations from significant changes in the process, whereas performance indices aim at quantifying the ability of the process to produce data that are within the clinical tolerances, at a precise moment. The authors retrospectively showed that the analysis of three selected control charts ͑individual value, moving-range, and EWMA control charts͒ allowed efficient drift detection of the dose delivery process for prostate and head-and-neck treatments before the quality controls were outside the clinical tolerances. Therefore, when analyzed in real time, during quality controls, they should improve the security of treatments. They also showed that the dose delivery processes in the cancer center were in control for prostate and head-and-neck treatments. In parallel, long term process performance indices ͑P p , P pk , and P pm ͒ have been analyzed. Their analysis helped defining which actions should be undertaken in order to improve the performance of the process. The prostate dose delivery process has been shown statistically capable ͑0.08% of the results is expected to be outside the clinical tolerances͒ contrary to the head-and-neck dose delivery process ͑5.76% of the results are expected to be outside the clinical tolerances͒.

Can a commercially available EPID dosimetry system detect small daily patient setup errors for cranial IMRT/SRS?

“…Our next step after the implementation of EPID dosimetry for cranial IMRT/SRS patients is to implement a more comprehensive quality control program for these patients through using statistical process control. 16 Statistical process control will provide the data to move from consensus-based pass/fail criteria agreed on by the medical physics community to criteria that better reflect the uncertainties of a delivery system.…”

Dosimetric and Mechanical Stability of CyberKnife Robotic Radiosurgery Unit: 5 Years’ Clinical Experience

“…[2,[4][5][6][7][8][9][10] Moreover, control charts, as used in industrial manufacturing and other healthcare systems, have recently been applied to QA for RT to determine shortand long-term stability of conventional linacs. [11][12][13][14][15][16][17][18][19][20][21][22] However, there does not appear to be a study analyzing short-and long-term output stability of CyberKnife Robotic Radiosurgery System (Accuray Inc., Sunnyvale, CA, USA).…”

Treatment Delivery Validation

“…Gerard et al [ 7 ] further investigated the use of two complementary machine learning tools-control charts and performance indices-to accurately analyze the dose delivery process in IMRT. Control charts aim at monitoring the process over time using statistical control limits, whereas performance indices aim at quantifying the ability of the process to produce data that are within the clinical specifi cation limits at a precise moment.…”