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Oh, Yea, Kim, Lee, and Kim: Analysis of Output Constancy Checks Using Process Control Techniques in Linear Accelerators
Original Article

Progress in Medical Physics 2014;25(3):185-192.

Published online: 20 September 2014

DOI: https://doi.org/10.14316/pmp.2014.25.3.185

Analysis of Output Constancy Checks Using Process Control Techniques in Linear Accelerators

Se An Oh∗, Ji Woon Yea∗, †, Sang Won Kim∗, Rena Lee‡, Sung Kyu Kim∗, †

∗Department of Radiation Oncology, Yeungnam University Medical Center, Korea

†Department of Radiation Oncology, Yeungnam University College of Medicine, Daegu, Korea

‡Department of Radiation Oncology, School of Medicine, Ewha Womans University, Seoul, Korea

Correspondence: Sung Kyu Kim(skkim3@ynu.ac.kr) Tel:82-53-620-3373, Fax:82-53-624-3599

This work was supported by the Nuclear Safety Research Program through the Korea Radiation Safety Foundation (KORSAF) and the Nuclear Safety and Security Commission (NSSC), Republic of Korea (Grant No. 1305033).

Received 29 April 2014       Revised 1 September 2014       Accepted 4 September 2014

Copyright © 2014 Korean Society of Medical Physics

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

The purpose of this study is to evaluate the results for the quality assurance through a statistical analysis on the output characteristics of linear accelerators belonging to Yeungnam University Medical Center by using the Shewhart-type chart, Exponentially weighted moving average chart (EWMA) chart, and process capability indices Cp and Cpk. To achieve this, we used the output values measured using respective treatment devices (21EX, 21EX-S, and Novalis Tx) by medical physicists every month from September, 2012 to April, 2014. The output characteristics of treatment devices followed the IAEA TRS-398 guidelines, and the measurements included photon beams of 6 MV, 10 MV, and 15 MV and electron beams of 4 MeV, 6 MeV, 9 MeV, 12 MeV, 16MeV, and 20 MeV. The statistical analysis was done for the output characteristics measured, and was corrected every month. The width of control limit of weighting factors and measurement values were calculated as ＝ 0.10 and L＝2.703, respectively; and the process capability indices Cp and Cpk were greater than or equal to 1 for all energies of the linear accelerators (21EX, 21EX-S, and Novalis Tx). Measured values of output doses with drastic and minor changes were found through the Shewhart-type chart and EWMA chart, respectively. The process capability indices Cp and Cpk of the treatment devices in our institution were, respectively, 2.384 and 2.136 for 21EX, 1.917 and 1.682 for 21EX-S, and 2.895 and 2.473 for Novalis Tx, proving that Novalis Tx has the most stable and accurate output characteristics.

Keywords: Output characteristics, Linear accelerator, Quality assurance, Process capability, Processability

References

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Fig. 1.
Shewhart-type chart of the Output for the 6 MV, 10 MV photon beams and 6 MeV, 9 MeV electrons in the 21EX linear accelerator. (a) 6 MV Photons, (b) 10 MV Photons, (c) 6 MeV electrons, and (d) 9 MeV electrons.
pmp-25-185f1.tif
Fig. 2.
Exponentially weighted moving average (EWMA) chart of the Output for the 6 MV, 10 MV photon beams and 6 MeV, 9 MeV electrons in the 21EX linear accelerator. (a) 6 MV Photons, (b) 10 MV Photons, (c) 6 MeV electrons, and (d) 9 MeV electrons.
pmp-25-185f2.tif
Fig. 3.
Process capability Cp and Cpk for the process of 21EX, 21EX-S, and Novalis Tx.
pmp-25-185f3.tif
Table 1.
Upper control limit (UCL), Center line (CL), and Lower control limit (LCL) of the Shewhart-type chart for the 21EX 21EX-S, and Novalis Tx linear accelerator.
21EX Photons Electrons
6 MV 10 MV 6 MeV 9 MeV 12 MeV 16 MeV 20 MeV
UCL 1.012 1.014 1.013 1.016 1.014 1.020 1.015
CL 1.003 1.003 1.001 1.004 1.004 1.004 1.003
LCL 0.994 0.992 0.990 0.992 0.994 0.987 0.991
21EX-S Photons Electrons
6 MV 10 MV 6 MeV 9 MeV 12 MeV 16 MeV 20 MeV
UCL 1.013 1.014 1.015 1.009 1.015 1.013 1.015
CL 1.003 1.004 1.003 1.003 1.003 1.006 1.005
LCL 0.993 0.993 0.991 0.996 0.990 0.999 0.995
Novalis Tx Photons Electrons
6 MV 15 MV 4 MeV 6 MeV 9 MeV 12 MeV 16 MeV 20 MeV
UCL 1.013 1.011 1.018 1.013 1.014 1.013 1.013 1.010
CL 1.005 1.003 1.000 1.004 1.002 1.005 1.006 1.005
LCL 0.997 0.995 0.981 0.996 0.990 0.998 0.999 1.000
Table 2.
Process capability (Cp) and acceptability (Cpk) of the output measurements for the 21EX, 21EX-S, and Novalis Tx linear accelerator from September 2012 to April 2014.
21EX Photons Electrons
6 MV 10 MV 6 MeV 9 MeV 12 MeV 16 MeV 20 MeV
Cp 2.889 2.223 2.870 2.284 2.367 1.167 2.436
Cpk 2.574 1.999 2.748 1.976 2.051 1.417 2.185
21EX-S Photons Electrons
6 MV 10 MV 6 MeV 9 MeV 12 MeV 16 MeV 20 MeV
Cp 1.919 1.614 1.946 2.281 1.799 2.098 1.765
Cpk 1.716 1.410 1.779 2.079 1.642 1.671 1.477
Novalis Tx Photons Electrons
6 MV 15 MV 4 MeV 6 MeV 9 MeV 12 MeV 16 MeV 20 MeV
Cp 2.996 2.296 1.995 2.292 2.458 2.718 3.986 4.420
Cpk 2.490 2.243 1.967 1.951 2.270 2.240 3.181 3.623
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