Prog Med Phys.  2014 Sep;25(3):185-192. 10.14316/pmp.2014.25.3.185.

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

Affiliations
  • 1Department of Radiation Oncology, Yeungnam University Medical Center, Daegu, Korea. skkim3@ynu.ac.kr
  • 2Department of Radiation Oncology, Yeungnam University College of Medicine, Daegu, Korea.
  • 3Department of Radiation Oncology, School of Medicine, Ewha Womans University, Seoul, Korea.

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 lambda = 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.

Keyword

Output characteristics; Linear accelerator; Quality assurance; Process capability; Processability

MeSH Terms

Academic Medical Centers
Particle Accelerators*

Figure

  • 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.

  • 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.

  • Fig. 3. Process capability Cp and Cpk for the process of 21EX, 21EX-S, and Novalis Tx.


Reference

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