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Utilizing statistical process control analysis and EPID for routine QA of medical linear accelerators

Andersson, Patrik (2011)
Medical Physics Programme
Abstract (Swedish)
Purpose: The main purposes of this study is to a) evaluate the suitability of a Varian aS1000 EPID as a safe quality assurance device for control of machine specific parameters, such as linac output, beam quality and beam symmetry, and b) to investigate the usefulness of the SPC concept for evaluation of the obtained data.

Method and Materials: Measurements were performed using a Varian Clinac 2300iX linear accelerator (Varian, Inc., Palo Alto, CA, USA), equipped with an a-Si EPID (Varian a-Si1000). The suitability of the EPID as a device for daily quality control of the linac output was investigated
for 6 MV and 15 MV. The EPID measurements were compared to Linacheck (PTW
GmBh, Freiburg, Germany) as well as ionization chamber... (More)
Purpose: The main purposes of this study is to a) evaluate the suitability of a Varian aS1000 EPID as a safe quality assurance device for control of machine specific parameters, such as linac output, beam quality and beam symmetry, and b) to investigate the usefulness of the SPC concept for evaluation of the obtained data.

Method and Materials: Measurements were performed using a Varian Clinac 2300iX linear accelerator (Varian, Inc., Palo Alto, CA, USA), equipped with an a-Si EPID (Varian a-Si1000). The suitability of the EPID as a device for daily quality control of the linac output was investigated
for 6 MV and 15 MV. The EPID measurements were compared to Linacheck (PTW
GmBh, Freiburg, Germany) as well as ionization chamber measurements in water (NE2571 farmer type ionization chamber, NE Technology Ltd., Reading, UK). The ability of an a-Si EPID to verify the beam quality and beam profile parameters was investigated by adjustments of the bending current shunt voltage and the steering current, respectively. A MATLAB© script was developed for analysis of the acquired EPID images. Statistical process control (SPC) was applied on the daily measurements and obtained action levels were compared to fixed limits and the standard deviation of the data. Results: The linear response of the EPID with varying number of monitor units (r2=0.999) made it suitable for detection of output deviations during the daily measurements. These detected deviations could be confirmed as true deviations by follow-up measurements using both the Linacheck as well as ionization chamber. The SPC provide determination of the stability of the process. A drift in the linac output was for example detected by the SPC analysis, indicating an
unstable process. Adjustments of the beam quality and symmetry could be readily detected by the a-Si EPID and was traceable to water measurements and data collected during commissioning of the linac. A deviation of 0.3% in the TPR measured in water corresponded to a 0.5% change in the hump measured with EPID.

Conclusion: The EPID is suitable as a device for daily quality control of linac output. The user independence and the flexibility of the EPID as a two-dimensional detector are the main advantages. The latter meaning it can be used for constancy control of beam quality and symmetry. Using SPC provides better knowledge on when to act on varying data points, in comparison to the use of for example fixed limits or to limits obtained by calculating the standard deviation based on a set of data points without any consideration to any time order. The stability of a process can be monitored and it gives a good balance between sensitivity and resources available for corrective actions. (Less)
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author
Andersson, Patrik
supervisor
organization
year
type
H2 - Master's Degree (Two Years)
subject
language
English
id
2273039
date added to LUP
2012-01-02 15:16:20
date last changed
2012-02-21 09:13:45
@misc{2273039,
  abstract     = {Purpose: The main purposes of this study is to a) evaluate the suitability of a Varian aS1000 EPID as a safe quality assurance device for control of machine specific parameters, such as linac output, beam quality and beam symmetry, and b) to investigate the usefulness of the SPC concept for evaluation of the obtained data.

Method and Materials: Measurements were performed using a Varian Clinac 2300iX linear accelerator (Varian, Inc., Palo Alto, CA, USA), equipped with an a-Si EPID (Varian a-Si1000). The suitability of the EPID as a device for daily quality control of the linac output was investigated
for 6 MV and 15 MV. The EPID measurements were compared to Linacheck (PTW
GmBh, Freiburg, Germany) as well as ionization chamber measurements in water (NE2571 farmer type ionization chamber, NE Technology Ltd., Reading, UK). The ability of an a-Si EPID to verify the beam quality and beam profile parameters was investigated by adjustments of the bending current shunt voltage and the steering current, respectively. A MATLAB© script was developed for analysis of the acquired EPID images.  Statistical process control (SPC) was applied on the daily measurements and obtained action levels were compared to fixed limits and the standard deviation of the data. Results: The linear response of the EPID with varying number of monitor units (r2=0.999) made it suitable for detection of output deviations during the daily measurements. These detected deviations could be confirmed as true deviations by follow-up measurements using both the Linacheck as well as ionization chamber. The SPC provide determination of the stability of the process. A drift in the linac output was for example detected by the SPC analysis, indicating an
unstable process. Adjustments of the beam quality and symmetry could be readily detected by the a-Si EPID and was traceable to water measurements and data collected during commissioning of the linac. A deviation of 0.3% in the TPR measured in water corresponded to a 0.5% change in the hump measured with EPID. 

Conclusion: The EPID is suitable as a device for daily quality control of linac output. The user independence and the flexibility of the EPID as a two-dimensional detector are the main advantages. The latter meaning it can be used for constancy control of beam quality and symmetry. Using SPC provides better knowledge on when to act on varying data points, in comparison to the use of for example fixed limits or to limits obtained by calculating the standard deviation based on a set of data points without any consideration to any time order. The stability of a process can be monitored and it gives a good balance between sensitivity and resources available for corrective actions.},
  author       = {Andersson, Patrik},
  language     = {eng},
  note         = {Student Paper},
  title        = {Utilizing statistical process control analysis and EPID for routine QA of medical linear accelerators},
  year         = {2011},
}