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Evaluation of current quality assurance (QA) system for volumetric modulated arc therapy (VMAT) by studying the impact of introduced errors

Krantz, Marcus (2010)
Medical Physics Programme
Abstract (Swedish)
Introduction: Currently, patient specific dosimetric measurements on diode arrays such as Delta4® (ScandiDos AB) is a part of standard quality assurance (QA) for RapidArc® (RA) at Herlev University hospital. However, a standing issue is the interpretation of the results from dosimetric measurements into consequences in the delivered absorbed dose to the patient. Furthermore, measurements are time con-suming. Thus, alternative QA systems are desirable for RA delivery. The aim of this study was to investigate (a) the impact of introduced known errors, (b) whether clinically important errors are detectable by the current QA system and (c) the benefits and drawbacks of dosimetric measurements and Monte Carlo (MC) ab-sorbed dose calculations... (More)
Introduction: Currently, patient specific dosimetric measurements on diode arrays such as Delta4® (ScandiDos AB) is a part of standard quality assurance (QA) for RapidArc® (RA) at Herlev University hospital. However, a standing issue is the interpretation of the results from dosimetric measurements into consequences in the delivered absorbed dose to the patient. Furthermore, measurements are time con-suming. Thus, alternative QA systems are desirable for RA delivery. The aim of this study was to investigate (a) the impact of introduced known errors, (b) whether clinically important errors are detectable by the current QA system and (c) the benefits and drawbacks of dosimetric measurements and Monte Carlo (MC) ab-sorbed dose calculations for RA QA.

Materials and methods: RA beam delivery was performed using a Varian Clinac 2300 iX for nine prostate treatment plans and one H&N treatment plan with and without introduced errors. The DICOM plan files were edited and systematic errors were introduced by shifting the position of one of the MLC banks. Systematic er-rors introduced were ±0.5 mm, ±1.0 mm, ±1.5 mm and ±2.0 mm. The minus and plus signs corresponds to the direction that results in larger and smaller aperture, respectively. The erroneous treatment plans were calculated in Eclipse™ using the analytical anisotropic algorithm (AAA) and the planes were subsequently meas-ured employing the Delta4® diode array. The impact of erroneous treatment plans was evaluated in terms of clinical importance by comparison of the planning target volume enclosed by ≥95% (PTV95%) of the prescribed absorbed dose, the volume enclosed by ≥95% and ≥105% of the prescribed absorbed dose (V95%, V105%). A qualitative evaluation of calculated reference and erroneous treatment plans were carried out in terms of 2D percental absorbed dose difference with the objective to visualize the impact of introduced errors. Delivered absorbed dose distributions were evaluated using gamma analysis as built into the Delta4® software with accep-tance criteria set to 3% dose difference/3 mm distance-to-agreement. Furthermore, dose volume histograms were evaluated for all treatment plans both in Eclipse™ and the Delta4® software’s. All treatment plans were also calculated with MC as an alternative QA protocol.

Results: The results disclose that systematic errors < +1.0 mm and ≤ -1.0 mm in the investigated prostate treatment plans, are in general not detected with the Delta4® diode array. Moreover, not all introduced errors ≥ +1.0 mm and > -1.0 mm were detected using the gamma index distribution exclusively, but by means of the absorbed dose deviation tool, all errors were detected. Also, for the investigated H&N treatment plan the gamma evaluation failed to find the introduced errors. However, using the absorbed dose deviation tool the investigated introduced errors of +1.0 mm, or larger, were detected. Furthermore, the MC based QA protocol was able to visualize introduced errors.

Conclusions: Small changes in position of the MLC-bank could be of clinical importance and dosimetric measurements of introduced errors demonstrated that clinically important errors are not always detected with the currently used RA QA system. This study showed that small systematic errors could be visualized with MC. Thus, MC can be used as a part of RA QA system in the future. (Less)
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author
Krantz, Marcus
supervisor
organization
year
type
H2 - Master's Degree (Two Years)
subject
keywords
Strålterapi
language
English
id
2292130
date added to LUP
2012-01-10 18:00:49
date last changed
2013-09-05 10:20:25
@misc{2292130,
  abstract     = {Introduction: Currently, patient specific dosimetric measurements on diode arrays such as Delta4® (ScandiDos AB) is a part of standard quality assurance (QA) for RapidArc® (RA) at Herlev University hospital. However, a standing issue is the interpretation of the results from dosimetric measurements into consequences in the delivered absorbed dose to the patient. Furthermore, measurements are time con-suming. Thus, alternative QA systems are desirable for RA delivery. The aim of this study was to investigate (a) the impact of introduced known errors, (b) whether clinically important errors are detectable by the current QA system and (c) the benefits and drawbacks of dosimetric measurements and Monte Carlo (MC) ab-sorbed dose calculations for RA QA.

Materials and methods: RA beam delivery was performed using a Varian Clinac 2300 iX for nine prostate treatment plans and one H&N treatment plan with and without introduced errors. The DICOM plan files were edited and systematic errors were introduced by shifting the position of one of the MLC banks. Systematic er-rors introduced were ±0.5 mm, ±1.0 mm, ±1.5 mm and ±2.0 mm. The minus and plus signs corresponds to the direction that results in larger and smaller aperture, respectively. The erroneous treatment plans were calculated in Eclipse™ using the analytical anisotropic algorithm (AAA) and the planes were subsequently meas-ured employing the Delta4® diode array. The impact of erroneous treatment plans was evaluated in terms of clinical importance by comparison of the planning target volume enclosed by ≥95% (PTV95%) of the prescribed absorbed dose, the volume enclosed by ≥95% and ≥105% of the prescribed absorbed dose (V95%, V105%). A qualitative evaluation of calculated reference and erroneous treatment plans were carried out in terms of 2D percental absorbed dose difference with the objective to visualize the impact of introduced errors. Delivered absorbed dose distributions were evaluated using gamma analysis as built into the Delta4® software with accep-tance criteria set to 3% dose difference/3 mm distance-to-agreement. Furthermore, dose volume histograms were evaluated for all treatment plans both in Eclipse™ and the Delta4® software’s. All treatment plans were also calculated with MC as an alternative QA protocol.

Results: The results disclose that systematic errors < +1.0 mm and ≤ -1.0 mm in the investigated prostate treatment plans, are in general not detected with the Delta4® diode array. Moreover, not all introduced errors ≥ +1.0 mm and > -1.0 mm were detected using the gamma index distribution exclusively, but by means of the absorbed dose deviation tool, all errors were detected. Also, for the investigated H&N treatment plan the gamma evaluation failed to find the introduced errors. However, using the absorbed dose deviation tool the investigated introduced errors of +1.0 mm, or larger, were detected. Furthermore, the MC based QA protocol was able to visualize introduced errors.

Conclusions: Small changes in position of the MLC-bank could be of clinical importance and dosimetric measurements of introduced errors demonstrated that clinically important errors are not always detected with the currently used RA QA system. This study showed that small systematic errors could be visualized with MC. Thus, MC can be used as a part of RA QA system in the future.},
  author       = {Krantz, Marcus},
  keyword      = {Strålterapi},
  language     = {eng},
  note         = {Student Paper},
  title        = {Evaluation of current quality assurance (QA) system for volumetric modulated arc therapy (VMAT) by studying the impact of introduced errors},
  year         = {2010},
}