Dosimetric pre-treatment verification of IMRT using an EPID; clinical experience

doi:10.1016/j.radonc.2006.09.008

Radiotherapy and Oncology

Volume 81, Issue 2, November 2006, Pages 168-175

https://doi.org/10.1016/j.radonc.2006.09.008 Get rights and content

Abstract

Background and purpose

In our clinic a QA program for IMRT verification, fully based on dosimetric measurements with electronic portal imaging devices (EPID), has been running for over 3 years. The program includes a pre-treatment dosimetric check of all IMRT fields. During a complete treatment simulation at the linac, a portal dose image (PDI) is acquired with the EPID for each patient field and compared with a predicted PDI. In this paper, the results of this pre-treatment procedure are analysed, and intercepted errors are reported. An automated image analysis procedure is proposed to limit the number of fields that need human intervention in PDI comparison.

Materials and methods

Most of our analyses are performed using the γ index with 3% local dose difference and 3 mm distance to agreement as reference values. Scalar parameters are derived from the γ values to summarize the agreement between measured and predicted 2D PDIs. Areas with all pixels having γ values larger than one are evaluated, making decisions based on clinically relevant criteria more straightforward.

Results

In 270 patients, the pre-treatment checks revealed four clinically relevant errors. Calculation of statistics for a group of 75 patients showed that the patient-averaged mean γ value inside the field was 0.43 ± 0.13 (1 SD) and only 6.1 ± 6.8% of pixels had a γ value larger than one. With the proposed automated image analysis scheme, visual inspection of images can be avoided in 2/3 of the cases.

Conclusion

EPIDs may be used for high accuracy and high resolution routine verification of IMRT fields to intercept clinically relevant dosimetric errors prior to the start of treatment. For the majority of fields, PDI comparison can fully rely on an automated procedure, avoiding excessive workload.

Section snippets

IMRT pre-treatment verification: current practice

In our institute, the fluoroscopic Theraview NT (TNT) EPID (Cablon Medical-Theraview Technology, Leusden, The Netherlands) is used for portal imaging. This system is equipped with a low-noise cooled CCD camera. Its stable response [0.4% (1 SD)], the simultaneous integration of signal in 1024 × 1024 pixels, and a dead time between the acquisition of frames of only 0.2 ms make this EPID well suited for dosimetric measurements in IMRT fields produced with dynamic multileaf collimation [9].

For the

Intercepted errors

In 270 patients, severe errors in the fluence delivery were detected four times: once a wrong plan was accidentally sent to the accelerator, and three times one of the leaves was malfunctioning. For the wrong treatment plan all five treatment fields showed a γ image with most values larger than one, clearly showing that there was a serious problem. Fig. 1 shows dose profiles for one of the leaf pairs, for the wrong (1A) and the correctly transferred (1B) treatment plan, respectively. Similar

Discussion

In the literature, there is no consensus of the criteria to be used for calculation of the γ index when comparing two dose distributions. For our clinical practice, the 3% local/3 mm criterion has been selected. However, in other studies, the dose difference criterion is taken with respect to the maximum global dose instead [2], [23]. Low and Dempsey suggest using 5% global/2–3 mm criteria for IMRT verification in clinical practice, based on their experience that for a 3% dose difference the

Conclusion

Fluoroscopic EPIDs may be used for high accuracy and high resolution routine verification of IMRT fields to intercept clinically relevant dosimetric errors prior to the start of treatment. Such errors have been found for 4 patients in a total of 270. An automated image analysis procedure has been proposed to limit the involved workload. Only the images that do not pass the tests are selected for further review by a physicist. With the proposed test criteria only 1/3 of the images require this

Acknowledgements

We thank Martijn Hol and Paul Jacobs for development of software tools and Adri van den Biggelaar and Henk de Swarte for carrying out the pre-treatment measurements. This work was financially supported by the Dutch Cancer Society (Grant DDHK 2004-3107).

References (28)

N.L. Childress et al.
The design and testing of novel clinical parameters for dose comparison
Int J Radiat Oncol Biol Phys
(2003)
T. Depuydt et al.
A quantitative evaluation of IMRT dose distributions: refinement and clinical assessment of the gamma evaluation
Radiother Oncol
(2002)
A. VanEsch et al.
The use of an aSi-based EPID for routine absolute dosimetric pre-treatment verification of dynamic IMRT fields
Radiother Oncol
(2004)
M. Essers et al.
Commissioning of a commercially available system for intensity-modulated radiotherapy dose delivery with dynamic multileaf collimation
Radiother Oncol
(2001)
M. Kroonwijk et al.
In vivo dosimetry for prostate cancer patients using an electronic portal imaging device (EPID): detection of internal organ motion
Radiother Oncol
(1998)
M. Bucciolini et al.
Verfication of IMRT fields by film dosimetry
Med Phys
(2004)
G.J. Budgell et al.
Quantitative analysis of patient-specific dosimetric IMRT specification
Phys Med Biol
(2005)
N.L. Childress et al.
Retrospective analysis of 2D patient-specific IMRT verification
Med Phys
(2005)
N.L. Childress et al.
Detection of IMRT delivery errors using a quantitative 2D dosimetric verification system
Med Phys
(2005)
E.M. Franken et al.
Characteristics relevant to portal dosimetry of a cooled CCD camera-based EPID
Med Phys
(2004)

E.M. Franken et al.

A novel approach to accurate dosimetry using CCD-camera based EPIDs

Med Phys

(2006)

B.J.M. Heijmen et al.

Portal dose measurement in radiotherapy using an electronic portal imaging device

Phys Med Biol

(1995)

T. LoSasso et al.

Physical and dosimetric aspects of a multileaf collimation system used in the dynamic mode for implementing intensity modulated radiotherapy

Med Phys

(1998)

D.A. Low et al.

A technique for the quantitative evaluation of dose distributions

Med Phys

(1998)

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Citation Excerpt :
Caution should be exercised when using EPIDs as they are not ideal absolute dosimeters. Calibration and commissioning should be performed before use, following vendor recommendations [1,6–10]. The accuracy and stability of the EPID should be regularly monitored.
To use statistical process control for intensity-modulated radiation therapy (IMRT) quality assurance (QA) and improve tolerance limits and action limits.
An electronic portal imaging device (EPID) was selected to verify IMRT QA. The I-chart and the exponentially weighted moving averages (EWMA) chart were used to analyze the corresponding results.
Twenty samples were used to enable the sampling requirements for building the control limits to be met. The I-chart showed that isolated data points beyond the control limits were mainly derived from complex plans. The EWMA made predictions of systematic errors earlier than the I-chart. Systematic errors primarily originated from the dose calibration on the EPID, and recalibrating the EPID could eliminate such errors.
Statistical process control is an effective tool to detect controllable and can be used in IMRT QA. After calibrating the EPID, the tolerance and action limits all improved and satisfied the requirements/recommended values of the AAPM TG-218 report.
Il vise à utiliser le contrôle de processus statistique pour l’assurance qualité (AQ) de la radiothérapie conformationnelle avec modulation d’intensité (RCMI) et d’améliorer les limites de tolérance et les limites d’action.
Un dispositif d’imagerie portale électronique a été sélectionné pour vérifier l’AQ de la RCMI. L’I-chartet le graphique exponentiel des moyennes mobiles pondérées ont été utilisés pour analyser les résultats correspondants.
Vingt échantillons ont été appliqués pour répondre aux exigences d’échantillonnage pour l’établissement des limites de contrôle. L’I-chart montrait que les points de données isolés en dehors de la plage de contrôle provenaient principalement de plans complexes. Le graphique exponentiel des moyennes mobiles pondérées prédisait les erreurs système plus tôt que l’I-chart. Les erreurs du système provennaient principalement de l’étalonnage de la dose sur le dispositif d’imagerie portale électronique, et le recalibrage du dispositif d’imagerie portale électronique peut éliminer ces erreurs.
Le contrôle statistique des processus s’agit d’un outil efficace pour détecter la contrôlabilité et peut être utilisé pour l’AQ de la RCMI. Après étalonnage du dispositif d’imagerie portale électronique, les limites de tolérance et d’action ont toutes été améliorées et ont satisfait aux exigences/valeurs recommandées du rapport de l’American Association of Physicists in Medicine (AAPM) TG-218.
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At our institute, a transit back-projection algorithm is used clinically to reconstruct in vivo patient and in phantom 3D dose distributions using EPID measurements behind a patient or a polystyrene slab phantom, respectively. In this study, an extension to this algorithm is presented whereby in air EPID measurements are used in combination with CT data to reconstruct ‘virtual’ 3D dose distributions. By combining virtual and in vivo patient verification data for the same treatment, patient-related errors can be separated from machine, planning and model errors.
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Virtual dose reconstructions agree within 1% with ionization chamber measurements. The average γ-pass rate values (3% global dose/3 mm) in the 3D dose comparison with the OCTAVIUS 4D system and the TPS patient data are 98.5 ± 1.9%(1SD) and 97.1 ± 2.9%(1SD), respectively. For virtual patient dose reconstructions, the differences with the TPS in median dose to the PTV remain within 4%.
Virtual patient dose reconstruction makes pre-treatment verification based on deviations of DVH parameters feasible and eliminates the need for phantom positioning and re-planning. Virtual patient dose reconstructions have additional value in the inspection of in vivo deviations, particularly in situations where CBCT data is not available (or not conclusive).
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Retrospective dosimetry study was performed to test a patient-specific QA system for preoperative endorectal brachytherapy that uses a radiochromic film dosimetry system. A dedicated phantom for brachytherapy applicator used for rectal cancer treatment was fabricated enabling us to compare calculated-to-measured dose distributions. Starting from the same criteria used for external beam intensity-modulated radiation therapy QA (3%, 3 mm), passing criteria for high- and low-dose gradient regions were subsequently determined. Finally, we investigated the QA system's sensitivity to controlled source positional errors on selected patient plans.
In low-dose gradient regions, measured dose distributions with criteria of 3%, 3 mm barely passed the test, as they showed 95% passing pixels. However, in the high-dose gradient region, a more stringent condition could be established. Both criteria of 2%, 3 mm and 3%, 2 mm with gamma function calculated using normalization to the same absolute dose value in both measured and calculated dose distributions, and matrix sizes rescaled to match each other showed more than 95% of pixels passing, on average, for 15 patient plans analyzed.
Although the necessity of the patient-specific brachytherapy QA needs yet to be justified, we described a radiochromic film dosimetry–based QA system that can be a part of the brachytherapy commissioning process, as well as yearly QA program.
Quality assurance of intensity-modulated radiotherapy treatment planning: A dosimetric comparison
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Dose reconstruction of electronic portal imaging device based on calibration and calculation
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AAPM Task Group Report 307: Use of EPIDs for Patient-Specific IMRT and VMAT QA
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View full text

EPID and IMRTDosimetric pre-treatment verification of IMRT using an EPID; clinical experience

Abstract

Background and purpose

Materials and methods

Results

Conclusion

Section snippets

IMRT pre-treatment verification: current practice

Intercepted errors

Discussion

Conclusion

Acknowledgements

Int J Radiat Oncol Biol Phys

Radiother Oncol

Radiother Oncol

Radiother Oncol

Radiother Oncol

Verfication of IMRT fields by film dosimetry

Med Phys

Quantitative analysis of patient-specific dosimetric IMRT specification

Phys Med Biol

Retrospective analysis of 2D patient-specific IMRT verification

Med Phys

Detection of IMRT delivery errors using a quantitative 2D dosimetric verification system

Med Phys

Characteristics relevant to portal dosimetry of a cooled CCD camera-based EPID

Med Phys

A novel approach to accurate dosimetry using CCD-camera based EPIDs

Med Phys

Portal dose measurement in radiotherapy using an electronic portal imaging device

Phys Med Biol

Physical and dosimetric aspects of a multileaf collimation system used in the dynamic mode for implementing intensity modulated radiotherapy

Med Phys

A technique for the quantitative evaluation of dose distributions

Med Phys

EPID and IMRT
Dosimetric pre-treatment verification of IMRT using an EPID; clinical experience