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Medical Physics / Volume 38 / Issue 9 / RADIATION THERAPY PHYSICS

Med. Phys. 38, 5146 (2011); http://dx.doi.org/10.1118/1.3622672 (21 pages)

Implementing RapidArc into clinical routine: A comprehensive program from machine QA to TPS validation and patient QA

Ann Van Esch and Dominique P. Huyskens

7Sigma, QA-team in Radiotherapy Physics, 3150 Tildonk, Belgium and Department of Radiotherapy, Clinique Ste. Elisabeth, 5000 Namur, Belgium

Claus F. Behrens, Eva Samsøe, Maria Sjölin, Ulf Bjelkengren, and David Sjöström

Department of Oncology, Division of Radiophysics, Copenhagen University Hospital, 2730 Herlev, Denmark

Christian Clermont, Lionel Hambach, and François Sergent

Department of Radiotherapy, Clinique Ste. Elisabeth, 5000 Namur, Belgium

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(Received 27 March 2011; accepted 15 July 2011; revised 20 June 2011; published online 24 August 2011)

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Purpose: With the increased commercial availability of intensity modulated arc therapy (IMAT) comes the need for comprehensive QA programs, covering the different aspects of this newly available technology. This manuscript proposes such a program for the RapidArc (RA) (Varian Medical Systems, Palo Alto) IMAT solution.
Methods: The program was developed and tested out for a Millennium120 MLC on iX Clinacs and a HighDefinition MLC on a Novalis TX, using a variety of measurement equipment including Gafchromic film, 2D ion chamber arrays (Seven29 and StarCheck, PTW, Freiburg, Germany) with inclinometer and Octavius phantom, the Delta4 systam (ScandiDos, Uppsala, Sweden) and the portal imager (EPID). First, a number of complementary machine QA tests were developed to monitor the correct interplay between the accelerating/decelerating gantry, the variable dose rate and the MLC position, straining the delivery to the maximum allowed limits. Second, a systematic approach to the validation of the dose calculation for RA was adopted, starting with static gantry and RA specific static MLC shapes and gradually moving to dynamic gantry, dynamic MLC shapes. RA plans were then optimized on a series of artificial structures created within the homogeneous Octavius phantom and within a heterogeneous lung phantom. These served the double purpose of testing the behavior of the optimization algorithm (PRO) as well as the precision of the forward dose calculation. Finally, patient QA on a series of clinical cases was performed with different methods. In addition to the well established in-phantom QA, we evaluated the portal dosimetry solution within the Varian approach.
Results: For routine machine QA, the “Snooker Cue” test on the EPID proved to be the most sensitive to overall problem detection. It is also the most practical one. The “Twinkle” and “Sunrise” tests were useful to obtain well differentiated information on the individual treatment delivery components. The AAA8.9 dose calculations showed excellent agreement with all corresponding measurements, except in areas where the 2.5 mm fixed fluence resolution was insufficient to accurately model the tongue and groove effect or the dose through nearly closed opposing leafs. Such cases benefited from the increased fluence resolution in AAA10.0. In the clinical RA fields, these effects were smeared out spatially and the impact of the fluence resolution was considerably less pronounced. The RA plans on the artificial structure sets demonstrated some interesting characteristics of the PRO8.9 optimizer, such as a sometimes unexpected dependence on the collimator rotation and a suboptimal coverage of targets within lung tissue. Although the portal dosimetry was successfully validated, we are reluctant to use it as a sole means of patient QA as long as no gantry angle information is embedded.
Conclusions: The all-in validation program allows a systematic approach in monitoring the different levels of RA treatments. With the systematic approach comes a better understanding of both the capabilities and the limits of the used solution. The program can be useful for implementation, but also for the validation of major upgrades.

© 2011 American Association of Physicists in Medicine

ACKNOWLEDGMENTS

The authors would like to thank PTW (Freiburg, Germany) for the fruitful collaboration and for providing dosimetric equipment. 7Sigma also has a research collaboration with Varian Medical Systems. The authors wish to thank Dr. V. Remouchamps for his enthusiastic support and for allowing the necessary time slots at the Novalis TX treatment unit.

Article Outline

  1. INTRODUCTION
  2. METHODS AND MATERIALS
    1. Machine QA
      1. Static MLC Twinkle: assessing the accuracy of dose rate modulation versus gantry angle (maximum acceleration and deceleration).
      2. Dynamic MLC Twinkle: assessing the accuracy of MLC movement versus gantry angle (maximum MLC speed)
      3. Sunrise: assessing the impact of gantry speed, gravity and inertia on the gantry angle precision
      4. Snooker Cue: combining MU versus gantry angle and MLC movement in one single test
        1. Gafchromic film.
        2. 2D ion chamber array with additional inclinometer.
        3. EPID:
    2. TPS validation
      1. AAA validation for manually programmed RA-specific fields
      2. Performance assessment of the RA optimization algorithm
      3. AAA validation of RA plans on artificial structures
    3. Patient QA
      1. Phantom QA
      2. Portal dosimetry
  3. RESULTS
    1. Machine QA
    2. TPS validation
      1. AAA validation for manually programmed RA-specific fields
      2. Performance assessment of the RA optimization algorithm
      3. AAA validation of RA plans on artificial structures
    3. Patient QA
      1. Phantom QA
      2. Portal dosimetry
  4. DISCUSSION
  5. CONCLUSION
Digital Object Identifier
http://dx.doi.org/10.1118/1.3622672

KEYWORDS and PACS

Keywords

collimators, dosimetry, lung, medical computing, optimisation, phantoms, radiation monitoring, radiation therapy

PACS

  • 87.53.Jw

    Therapeutic applications, including brachytherapy

  • 87.55.dk

    Dose-volume analysis

  • 87.53.Bn

    Dosimetry/exposure assessment

PUBLICATION DATA

ISSN

0094-2405 (print)  

Publisher

$abstract.society.value is a Member of CrossRef American Association of Physicists in Medicine

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Figures (11) Tables (2)

Figures (click on thumbnails to view enlargements)

FIG.1
Polar graphs displaying the programmed dose rate, gantry speed and MLC leaf positions as a function of gantry angle for (a) the Static MLC Twinkle, (b) the Dynamic MLC Twinkle, (c) the Sunrise and (d) the Snooker Cue for machine QA.

FIG.1 Download High Resolution Image (.zip file) | Export Figure to PowerPoint

FIG.2
Experimental setups for the machine QA tests: (a) Gafchromic film in transversal plane through the isocenter in 10 × 10 cm2 solid water blocks, (b) 2D ion chamber array with inclinometer mounted to the tray holder of the Clinac and (c) EPID with metal rod placed on the treatment couch.

FIG.2 Download High Resolution Image (.zip file) | Export Figure to PowerPoint

FIG.3
Schematic overview of the MLC shapes and the field sizes used for the basic validation of the dose calculation. The drawings correspond to the Millenium120 MLC setup, but are very similar for the HD MLC. The cross marks the CAX and the δ indicates the leaf gap(s) between opposing leaf tips. The collimator settings are indicated first for the central setup and second for the laterally or longitudinally shifted setup. All measurements were performed at a depth of 5 cm and SSD = 95 cm, except the Static Twinkle and Sunrise test which were performed in the setup shown in Fig. 2.

FIG.3 Download High Resolution Image (.zip file) | Export Figure to PowerPoint

FIG.4
Some illustrations of the artificial structures contoured in the Octavius: (a) central and off-axis cylinder, (b) spherical prostate with bladder and rectum, (c) horseshoe-shaped head and neck volume with two PTVs, (d) oesophagus and spinal cord tilted cylinders, (e) spiral shaped Snake structure. Heterogeneity tests were performed in the home made inhomogeneous lung phantom displayed in (f).

FIG.4 Download High Resolution Image (.zip file) | Export Figure to PowerPoint

FIG.5
Examples of machine QA measurements: (a) StarCheck and inclinometer data obtained for the Static MLC Twinkle data for correct delivery (upper polar plot) and delivery with intended errors (lower polar plot). The gray bars indicate the theoretically expected dose rate as a function of gantry angle. The errors shown in the lower part correspond to an artificially induced gantry inertia effect of 3° and a 2° smoothening effect of the gantry angle motion. (b) Film data obtained for the Static MLC Twinkle displaying correct delivery, induced inertia effect and overly smoothened delivery. All comparisons show the expected image, the measured film and the isodoses of the measurement overlayed on the expected image.

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FIG.6
Integrated images of one of the subarcs of the Snooker Cue test: displaying the rod in the center of the projected MLC gaps for all gantry angles for the correct delivery (a) and the displaced projection of the metal rod in the vertical lines for the simulated inertia error of 1° (b) and 2° (c) (for all gantry angles except the starting angle).

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FIG.7
Effect of the resolution on the calculation accuracy: Measured (film) (black line) and calculated doses (AAA 8.9 with 2.5 mm (dotted line) or 1 mm (solid line) resolution and AAA 10.0 (dashed line) with a 0.3 mm fluence resolution and 1 mm dose calculation resolution) for (a) the central DLG test setup with a 3 mm gap between the leaf tips, (b) the off-axis test setup with a 3 mm gap between the leaf tips and (c) the tongue and groove setup.

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FIG.8
Effect of the angular resolution on the calculation accuracy: (a) Sunrise test calculated with 15° (dashed line), 3° (solid line) and 1° (solid black line) angular resolution. The position of the extracted line profiles is shown on the 2D dose images displayed on the right of the graph. (b) Dynamic gantry, sweeping gap test results for film (black line), 2D array (black squares), and AAA 8.9 dose calculations with an angular resolution of 15° (solid line) and 1° (dashed line). (c) Line profiles corresponding to the Tongue and groove arc, measured with film (solid black line) and the 2D array (black squares) and calculated with 1° angular resolution and 1 mm dose grid resolution for a 2.5 mm (dashed line) and 0.3 mm (solid line) fluence map resolution.

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FIG.9
Examples of a comparison of calculated and measured data for a number of RapidArc plans made on the artificial structures. (a) Line profiles in the upper part of each quadrant show the film data while the lower part displays the 2D array measurement points compared to AAA 8.9. (b) Isodose overlays and gamma evaluation maps from which the line profiles were extracted. Red points indicate measurement points with a gamma value larger than unity. The black lines indicate the position of the lineprofiles shown in (a).

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FIG.10
Measured and calculated data obtained in the heterogeneous lung phantom for the mediastinal and lateral lobe PTV structures. The ion chamber absolute point dose measurement is indicated with a diamond.

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FIG.11
Examples of the comparison of predicted (PDIP) and measured (aSi) portal dose images for two arc deliveries (on the Novalis TX treatment unit). The position of the displayed line profiles is indicated on the gamma evaluation map. The upper graph shows the area of poor gamma agreement in the “_v” line profile. The lower graph shows the typical extreme modulation observed in the RA integrated images.

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Tables

Table I. Overview of the used measurement equipment for the different parts of the RA validation protocol. Letters indicate during which phases the setups are used: I = implementation, R = routine, P = problem investigation or U = major upgrade.

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Table II. Overview of the planning results for a number of RA plans on the artificial structure sets. Different plans listed for the same structure set were all obtained with identical constraints and priorities during the optimization process for meaningful inter comparison. The confirmity Index CI, the lesion coverage fraction, LCF, and the normal tissue overdosage fraction, NTOF, are calculated for the 95% isodose, according to the formulas listed in the manuscript.

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