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Remote dosimetric auditing for intensity modulated radiotherapy: A pilot study

  • Narges Miri
    Correspondence
    Corresponding author at: School of Mathematical & Physical Sciences, Mathematics Building – V123, University of Newcastle, University Drive, Callaghan, NSW 2308, Australia.
    Affiliations
    School of Mathematical and Physical Sciences, University of Newcastle, Callaghan, NSW, Australia
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  • Joerg Lehmann
    Affiliations
    School of Mathematical and Physical Sciences, University of Newcastle, Callaghan, NSW, Australia

    Calvary Mater Newcastle Hospital, Newcastle, New South Wales, Australia
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  • Kimberley Legge
    Affiliations
    School of Mathematical and Physical Sciences, University of Newcastle, Callaghan, NSW, Australia
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  • Benjamin J. Zwan
    Affiliations
    School of Mathematical and Physical Sciences, University of Newcastle, Callaghan, NSW, Australia

    Central Coast Cancer Centre, Gosford Hospital, Gosford, NSW, Australia
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  • Philip Vial
    Affiliations
    Department of Medical Physics, Liverpool and Macarthur Cancer Therapy Centres, Sydney, Australia

    Ingham Institute of Applied Medical Research, Sydney, Australia

    Institute of Medical Physics, School of Physics, University of Sydney, Australia
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  • Peter B. Greer
    Affiliations
    School of Mathematical and Physical Sciences, University of Newcastle, Callaghan, NSW, Australia

    Calvary Mater Newcastle Hospital, Newcastle, New South Wales, Australia
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Open AccessPublished:December 06, 2017DOI:https://doi.org/10.1016/j.phro.2017.11.004

      Abstract

      Background and Purpose

      Electronic portal imaging devices (EPIDs) can be used to reconstruct dose inside a virtual phantom. This work aims to study the feasibility of using this method for remote dosimetry auditing of clinical trials.

      Materials and Methods

      Six centres participated in an intensity modulated radiotherapy (IMRT) pilot study of this new audit approach. Each centre produced a head and neck (HN) and post-prostatectomy (PP) trial plan and transferred the plans to virtual phantoms to calculate a reference dose distribution. They acquired in-air images of the treatment fields along with calibration images using their EPID. These data were sent to the central site where the images were converted to 2D field-by-field doses in a flat virtual water phantom and to 3D combined field doses in a cylindrical virtual phantom for comparison with corresponding reference dose distributions. Additional test images were used to assess the accuracy of the method when using different EPIDs.

      Results

      Field-by-field 2D analysis yielded mean gamma pass-rates of 99.6% (±0.3%) and 99.6% (±0.6%) for HN and PP plans respectively (3%/3 mm, doses greater than 10% global max). 3D combined field analysis gave mean pass-rates of 97.9% (±2.6%) and 97.9% (±1.8%) for the HN and PP plans. Dosimetry tests revealed some field size limitations of the EPIDs.

      Conclusions

      The remote auditing methodology using EPIDs is feasible and potentially an inexpensive method.

      1. Introduction

      In radiotherapy clinical trials, the complex nature of planning and delivery systems may result in variations in the dose deliveries among participant centres. Studies have demonstrated the clinical relevance of poor quality planning and treatment outcomes [
      • Bentzen S.M.
      • et al.
      Clinical impact of dosimetry quality assurance programmes assessed by radiobiological modelling of data from the thermoluminescent dosimetry study of the European organization for research and treatment of cancer.
      ]. The benefit of rigorous pre-treatment patient specific quality assurance (QA) and external dosimetric audits for clinical trials has been well demonstrated [
      • Pettersen M.N.
      • Aird E.
      • Olsen D.R.
      Quality assurance of dosimetry and the impact on sample size in randomized clinical trials.
      ,
      • Ibbott G.S.
      • Haworth A.
      • Followill D.S.
      Quality assurance for clinical trials.
      ]. On-site pre-treatment QA for intensity modulated radiotherapy (IMRT) has been shown to not always detect discrepancies between planning and delivery systems found in external audits [
      • Kry S.F.
      • et al.
      Institutional patient-specific IMRT QA does not predict unacceptable plan delivery.
      ]. Participating centres’ treatment delivery should be assessed to reduce variability thus improving the reliability of trial results. Conventionally, an independent centre performs the audit by site visit(s) or by mailing phantoms and dosimeters [
      • Clark C.H.
      • et al.
      Radiotherapy dosimetry audit: three decades of improving standards and accuracy in UK clinical practice and trials.
      ,
      • Ibbott G.S.
      • Molineu A.
      • Followill D.S.
      Independent evaluations of IMRT through the use of an anthropomorphic phantom.
      ,
      • Molineu A.
      • et al.
      Credentialing results from IMRT irradiations of an anthropomorphic head and neck phantom.
      ]. The most comprehensive audit is an ‘end-to-end audit’ that tests the full treatment chain from CT scanning to delivery using an anthropomorphic phantom. For example, the Imaging and Radiation Oncology Core (IROC) sends a head and neck phantom to participant centres. It has used pass criteria of 7% point dose difference in the planning target volume (PTV) and 4 mm distance to agreement in the high dose gradient area [
      • Molineu A.
      • et al.
      Credentialing results from IMRT irradiations of an anthropomorphic head and neck phantom.
      ]. While successfully established, the mailing audit approach is limited by the resources and costs involved in transporting equipment to and from each centre. As the measurement is responsibility of the local physicists, phantom and dosimeter set-up errors can result in measurements out of tolerance and therefore the need for repetition.
      Site-visit audits, on the other hand, are performed by external auditors which reduce set-up errors and increase the consistency of measurements. More recent approaches targeting specific technology such as volumetric modulated arc therapy (VMAT) have used a ‘TPS planned audit’ approach where centres use benchmark CT data sets and planning instructions to produce a local treatment plan. This plan is then transferred to CT data sets of the audit QA phantoms or 2D/3D detectors, then the dose distribution is calculated and compared to measurements performed during the site-visit. Examples of this approach include an audit that used a head and neck IMRT plan transferred to a solid water block to perform film measurements, where 75% of the films passed a gamma criteria of 4%/3 mm [
      • Clark C.H.
      • et al.
      Dosimetry audit for a multi-centre IMRT head and neck trial.
      ]. Another audit found that 42 out of 43 trial plans achieved greater than 95% pass rates at 3%/3 mm criteria for measured dose planes [
      • Clark C.H.
      • et al.
      A multi-institutional dosimetry audit of rotational intensity-modulated radiotherapy.
      ]. Site visits however can be expensive, time-consuming and logistically difficult to perform [
      • Ebert M.A.
      • et al.
      Comprehensive Australasian multicentre dosimetric intercomparison: issues, logistics and recommendations.
      ].
      Some alternative remote methods have been proposed to reduce cost and increase efficiency. The European Organisation for the Research and Treatment of Cancer (EORTC) in conjunction with IROC, termed the use of “institutional virtual phantoms”. In this method, participant centres sent CT data sets of their institutional phantom and their measured and planned dose distributions to the auditing site for analysis with standardised software. Although 6 out of 12 centres demonstrated more than 95% pass rates at 3%/3 mm criteria, 1/3rd of the centres could not be analysed centrally due to the variation of employed techniques and dosimeters [
      • Weber D.C.
      • et al.
      IMRT credentialing for prospective trials using institutional virtual phantoms: results of a joint European organization for the research and treatment of cancer and radiological physics center project.
      ].
      Recently we proposed a novel concept to perform remote dosimetric auditing for clinical trials using the ‘TPS planned audit’ model and EPID measurements [
      • Miri N.
      • et al.
      Virtual EPID standard phantom audit (VESPA) for remote IMRT and VMAT credentialing.
      ]. In that work an overview of the concept was presented. In current work, the specific details of the auditing method are outlined including EPID image and calibration plan acquisition details, image processing and conversion to dose methodology. The results from a pilot study of IMRT for six centres are presented including a separate analysis of the dose conversion model performance for each centre using open-field image data. The method combines the cost and efficiency benefit of remote audits with a standardised measurement and analysis process using EPID. The approach is termed the Virtual Epid Standard Phantom Audit (VESPA) and is based on EPID to dose conversion model [
      • King B.W.
      • et al.
      Development and testing of an improved dosimetry system using a backscatter shielded electronic portal imaging device.
      ]. In this paper we investigate the feasibility of this concept for IMRT auditing using data from six participant pilot centres.

      2. Materials and methods

      2.1 Equipment

      Six centres equipped with linear accelerators (linacs) from two vendors participated in this pilot study. Vendor 1 was Varian (Varian Medical Systems, Palo Alto, CA) with aS1000 type EPIDs, centres A, B and C and Vendor 2 was Elekta (Elekta AB, Stockholm, Sweden) with iViewGT EPIDs, centres D, E and F. Three different treatment planning systems (TPSs) were used. Comprehensive audit instructions were provided to the centres including a separate EPID guide to assist with correct calibration and operation. The Trans-Tasman Radiation Oncology Group (TROG) supplied IMRT head and neck (HN) and post-prostatectomy (PP) trial benchmarking plan instructions and CT data sets. Prescriptions, PTV and OAR constrains for both cases are shown in Supplementary Table 1. CT datasets of two standard virtual water-equivalent QA phantoms were also provided; a virtual flat phantom (VFP) and a virtual cylindrical phantom (VCP). The VFP was 41 cm in length (superior-inferior direction) and 43 cm × 35 cm in cross-section and the VCP was 40 cm in length and 20 cm diameter in cross-section. The VESPA process has been summarised in Supplementary Fig. 1.

      2.2 EPID to dose conversion method

      2.2.1 2D dose planes in VFP

      The conversion of EPID signal to dose at 10 cm depth within the virtual phantom was performed using an in-house software, based on the method of King et al. [
      • King B.W.
      • et al.
      Development and testing of an improved dosimetry system using a backscatter shielded electronic portal imaging device.
      ], developed at Calvary Mater Newcastle Hospital (CMNH). The software included a model that does not require parameter adjustment by the participating centres. However, an individual machine specific file, using information provided by each centre, was used to refine the model and adapt it to each machine/delivery type. The machine specific file used calibration images from each centre to determine central axis coordinate on the EPID and EPID sag as described below.
      Two sets of dose conversion parameters have been developed for images acquired with a Varian aS1000 EPID and aS1200 EPID and validated by comparison to 2D dose-planes measured with MapCheck diode array (Sun Nuclear Corporation, Melbourne, FL) [
      • Miri N.
      • et al.
      EPID-based dosimetry to verify IMRT planar dose distribution for the aS1200 EPID and FFF beams.
      ]. This study used the model developed for Vendor 1. The model required an additional EPID support-arm backscatter correction for the Varian images [
      • King B.W.
      • Greer P.B.
      A method for removing arm backscatter from EPID images.
      ]. It was benchmarked for 14 prostate fields and 22 head and neck fields with mean gamma pass-rates of respectively 99.4% (SD 1.0%) and 99.3% (SD 1.3%) at 2%/2 mm criteria. The model was applied to images from Vendor 2 for the first time in this study. In conjunction with the IMRT field images, a series of dosimetry test images with varying field size were also acquired in this study to examine the model performance for auditing of both vendors.

      2.2.2 3D dose distribution in VCP

      For calculation of 3D dose in the VCP, the method of Ansbacher was used [
      • Ansbacher W.
      Three-dimensional portal image-based dose reconstruction in a virtual phantom for rapid evaluation of IMRT plans.
      ]. Images acquired at actual gantry angles were converted to planar dose at 10 cm depth at isocentre in the flat phantom as described above. The planar dose was converted to 3D dose inside a VCP by multiplying it by a 2D off-axis correction matrix and applying an exponential percentage depth dose (PDD) and buildup factor. The dose was calculated for each individual image at the actual gantry angle then it was added to the total dose matrix to give the combined 3D dose distribution. The combined dose was stored in coordinates of the TPS dose matrix so that the dose distributions could be quantitatively compared.

      2.3 Treatment planning

      Each participating centre planned a prostate and a head neck trial case following the benchmarking instructions on the provided patient datasets. A dose of 70 Gy in 35 fractions were prescribed to the head and neck case with D95% constraint for the PTVs and the PP plan prescribed a total dose of 64 Gy in 32 fractions using D98% for the PTV; both being delivered at 6 MV energies. They then transferred the trial plan onto the VFP at perpendicular incidence for individual field analysis and onto the VCP at actual treatment gantry angles for the combined 3D dose distribution (Fig. 1). The isocentre was placed at 10 cm depth at 90 cm source to surface distance (SSD), which was at the centre of the VCP. These verification plan doses were then exported in DICOM format.
      Figure thumbnail gr1
      Fig. 1An example of an axial 2D plane of the head & neck (top row) and post-prostatectomy (bottom row) VCP doses. Left and right images show respectively delivery and treatment planning system (TPS) dose for each plan from centre F.
      A DICOM-RT format TPS calibration plan was also provided. This plan was only used for calculation of doses in the TPS. The plan consisted of a series of open jaw defined field sizes to calculate dose in the VFP, and a 10 × 10 cm2 field to calculate dose in the VCP, all at gantry zero incidence. The TPS open field doses were compared to doses derived with the model acquired from open field images to investigate model performance for each centre as described below. The dose at isocentre for the VCP provided a calibration dose for the 3D model.

      2.4 EPID measurements

      Integrated images of the IMRT fields were acquired both at gantry vertically downward and at actual gantry angles. All images were acquired with the clinical or QA mode operating. The images from Vendor 2 were acquired at 160 cm source to EPID distance and exported in Hamamatsu Image Sequence (HIS) format as the DICOM export does not retain pixel scaling information. Images from Vendor 1 however were acquired at 105 cm source to EPID distance.
      An EPID calibration plan in DICOM-RT format was provided for the centres. This plan consisted of a series of 10 × 10 cm2 fields at 45° gantry angles to provide data to 1) calibrate EPID response to dose; 2) determine EPID central axis position at gantry zero; and 3) correct EPID sag with gantry angle. The fields and analysis method are described below.
      As this method was not previously applied to the systems from Vendor 2, a dosimetry test plan was also provided in DICOM-RT format consisting of a series of open jaw defined fields of size 2 × 2, 3 × 3, 4 × 4, 6 × 6, 10 × 10, 15 × 15, 20 × 20, 25 × 25 (cm2) to compare to TPS doses following image to dose conversion. The plan also included a set of 10 × 10 cm2 fields with different monitor unit (MU) settings for EPID linearity assessment.
      The centres exported their images and TPS doses and uploaded them via the cloud to the central site for assessment. HIS format images were converted to DICOM format. For each centre, the following procedures were performed to determine the centre specific machine parameter file before dose was calculated in the virtual phantoms from the EPID images.

      2.4.1 Coordinate system

      Two EPID images of a 10 × 10 cm2 field with 9° and 270° collimator angles at gantry zero (gantry pointing vertically down) were used to determine the sub-pixel central axis (CAX) location on the EPID and hence an EPID coordinate system referenced to radiation isocentre. The field edges (50% dose points) of each image were determined using linear interpolation between pixels. The average mid-point of the two images gives the CAX location independent of jaw positioning [
      • King B.W.
      • et al.
      Development and testing of an improved dosimetry system using a backscatter shielded electronic portal imaging device.
      ].

      2.4.2 EPID sag correction

      To characterise EPID sag, several methods have been presented [
      • Rowshanfarzad P.
      • et al.
      Detection and correction for EPID and gantry sag during arc delivery using cine EPID imaging.
      ]. In the current study, EPID images of a 10 × 10 cm2 field were acquired at either 450 or 900 gantry angles. The field mid-point location was determined on each image as described above and compared to the mid-point at gantry zero to calculate sag relative to gantry zero (where the CAX position is known). The difference versus gantry angle showed best fit with a first order Fourier series, Sag(θ)=a0+a1cos(θ)+b1sin(θ). This fit was then used to correct the coordinate system for each acquired image depending on its gantry angle, Supplementary Fig. 2.

      2.4.3 Calibration factor

      The dose conversion method required a calibration factor [
      • King B.W.
      • Morf D.
      • Greer P.B.
      Development and testing of an improved dosimetry system using a backscatter shielded electronic portal imaging device.
      ]. Images of a 10 × 10 cm2 field were acquired with 20 MU for Vendor 2 systems and 100 MU for Vendor 1 systems. This difference was related to the methods employed for IMRT image acquisition on these systems. Deshpande et al. demonstrated that calibrating pixel to dose at 100 MU for linacs from Vendor 2 would introduce calibration errors of 2–4% for the typical range of IMRT segment (4–20) MUs and recommended 20 MU for calibration [
      • Deshpande S.
      • et al.
      Dose calibration of EPIDs for segmented IMRT dosimetry.
      ]. The converted dose value in a region of interest at central axis was compared to the corresponding TPS value for calibration factor determination.

      2.5 Dose analysis

      IMRT images of each individual field delivery acquired at gantry zero were used to reconstruct planar dose at 10 cm depth in the VFP and compared to TPS calculations with 2D gamma analysis. The images acquired at actual gantry angles were used to reconstruct the 3D dose distribution in the VCP and compared to TPS calculations with 3D gamma analysis. An in-house developed gamma algorithm was used for the dose comparison. All doses above 10% of the maximum dose were assessed with a search region of 6 mm radius. The gamma function used a global dose difference (DD) criteria defined by percentage of maximum dose of each measured image. For individual fields, 2D gamma analysis was employed while for combined dose distributions, 3D gamma analysis was used. The dose comparisons in this work were performed with 2%/2 mm, 3%/2 mm and 3%/3 mm criteria.
      To gain insight into the consistency of response and model performance and uncertainties for the different linac vendors, the dose converted from EPID images of open fields calculated in the VFP was compared to TPS calculations for each centre. Dose at isocentre at 10 cm depth was modelled for a set of square field images with different sizes, 2 × 2, 3 × 3, 4 × 4, 6 × 6, 10 × 10, 15 × 15, 20 × 20, 25 × 25 (cm2), then compared with their corresponding TPS dose.
      Finally, to ensure that the EPIDs from different centres were responding linearly to dose and to assess inter-centre response differences, each centre acquired a set of 10 × 10 cm2 images at incremental MU irradiations, (5–400) MU. The mean integrated pixel value (IPV) was calculated for 11 × 11 central pixel region of each image and normalised to the IPV at 100 MU.

      3. Results

      3.1 Audit: 2D/3D dose planes

      The gamma results for the pilot study audit are shown in Table 1. For the 2D field-by-field analysis, the mean of all centres was 95.6% at 2%/2 mm criteria with the lowest being 91.6% for Centre A. For the 3D combined dose analysis, the results were lower with the lowest being Centre E with 92.7% at 3%/3 mm criteria (see Table 2). Fig. 1 shows examples of an axial plane of the 3D dose distributions in the VCP for both the HN and PP plans.
      Table 1Mean (with standard deviation) gamma pass rates of the pilot centres for head and neck (HN) and post-prostatectomy (PP) individual fields. 2D dose planes were compared at 10 cm depth in the VFP for each field.
      CentresHN pass-rate (±SD) (%)PP pass-rate (±SD) (%)
      3%/3 mm3%/2 mm2%/2 mm3%/3 mm3%/2 mm2%/2 mm
      A99.2 (1.3)97.9 (2.0)91.6 (5.9)100.0 (0.0)99.9 (0.2)99.4 (0.5)
      B100.0 (0.0)100.0 (0.0)99.7 (0.2)100.0 (0.0)100.0 (0.0)99.8 (0.1)
      C99.3 (0.9)98.8 (1.1)96.5 (2.0)99.8 (0.2)99.7 (0.4)99.2 (0.8)
      D99.5 (0.3)97.4 (1.4)94.6 (2.4)99.4 (0.7)98.0 (1.3)93.4 (2.9)
      E99.8 (0.2)96.7 (2.5)93.2 (3.2)99.8 (0.3)98.9 (0.8)95.8 (2.3)
      F99.8 (0.2)99.3 (0.4)98.2 (1.0)98.5 (0.3)96.1 (0.4)90.7 (0.5)
      Mean (SD)99.6 (0.3)98.4 (1.2)95.6 (3.1)99.6 (0.6)98.8 (1.5)96.4 (3.8)
      Table 2Mean gamma pass rates (with standard deviation) of the pilot centres for head and neck (HN) and post-prostatectomy (PP) combined dose distributions in the VCP, 3D dose gamma analysis.
      CentresHN pass-rate (±SD) (%)PP pass-rate (±SD) (%)
      3%/3 mm3%/2 mm2%/2 mm3%/3 mm3%/2 mm2%/2 mm
      A99.897.787.598.793.479.2
      B98.996.989.498.798.095.5
      C98.195.787.699.899.396.8
      D98.987.372.095.692.282.3
      E92.777.654.498.993.678.5
      F99.194.079.695.888.777.0
      Mean (SD)97.9 (2.6)91.5 (7.8)78.4 (13.5)97.9 (1.8)94.2 (3. 9)84.9 (8.9)

      3.2 Dosimetry verification

      Fig. 2 demonstrates the EPID converted dose versus field size for open jaw defined fields compared to TPS dose for each centre. The Vendor 1 centres showed consistent differences between EPID dose and TPS dose. The image converted dose was similar to the TPS calculation at fields smaller than 10 × 10 cm2 and slightly higher at larger fields. Similarly, the Vendor 2 centres showed consistent differences with these being larger than the differences for Vendor 1. The image converted dose was slightly lower than the TPS calculation at fields smaller than 10 × 10 cm2 and slightly higher at larger fields. Centres D and E demonstrated large differences at the largest field size, 25 × 25 cm2.
      Figure thumbnail gr2
      Fig. 2Calculated dose at isocentre in the VFP for jaw defined open field sizes using EPID images (stars) and TPS (circles) for different centres. The insets correspond to the difference defined by (Dmodel − DTPS/DTPS%). All values were normalised to the values of 10 × 10 cm2 field size.
      The EPID response versus MU is shown in Fig. 3. As expected the response of the EPID was not completely linear. Apart from the response at small MUs, the Vendor 1 centres showed similar response while for the Vendor 2 centres, centre D demonstrated different response than the other two centres.
      Figure thumbnail gr3
      Fig. 3Central integrated pixel value (IPV) per MU versus MU for 10 × 10 cm2 images acquired at different MU settings. All IPVs were normalised to the values of 100 MU.

      4. Discussion

      The 2D field-by-field analysis resulted in mean (of all centres) gamma pass rates over 99.5% at 3%/3 mm criteria and over 95.5% at 2%/2 mm. The EPID signal to dose in water conversion model was not adapted for individual centres. The open field comparisons in Fig. 2 suggest that improvement to the results could potentially be made by deriving an Elekta specific set of model parameters. This could explain the slightly lower gamma results obtained for the Elekta systems in the study.
      For 3D dose analysis of the centres, only 1 of the 12 plans had a gamma pass rate below 95% at 3%/3 mm criteria. The pass rates were lower for the 3D analysis reflecting the larger uncertainty in the 3D model where depth-dose modelling is required. The current algorithm does not use vendor or centre-specific beam information. A detailed investigation into the contributing uncertainty components of the VESPA model when implemented across multiple types of linacs is underway and shall be reported separately.
      As Fig. 2 demonstrates, a large discrepancy is observed at large field sizes for two centres from Vendor 2. The reason for this could be an EPID signal artefact introduced by scatter close to the peripheral electronics. The imager response from centre D was re-measured and it confirmed that the artefact exists for fields larger than 23 × 23 cm2, Supplementary Fig. 3. This did not influence the gamma results in this study as smaller field sizes were used for the HN and PP fields. Future studies will restrict measurements for systems from Vendor 2 to a maximum field size of 23 × 23 cm2. Furthermore, the EPID response versus MU demonstrated non-linearity at low MUs for the EPIDs. This could be due to the failure of the acquisition system in integrating all EPID frames [
      • Podesta M.
      • et al.
      Measured vs simulated portal images for low MU fields on three accelerator types: possible consequences for 2D portal dosimetry.
      ], however the magnitude varies for the centres. Further investigations and data are required in order to determine the causes of these variations. A centre-specific calibration to dose that varies with irradiated MU could also be employed.
      The VESPA method provides a potentially inexpensive and rapid method to perform dosimetric auditing for specific assessments of new technologies. To be consistent with previous auditing methodologies, each centre produced their own treatment plan using their own planning techniques. This can introduce variation in the deliveries compared with providing each centre with an identical plan. However, technically it would not be possible to deliver an identical plan on different vendor systems, and this auditing approach assesses the individual centres planning methods. The measurements can be performed in 2–3 h while one calibration process suffices, if the measurements are performed in one session.
      However, VESPA is not as comprehensive as an ‘end-to-end’ audit and cannot assess absolute beam output, beam profile or inhomogeneity modelling. In some cases, site visits or ‘end-to-end’ audits may still be preferable. The VESPA method has not yet been implemented for flattening-filter-free deliveries or small-field auditing. The method follows the TPS planned audit approach which specifically targets a new technique such as IMRT or VMAT. It aims to combine the cost effectiveness of, for example, the EORTC “institutional virtual phantoms” method [
      • Weber D.C.
      • et al.
      IMRT credentialing for prospective trials using institutional virtual phantoms: results of a joint European organization for the research and treatment of cancer and radiological physics center project.
      ] with a more standardised approach to the dosimetry and analysis. In principle, it is attempting to mimic audits of IMRT performed with a pre-treatment verification type dosimeter but with extension to 3D dose estimation [
      • Clark C.H.
      • et al.
      A multi-institutional dosimetry audit of rotational intensity-modulated radiotherapy.
      ]. For the 3D dose volume analysis in the VCP the results are lower. It is likely due to uncertainties in the modelling of percentage depth dose as a single depth-dose model was used for these analyses. Improvement using a field-size specific and/or centre-specific depth dose model could be explored. Another approach that may improve results would be the use of a larger diameter virtual phantom to reduce high dose regions near the phantom surface.
      The applied gamma tolerances should consider the expected dosimetric uncertainties of the treatment chain as well as the audit method [
      • Clark C.H.
      • et al.
      Radiotherapy dosimetry audit: three decades of improving standards and accuracy in UK clinical practice and trials.
      ]. Clark et al. have suggested 3%/3 mm and 4%/3 mm criteria to compare respectively field-by-field and combined field dose distributions [
      • Clark C.H.
      • et al.
      Dosimetry audit for a multi-centre IMRT head and neck trial.
      ] and some recent studies have used 7%/4 mm criteria for end-to-end audit frameworks [
      • Molineu A.
      • et al.
      Credentialing results from IMRT irradiations of an anthropomorphic head and neck phantom.
      ,
      • Weber D.C.
      • et al.
      IMRT credentialing for prospective trials using institutional virtual phantoms: results of a joint European organization for the research and treatment of cancer and radiological physics center project.
      ]. The current study however analysed the results at 3%/3 mm, 3%/2 mm and 2%/2 mm criteria for analysis of both 2D and 3D dose distributions. The gamma pass rates were higher compared to the results from a similar audit based, albeit with a smaller number of centres [
      • Clark C.H.
      • et al.
      A multi-institutional dosimetry audit of rotational intensity-modulated radiotherapy.
      ]. The results suggest that analysis of the 3D dose delivery is feasible at 3%/3 mm which is stringent compared with other audit methods. Improvement to depth-dose modelling may allow this criteria to be tightened. It may also be possible to assess field-by-field deliveries at 2%/2 mm criteria for higher sensitivity. However a sensitivity analysis of the method should be performed to ensure that clinically significant dosimetric errors can be detected. We are currently in the process of assessment of the sensitivity at our centre and comparison of this with a dose-volume histogram approach instead of a gamma assessment.
      In conclusion, this pilot study assessed the methodology and feasibility of the VESPA method for remote verification of IMRT deliveries performed at different centres. Results of the current study demonstrate the feasibility of this method for clinical trial dosimetry auditing. The remote nature of the method promises a less expensive and more efficient alternative to those currently available. Further assessment and subsequent improvements will establish the method’s capabilities as an alternative to current IMRT and VMAT dosimetric audit methods.

      Conflict of interest

      There is no conflict of interest for this paper.

      Acknowledgments

      Funding has been provided from the Department of Radiation Oncology and TROG Cancer Research. Narges Miri is a recipient of a University of Newcastle postgraduate scholarship. We would like to thank the physicists and therapists from the pilot centres for their assistance in this study. We acknowledge the support of Melissa Crain, Alisha Moore, Monica Harris and Olivia Cook from TROG Cancer Research.

      Appendix A. Supplementary data

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      Linked Article

      • Erratum regarding previously published papers
        Physics and Imaging in Radiation OncologyVol. 13
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          The Publisher would like to point out that the papers listed below were mistakenly published without Declaration of Interest statements. Statements have now been added to each paper and are also gathered below within this erratum.
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