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Radiotherapy reference dose audit in the United Kingdom by the National Physical Laboratory: 20years of consistency and improvements

Open AccessPublished:August 11, 2017DOI:https://doi.org/10.1016/j.phro.2017.07.005

      Abstract

      Background and purpose

      Audit is imperative in delivering consistent and safe radiotherapy and the UK has a strong history of radiotherapy audit. The National Physical Laboratory (NPL) has undertaken audit measurements since 1994 and this work examines results from these audits.

      Materials and methods

      This paper reviews audit results from 209 separate beams from 82 on-site visits to National Health Service (NHS) radiotherapy departments conducted between June 1994 and February 2015. Measurements were undertaken following the relevant UK code of practice. The accuracy of the implementation of absorbed dose calibration across the UK is quantified for MV photon, MeV electron and kV X-ray radiotherapy beams.

      Results

      Over the measurement period the standard deviation of MV photon beam output has reduced from 0.8% to 0.4%. The switch from air kerma- to absorbed dose-based electron code of practice contributed to a reduction in the difference of electron beam output of 0.6% (p < 0.01). The mean difference in NPL to local measurement for radiation output calibration was less than 0.25% for all beam modalities.

      Conclusions

      The introduction of the 2003 electron code of practice based on absorbed dose to water decreased the difference between absolute dose measurements by the centre and NPL. The use of a single photon code of practice over the period of measurements has contributed to a reduction in measurement variation. Within the clinical setting, on-site audit visits have been shown to identify areas of improvement for determining and implementing absolute dose calibrations.

      1. Introduction

      The National Physical Laboratory (NPL) develops and maintains the primary standards for radiation dosimetry for the UK, including those which are for external beam radiotherapy. The radiotherapy treatment machines have dose traceable to the NPL primary standard graphite calorimeter which ensures accuracy and consistency. Treatment machines are usually calibrated against a tertiary dosimeter which has been calibrated against a secondary standard ionisation chamber (the NPL designed NE 2561 ionisation chambers
      The original NE 2561, was designed by NPL and manufactured by Nuclear Enterprises. That design was superseded by the type NE 2611, however manufacture and repair of the NE 2611 chamber type was taken over by NPL when Nuclear Enterprises stopped production. New secondary standard chambers are designated as being of type NPL 2611. All versions are radiologically equivalent.
      ) which itself has been calibrated at the NPL through comparison with the graphite calorimeter primary standard [
      • DuSautoy A.R.
      The UK primary standard calorimeter for photon-beam absorbed dose measurement.
      ]. Once calibrated, the accuracy of delivered dose is assessed through independent audit from another centre, the Radiotherapy Trials Quality Assurance team (RTTQA), or from NPL. Reference dose audit acts to verify that treatment machines have been calibrated correctly using the relevant UK Code of Practice (CoP) for megavoltage photon (MV) beams [
      • Lillicrap S.C.
      • Owen B.
      • Williams J.R.
      • Williams P.C.
      Code of Practice for high-energy photon therapy dosimetry based on the NPL absorbed dose calibration service.
      ], electron (MeV) beams [
      • Thwaites D.I.
      • DuSautoy A.R.
      • Jordan T.
      • McEwen M.R.
      • Nisbet A.
      • Nahum A.E.
      • et al.
      The IPEM code of practice for electron dosimetry for radiotherapy beams of initial energy from 4 to 25 MeV based on an absorbed dose to water calibration.
      ] or kilovoltage (kV) beams [
      • Klevenhagen S.C.
      • Aukett R.J.
      • Harrison R.M.
      • Moretti C.
      • Nahum A.E.
      • Rosser K.E.
      The IPEMB code of practice for the determination of absorbed dose for x-rays below 300 kV generating potential (0.035 mm Al-4 mm Cu HVL; 10–300 kV generating potential).
      ,
      • Aukett R.J.
      • Burns J.E.
      • Greener A.G.
      • Harrison R.M.
      • Moretti C.
      • Nahum A.E.
      • et al.
      Addendum to the IPEMB code of practice for the determination of absorbed dose for x-rays below 300 kV generating potential (0.035 mm Al-4 mm Cu HVL).
      ], as used by all National Health Service (NHS) centres. There are many methods used for dosimetric audit ranging from local audit groups to international groups with different measurement methods including TLDs, OSLDs, either involving a site visit or a postal audit system [
      • Hurkmans C.W.
      • Christiaens M.
      • Collette S.
      • Weber D.C.
      Beam output audit results within the EORTC Radiation Oncology Group network.
      ,
      • Lye J.
      • Dunn L.
      • Kenny J.
      • Lehmann J.
      • Kron T.
      • Oliver C.
      • et al.
      Remote auditing of radiotherapy facilities using optically stimulated luminescence dosimeters.
      ,
      • Alvarez P.
      • Kry S.F.
      • Stingo F.
      • Followill D.
      TLD and OSLD dosimetry systems for remote audits of radiotherapy external beam calibration.
      ].
      The current UK CoP for MV beam dosimetry is the Institute of Physical Sciences in Medicine (IPSM) 1990 Code of Practice [
      • Lillicrap S.C.
      • Owen B.
      • Williams J.R.
      • Williams P.C.
      Code of Practice for high-energy photon therapy dosimetry based on the NPL absorbed dose calibration service.
      ]. This was the world’s first absorbed dose to water based protocol and was developed in collaboration with the NPL, IPSM (now known as the Institute of Physics and Engineering in Medicine (IPEM)) and hospital physicists. This CoP provides a formalism based on absorbed dose to water providing reduced uncertainty, compared with the previous air kerma calibration method [
      • Thwaites D.I.
      • DuSautoy A.R.
      • Jordan T.
      • McEwen M.R.
      • Nisbet A.
      • Nahum A.E.
      • et al.
      The IPEM code of practice for electron dosimetry for radiotherapy beams of initial energy from 4 to 25 MeV based on an absorbed dose to water calibration.
      ,

      Andreo P, Burns DT, Hohlfeld K, Huq MS, Kanai T, Laitano F, et al. Technical Reports Series No. 398. Absorbed Dose Determination in External Beam Radiotherapy: An International Code of Practice for Dosimetry Based on Standards of Absorbed Dose to Water. International Atomic Energy Agency (IAEA), Vienna; 2000. doi:10.1097/00004032-200111000-00017.

      ].
      Audit of reference dosimetry for radiotherapy beams forms part of a complete quality assurance system, with the first national dosimetry intercomparison of MV photon beams completed in 1991 by Thwaites et al. [
      • Thwaites D.I.
      • Williams J.R.
      • Aird E.G.
      • Klevenhagen S.C.
      • Williams P.C.
      A dosimetric intercomparison of megavoltage photon beams in UK radiotherapy centres.
      ], followed by an electron dosimetry audit in 1996 by Nisbet and Thwaites [
      • Nisbet A.
      • Thwaites D.I.
      A dosimetric intercomparison of electron beams in UK radiotherapy centres.
      ]. In 1991 the IPSM formed regional audit groups, dividing the UK into eight geographical regions. These groups would then conduct audits between the associated centres on an annual basis, to escalate the number of audits performed across the country, thus increasing confidence in the UK radiotherapy service. In 1994, NPL was invited by IPEM to provide independent audits to act as a link to the primary standard, initially performing one MV photon dosimetry audit per region per year and later expanded to include electron and kV reference dosimetry audits, which are reported in this paper. Recently more specific and complex audits such as the national rotational radiotherapy audit [
      • Clark C.H.
      • Hussein M.
      • Tsang Y.
      • Thomas R.
      • Wilkinson D.
      • Bass G.
      • et al.
      A multi-institutional dosimetry audit of rotational intensity-modulated radiotherapy.
      ], the national lung SABR audit [
      • Distefano G.
      • Lee J.
      • Jafari S.
      • Gouldstone C.
      • Baker C.
      • Mayles H.
      • et al.
      A national dosimetry audit for stereotactic ablative radiotherapy in lung.
      ], the national stereotactic radiosurgery audit [
      • Dimitriadis A.
      Assessing the dosimetric and geometric accuracy of stereotactic radiosurgery.
      ], and the national rectal contact brachytherapy audit [
      • Humbert-vidan L.
      • Sander T.
      • Eaton D.J.
      • Clark C.H.
      National audit of a system for rectal contact brachytherapy.
      ] have been included. This definitive link between the regional audit groups and NPL has been shown to reduce the uncertainty of the regional dose measurements [
      • Palmer A.
      • Mzenda B.
      • Kearton J.
      • Wills R.
      Analysis of regional radiotherapy dosimetry audit data and recommendations for future audits.
      ], thus it has proven to be a key aspect in the confidence of accurate delivery of radiotherapy in the UK.
      There are numerous recommendations specifying that reference dosimetry audit should take place between radiotherapy centres to ensure beam output calibrations are correctly applied, and to maintain national consistency [
      ,

      The Royal College of Radiologists, Society and College of Radiographers, Institute of Physics and Engineering in Medicine, National Patient Safety Agency, British Institute of Radiology. Towards Safer Radiotherapy. Towards Safer Radiotherapy. The Royal College of Radiologists; 2008.

      ]. There is however no legislation in the UK which requires that external audit is performed, and hence are often conducted on an ad hoc basis [
      • Clark C.H.
      • Aird E.G.
      • Bolton S.
      • Miles E.A.
      • Nisbet A.
      • Snaith J.A.
      • et al.
      Radiotherapy dosimetry audit: three decades of improving standards and accuracy in UK clinical practice and trials.
      ].
      This paper reviews the dosimetry audits for reference conditions which the NPL has completed, from June 1994 to February 2015. This includes MV audits, which all follow the 1990 CoP [
      • Lillicrap S.C.
      • Owen B.
      • Williams J.R.
      • Williams P.C.
      Code of Practice for high-energy photon therapy dosimetry based on the NPL absorbed dose calibration service.
      ], MeV audits following both the 1996 CoP [
      • Burns D.T.
      • Klevenhagen S.C.
      • Nahum A.E.
      • Pitchford W.G.
      The IPEMB code of practice for electron dosimetry for radiotherapy beams of initial energy from 2 to 50 MeV based on an air kerma calibration. Institution of Physics and Engineering in Medicine and Biology.
      ] and 2003 CoP [
      • Thwaites D.I.
      • DuSautoy A.R.
      • Jordan T.
      • McEwen M.R.
      • Nisbet A.
      • Nahum A.E.
      • et al.
      The IPEM code of practice for electron dosimetry for radiotherapy beams of initial energy from 4 to 25 MeV based on an absorbed dose to water calibration.
      ] and kV audits which follow the 1996 CoP [
      • Klevenhagen S.C.
      • Aukett R.J.
      • Harrison R.M.
      • Moretti C.
      • Nahum A.E.
      • Rosser K.E.
      The IPEMB code of practice for the determination of absorbed dose for x-rays below 300 kV generating potential (0.035 mm Al-4 mm Cu HVL; 10–300 kV generating potential).
      ] and its 2005 addendum [
      • Aukett R.J.
      • Burns J.E.
      • Greener A.G.
      • Harrison R.M.
      • Moretti C.
      • Nahum A.E.
      • et al.
      Addendum to the IPEMB code of practice for the determination of absorbed dose for x-rays below 300 kV generating potential (0.035 mm Al-4 mm Cu HVL).
      ]. The analysis completed gives an overview of the state of the accuracy of absolute dose calibrations in radiotherapy across the UK throughout this time. The data included is for audits which were completed within National Health Service (NHS) radiotherapy departments and has been collated from a combination of the audit reports sent to the host centres and the original data as recorded during the audit.

      2. Methods and methods

      2.1 General procedure

      The NPL reference dosimetry audits have been conducted in the same general manner for over 20 years whether for MV, MeV or kV beams. NPL use their own calibrated chambers, electrometers, phantoms, barometers and thermometers. A member of the NPL dosimetry group visits the department and first takes measurements of the beam quality index (tissue phantom ratio TPR20,10 for MV, R50,D for MeV, Half Value Layer (HVL) for kV) from which calibration factors for the secondary standard ionisation chamber are derived. An output measurement in reference conditions for each beam quality under test was then made using both the NPL equipment and the tertiary standard supplied by the host department allowing a check of the calibration factor on the tertiary standard. These measurements are then compared to those given by the host centre, which may be a result measured on the day as part of a daily check, or calculated from a data table. As well as the absorbed dose to water, the calibration of the local thermometer, barometer and calculation of ion recombination are also checked, providing a complete independent verification of the delivered dose and all intermediate steps following the CoP, thus ensuring each aspect of beam output measurement is verified. This contrasts with postal audits which may use TLDs or Alanine in which only the dose delivered can be compared. Within a given audit cycle, the same NPL secondary standard chamber was used for all measurements wherever possible.
      In the clinical environment the chamber to be calibrated may be positioned using a side-by-side phantom in the same beam as that with a calibration. The measurements can then be compared using the geometric mean to determine the calibration factor for the field instrument. A method of direct replacement or substitution, where only one chamber is positioned within the beam at a time, and then the other is placed in the same location and further measurements taken, to transfer the calibration, may also be used and was the method employed for these audits.

      2.2 Set up

      2.2.1 Megavoltage photon (MV) set up

      Dose measurements were made in a specially constructed water-filled PMMA phantom that was the same as that used to calibrate the secondary standard chambers when sent from the hospitals to NPL (in the time period considered for these audits). This was a 30 × 30 × 30 cm open top box made from PMMA of wall thickness of 1.0 cm (considered as 1.2 cm water equivalent). The phantom contains inserts which allow the NE2561/NE2611 ionisation chamber to be positioned at water equivalent depths of 5.0, 7.0, 10.0 and 20.0 cm on the beam axis. The linac was then set to produce a horizontal beam with the collimators set to give a 10.0 × 10.0 cm field at the radiation isocentre. The phantom was positioned in a horizontal beam using the light field and cross hairs on the linac. A plane flat mirror was used to align the phantom surface perpendicular to the beam direction by reflecting the light field back on to the head of the machine and ensuring the shadow of the crosshair was coincident. The reference point of the chamber was positioned on the central axis of the light field at the required reference depth in water. The focal distance was set according to local practice and the physical pointer, projected scale readings and laser agreement were are compared.

      2.2.2 Megavoltage electron (MeV) set up

      Measurements were taken in NPL-owned WTe Solid Water phantom material (St Bartholomew’s Hospital, London, UK) using either a Roos or NACP-02 type ionisation chamber with matching WTe chamber holder plate. A selection of WTe sheets were used, ranging in thickness from 1 to 40 mm to enable the build-up depth to be changed to match the requirements of the beam quality under test. The WTe phantom was set up on the patient couch. A vertical beam (gantry = 0°) was used. The same arrangement was used for beam quality, beam output and tertiary standard calibration.

      2.2.3 Kilovoltage photon (kV) set up

      For the measurement of kV beam quality (HVL), an NPL-owned jig was used that enabled a narrow beam geometry arrangement for a horizontal beam. Radiographs of the NPL chamber used in this geometry were taken to confirm correct alignment of the chamber. High-purity sheets of aluminium (or copper, as appropriate) covering a range of (pre-measured) thicknesses were used to enable the assessment of the HVL.
      For beam output and tertiary standard calibration, an NPL-owned jig was used to place either the NPL secondary standard or the tertiary standard in air at the end of the required applicator for a horizontal beam.

      2.3 Beam quality

      The beam quality is defined in different ways depending on the type of beam; MV, MeV or kV. Full details of each are given in the relevant Code of Practice.

      2.3.1 Megavoltage photon (MV) beam quality

      For MV beams the beam quality is the TPR20,10 [
      • Lillicrap S.C.
      • Owen B.
      • Williams J.R.
      • Williams P.C.
      Code of Practice for high-energy photon therapy dosimetry based on the NPL absorbed dose calibration service.
      ]. This was measured using a NE2561/NE2611 chamber and taken to be the ratio of ionisation measurements at 20.0 cm and 10.0 cm deep in water with a fixed focus to chamber distance of 100 cm and constant field size (10.0 × 10.0 cm). Readings were corrected for temperature, pressure and ion recombination. In practice any differences in ion recombination with depth (dose per pulse) make only a small change in TPR, which in turn gives only a small change in absorbed dose calibration e.g. a change of 1.0% in the TPR leading to a change of 0.16% in the calibration [
      • Klevenhagen S.C.
      • Aukett R.J.
      • Harrison R.M.
      • Moretti C.
      • Nahum A.E.
      • Rosser K.E.
      The IPEMB code of practice for the determination of absorbed dose for x-rays below 300 kV generating potential (0.035 mm Al-4 mm Cu HVL; 10–300 kV generating potential).
      ]. It should be noted that this may not be the case in Flattening Filter Free (FFF) beams and this issue is addressed in the recent IPEM publication [
      • Budgell G.
      • Brown K.
      • Cashmore J.
      • Duane S.
      • Frame J.
      • Hardy M.
      • et al.
      IPEM topical report 1: guidance on implementing flattening filter free (FFF) radiotherapy.
      ], however no FFF beams were audited during the time period this work covers.

      2.3.2 Megavoltage electron (MeV) beam quality

      For MeV beams there are two codes of practice discussed within this work, referred to as the 1996 and 2003 CoPs [
      • Thwaites D.I.
      • DuSautoy A.R.
      • Jordan T.
      • McEwen M.R.
      • Nisbet A.
      • Nahum A.E.
      • et al.
      The IPEM code of practice for electron dosimetry for radiotherapy beams of initial energy from 4 to 25 MeV based on an absorbed dose to water calibration.
      ,
      • Burns D.T.
      • Klevenhagen S.C.
      • Nahum A.E.
      • Pitchford W.G.
      The IPEMB code of practice for electron dosimetry for radiotherapy beams of initial energy from 2 to 50 MeV based on an air kerma calibration. Institution of Physics and Engineering in Medicine and Biology.
      ]. Major differences between them will be noted here, but a detailed discussion is contained within the 2003 CoP. The 1996 CoP derives absorbed dose to water from a 2 MV or Co-60 Air Kerma calibration of the NE2561/NE2611 secondary standard chamber. E0 and Ez are required to select physical data to convert to absorbed dose to water and are determined from a measurement of R50,I/D. The 2003 MeV CoP greatly simplifies the required steps for the user giving the absorbed dose to water calibration of the chamber traceable to the therapy electron primary standard calorimeter [
      • McEwen M.R.
      • Williams A.J.
      • DuSautoy A.R.
      Determination of absorbed dose calibration factors for therapy level electron beam ionization chambers.
      ]. The calibration factor for a secondary standard is given over a range of beam qualities defined by the quality index R50,D which is the depth at which the dose in water falls to 50% of its maximum along the central axis. This is measured at 100 cm SSD in a field large enough to ensure the result is independent of field size (the required field size will generally increase with energy). Measurements were taken in WTe Solid Water using either Roos or NACP-02 chambers.

      2.3.3 Kilovoltage photon (kV) beam quality

      For kV beams the beam quality is specified using the concept of the HVL which, in the case of the calibration of the secondary standard chamber at NPL, is defined in terms of the thickness of Aluminium or Copper required to reduce the air kerma rate by 50%. This is then used to derive the calibration factor for the secondary standard. Measurements were taken in air using NE2561/NE2611 chambers using narrow beam geometry with a setup to minimise scatter conditions which was dependent upon the host centre’s equipment location.

      2.4 Radiation output

      Radiation output is defined as the absorbed dose to water per Monitor Unit (MU) (or on some units in terms of time usually seconds) measured for a given set of reference conditions. The 1990 MV photon code of practice did not explicitly state what conditions should be used for reference dosimetry. The situation has therefore arisen where different criteria are in use; for example either 100 cm source to surface distance (SSD) or to isocentre is used, with various measurement depths ranging from 5 to 10 cm. Hence for MV photon dosimetry there exist a number of different ways in which the reference conditions have been implemented. Care must always be taken in ensuring the interpretation of the reference conditions within a department is fully understood.

      2.4.1 Megavoltage photon (MV) radiation output

      Radiation output is measured at a reference depth in water of 5 cm, or 7 cm for beams with quality index greater than 0.75. The output is measured at the beginning and end of each visit to ensure consistency and detect any drift, the average of these is taken for comparison with the locally provided value. The locally provided value was generally measured on the same day and the results may also have been derived from the centre’s tabulated data. Dependent on the reference conditions on occasion a percentage depth dose correction from centre-measured data was required.

      2.4.2 Megavoltage electron (MeV) radiation output

      1996 CoP: The output in terms of absorbed dose to water of the electron beam is determined based on a chain of measurements from an air kerma calibration of the NE2611 secondary standard. The transfer of calibration is first by means of a cross calibration of the secondary standard to a “Farmer” type chamber in a photon beam, then transfer from the Farmer type chamber to plane parallel electron chambers in a high energy electron beam (note whilst the Farmer chamber may be used for calibration of higher energy electron beams it is not recommended for use below an R50,D of 4 cm of water). The reference measurement depth (Zref) of 0.6 R50,D – 0.1 cm is calculated for calibration of beams, from the beam quality index. Derivation of the calibration factor for other electron energies may then be calculated as described in the CoP.
      2003 CoP: The 2003 MeV CoP specifies absorbed dose to water calibration factors given as a function of R50,D from the NPL calibration service [
      • McEwen M.R.
      • Williams A.J.
      • DuSautoy A.R.
      Determination of absorbed dose calibration factors for therapy level electron beam ionization chambers.
      ]. It is recommended that parallel plate chambers are used to measure the depth dose curves which will be used in determining the calibration of the beam. Absorbed dose should be determined in water at the calculated reference depth. Unless a significant difference was found between the value given by the host and that derived by NPL for R50,D, and hence Zref, the value supplied by the host was used by NPL for the audit measurements.

      2.4.3 Kilovoltage photon (kV) radiation output

      The kV CoP (including its addendum in 2003) is split into three sections; medium (0.5–4 mm Cu HVL), low (1.0–8 mm Al HVL), and very low energy (0.035–1 mm Al HVL) X-rays. Methods for calibrating at the surface or at a depth of 2 cm in water are provided for medium energy x-rays, and the method chosen will depend upon the clinical need of a particular department. For calibration based on dose at the surface an in-air method is used, whereas dose at depth may be more accurately determined using measurements in water. The addendum included revised values for Kch, extension to the backscatter factors, mass energy absorption coefficients, and a methodology to allow for the determination of absorbed dose to water either at the surface or at 2 cm deep dependent on clinical need. Measurements during audits were always performed in air with the chamber positioned centrally at the end of the applicator. The known thickness of the chamber is used to correct the response of the chamber back to the end of the applicator using the inverse square law. The CoPs are then used to convert the air kerma measurements to dose at the surface of a phantom or at 2 cm deep in water dependent on the requirements of the centre.

      2.5 Reporting of results and treatment of data

      Results from each audit were reported as a ratio of the department’s derived value to that of NPL. The beam quality, radiation output and calibration factor of the field instrument are derived during the audit and compared with the values in use within the department. These measurements were all performed following the recommendations of the relevant CoP in use within the department. Audit regions were able to select which centre was audited. Some had the same centre audited each time and they in turn would audit the remaining regional centres. Others rotated the NPL audit amongst the group. Due to this there are a number of repeat visits to centres within this dataset, with the number of repeat visits varying between regional groups.
      Measurements have been categorised by beam type, electron audits are then further divided according to which CoP was used. For each beam type the mean difference in the dose between the host centre and NPL was determined. Within this review the set of audit measurement for an individual beam will be referred to as a measurement set.

      2.6 Statistical comparisons

      The mean output ratio, standard deviation and standard deviation of the mean of results have been compared. The output ratios were compared using an independent t-test, assuming both equal and unequal variances (often referred to as a Welch test).

      3. Results

      A summary of the audits included in the analysis is given in Table 1 with the standard deviation, minimum and maximum differences given. There were no significant differences between the modalities. A mean difference of <1% (<0.5% for absorbed dose to water based COPs) between the Host and NPL for the measured values of output was found. The standard deviation for all beams was 0.74% (range 0.42–0.88% for each beam type).
      Table 1Summary of NPL radiation output ratio measurements from audits carried out during the period 1994 to February 2015. To allow comparison, data from the two MeV CoPs has been included both separately and combined (as shown in italics).
      AuditsRadiation Output Measurements
      Beam TypeNo. VisitsNo. BeamsMean DifferenceStandard DeviationMinMax
      MV4781+0.05%0.68%−1.30%+1.99%
      MeV (1996)614+0.75%0.42%+0.10%+1.65%
      MeV (2003)1784+0.20%0.75%1.50%+2.70%
      MeV (All)2398+0.27%0.74%−1.50%+2.70%
      kV1230+0.24%0.88%−2.40%+2.10%
      All Beams82209+0.18%0.74%−2.40%+2.70%
      A plot of all the measured output ratios is given in Fig. 1which has been split by audit region and beam type. It is noted that there were no MeV audits completed within audit region 4, and there was a large range in the number of audits performed between the different regions (range of 11 to 51). All MV results were within ±2%. All but three of the MeV results were within ±2%.
      Figure thumbnail gr1
      Fig. 1Measured beam output ratios separated into the eight individual regional audit groups. Mean value for each beam type in each group is indicated by a horizontal line (−).
      The variation in standard deviation for the previous 20 audits is shown in Fig. 2 for MV and MeV results in order of completion. On these plots the rolling standard deviation of the previous 20 results shows that the variation in the MV results decreased over time. After the first 20 audits the standard deviation of results was approximately 0.8% which has fallen to less than 0.4 % through the period of this analysis.
      Figure thumbnail gr2
      Fig. 2Plot of MV (a) and MeV (b) output results in order of completion. The standard deviation of the previous 20 audits (after removing three known erroneous MeV results) is shown by the dashed line. Bars indicate the standard uncertainty as given in the NPL report of the audit produced for the department.
      The results of measured output ratios for each beam type are approximately normally distributed and centred about unity indicating good agreement throughout the centres audited.
      For the electron beams the measured output ratios obtained from audits following the 1996 and 2003 MeV CoPs have been compared. A statistically significant difference (p < 0.01) between output ratios was found in the mean results obtained, with an improvement of 0.64% seen with the 2003 CoP. There was no significant difference in measurements seen between energies of any modality.
      A summary of the beam quality measurement is shown in Table 2 demonstrating good consistency between centres. It should be noted that for the kV results, the NPL measurements have a standard uncertainty of ±2%, however this equates to a dose difference of less than 0.1%. For the MV results, the NPL measurements have a standard uncertainty of ±0.2%, equating to a dose difference of less than 0.1% in the calculated calibration factor. For the MeV results, the NPL measurements have a standard uncertainty of ±0.2 cm. The variations in measurements of the beam quality give rise to less than 0.5% variation in energy for all beam types. It is not possible to give measurement specific uncertainty values for individual measurements performed by the host centre. An indication of the possible uncertainties in the centres beam calibration is given in the CoPs which state the uncertainty in secondary standard dosemeter calibration is ±1.5% at the 95% confidence level for MV and MeV beams and ±3% (1 standard deviation) for kV beams.
      Table 2Summary of NPL beam quality measurements as measured during audit visits. MV are shown as percentage difference of TPR, MeV are shown as difference in the depth of R50,D and kV are shown as percentage difference of HVL as a range of energies is included.
      Beam TypeMeasure of Beam QualityNo. ResultsMeanMinMax
      MVTPR20,10 (NPL/Host)81−0.11%−1.5%+1.2%
      MeVR50,D (NPL − Host)61−0.05 cm−0.28 cm+0.14 cm
      kVHVL (NPL/Host)26−0.04%−2.7%+7.7%

      4. Discussion

      A total of 82 audit visits were made during the specified time period, with 209 individual beams being audited. There were only five results outside the ±2% tolerance. Two of these were from kV measurements, and three from MeV measurements. None were from MV beams which are by far the most commonly used during treatments. All of the MeV results which exceeded ±2% for the output were from a single centre during a single visit. After further investigation at the time of the audit it transpired that the host centres output measurements were derived using the 1996 CoP, whereas the audit was carried out using the 2003 CoP. Correcting for this reduced all but one of the measurements to within the tolerance, with a single result remaining at +2.7%. Generally any issues identified during the audit were investigated and resolved during the visit.
      The standard deviation of the measured MV output ratios reduced from 0.8% to 0.4% over the period the audits were conducted (Fig. 2). The initial standard deviation of 0.8% was already excellent, and for this to reduce by half is testament to the success of the process of dissemination that exists in the UK for dose from the primary standard to the clinic. This is achieved through the combination of the use of the CoP, dedicated secondary standard instrumentation and UK regional audit network. NPL reference dosimetry audits also make a valuable contribution in ensuring the continued development and improvement in the traceability, accuracy and precision of dose delivered to patients receiving external beam radiotherapy.
      This improvement in audit results can be compared to similar audits, both within the UK and internationally. The audit completed by Thwaites et al. [
      • Thwaites D.I.
      • Williams J.R.
      • Aird E.G.
      • Klevenhagen S.C.
      • Williams P.C.
      A dosimetric intercomparison of megavoltage photon beams in UK radiotherapy centres.
      ] included 64 UK centres, and measured 161 MV beams between 1987 and 1991 and found a standard deviation of 1.4% and 1.5% for Co-60 and Linac beams respectively, under reference conditions. It can be seen that the recent NPL results have improved on those achieved in that study. The MeV national audit conducted by Nisbet in 1996 [
      • Nisbet A.
      • Thwaites D.I.
      A dosimetric intercomparison of electron beams in UK radiotherapy centres.
      ] which included 156 beams from 52 centres, gave a standard deviation of 1.8%. The MeV results from the NPL audits also show less variation than this and have remained approximately constant at 0.7%. Audits that use dosimeters such as TLDs rather than ionisation chambers have larger uncertainties, for example the IAEA postal audits which are conducted worldwide show that the first time a centre is audited, approximately 65% of results are within a ±5% tolerance [
      • Izewska J.
      • Andreo P.
      The IAEA/WHO TLD postal programme for radiotherapy hospitals.
      ]. This improves with subsequent audits demonstrating one of the benefits of an audit program [
      • Izewska J.
      • Wanklyn M.
      • Grochowska P.
      • Dunscombe P.
      PD-0384: The XX Postal TLD Audit Programme: analysis of 10,660 results.
      ] not only for new, but also for established centres.
      The combined uncertainty for the secondary standard dosemeter calibration factor is ±1.5% at the 95% confidence level [
      • Lillicrap S.C.
      • Owen B.
      • Williams J.R.
      • Williams P.C.
      Code of Practice for high-energy photon therapy dosimetry based on the NPL absorbed dose calibration service.
      ]. This would equate to one standard deviation of 0.8% which closely matches the 0.7% standard deviation measured for the MV beams. An even closer match is seen for the 2003 MeV CoP which provides an uncertainty of 1.5% at the 95% level, corresponding to a standard deviation of 0.8%, which is that measured for the MeV results following the 2003 CoP.
      It can be seen from the analysis of the 1996 and 2003 MeV CoP data (Table 1) that changing from an air kerma to an absorbed dose to water based CoP has reduced the difference in dose measurements in electron beams between clinical radiotherapy centres and NPL. A common misconception is that there is greatest uncertainty for low energy electron beams. There was no significant difference observed between measured values, or their variation for different beam energies within any beam type which could be derived from this work. The change in MeV CoP from that based on air kerma to absorbed dose has improved consistency when implemented within a clinical environment, reducing the mean variation of the measured NPL:Host outputs by 0.6% (p < 0.01).
      In the UK the implementation of a new code is recommended to be taken up by users within 3 years of implementation. During NPL audit visits it has been found that in some cases this is not achieved, indeed there have been occasions noted where the “new” code has not been implemented for some time including two cases where the 1990 MV code and the 2003 electron code were not implemented for 10 years. On another occasion one department implemented the 2003 electron code of practice in good time but admitted they had never got round to implementing the 1996 code. It is sometimes cited that this is due to a desire to remain consistent within the department, but this is then at the detriment of consistency on a national perspective. Time pressure on staff and availability of machine time is also given as a barrier to prompt implementation. Whilst it is imperative that patient treatments are the priority, there is on occasion a lack of understanding between staff groups regarding the needs of all the professions in ensuring the delivery of best practice in patient care.
      These audits have been carried out by a small group of NPL staff over a 20 year period as a result the procedures and practices during these audits have remained consistent over that time. Further to measuring the beam quality and absolute dose, which are reported to the host centre through a written audit report, there are additional benefits worth noting such as being able to check the host’s temperature and atmospheric pressure readings against NPL’s own calibrated instruments.
      A more subtle benefit of “on site” audit is that there is time for informal discussion of practices, in which potential problems may be identified and these issues clarified or pre-empted. This is not easy through postal forms of audit. While not formally provided in the results, these discussions on general practice are a significant contributory factor in ensuring the robustness of the complete dosimetry chain from primary standard to patient dose delivery. These discussions also provide a valuable opportunity to observe and understand how the implementation and interpretation of recommendations have been conducted within individual departments. There is also great value in receiving feedback from the end user community regarding the clarity (or lack of) in the CoP or other recommendations regarding complexities and issues that may not have been identified when a document was written. For example the 1990 MV photon code of practice was undoubtedly significant in being the world’s first absorbed dose based protocol, further, it is often held up as an example of how simply a code can be written. However this is a double edged sword, as recommendations of reference conditions were open to interpretation, and variations therefore exist in the adopted reference conditions particularly regarding source to detector distance and field size at the chamber.
      The requirement for more clinically relevant quantities to define dose has led to the development and improvement of primary standards away from air kerma based protocols to absorbed dose to water. There is a continued requirement to drive further improvement through the development of absorbed to dose to medium protocols and through primary standards that closer reflect the effect of radiation on the tissue at the cellular or even DNA level [
      • Galer S.
      • Hao L.
      • Gallop J.
      • Palmans H.
      • Kirkby K.
      • Nisbet A.
      Design concept for a novel SQUID-based microdosemeter.
      ]. For this work to have impact there must be a rigorous, consistent and thorough method of dissemination from the primary standard via dedicated CoPs and designated secondary standard instrumentation. On occasion issues have arisen between mismatched labels on equipment and calibration certificates; better practice is to check the manufacturer’s unique serial number stamped on to the instrument. As part of this dose traceability chain, audit is a crucial part in ensuring the accurate and consistent implementation of these systems.
      This continued improvement in relevant quantity and the precision and accuracy of dose measurement feeds into clinical trials helping to give a clearer picture of patient outcome. With the increasing complexity of treatment techniques both from linacs and indeed particle therapy there is a need to ensure ever more consistency across departments and between techniques and modalities in ensuring clear and beneficial results from patient clinical trials. Consistency and comparability is also very relevant with the increased interest in interrogating large data sets.
      Continued development of UK primary standards for radiotherapy, implementation of specific guidance through recommendations and codes of practice combined with an extensive regional and national audit network provide a foundation for the implementation of safe and consistent radiotherapy dosimetry and ultimately for patient benefit.

      Conflicts of interest

      The authors declare to have no conflict of interest.

      Acknowledgements

      Audit is very much a collaborative process and the authors would like to extend their thanks and gratitude to NPL staff who have worked on the audit program and to our clinical colleagues who have welcomed us into their departments. All have contributed to the continued improvement of audit and hence radiotherapy provision within the UK. We would like to acknowledge funding from the National Measurement Systems.

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