Physics and Imaging in Radiation Oncology (2017) 1–5 Contents lists available at ScienceDirect Physics and Imaging in Radiation Oncology journal homepage: www.elsevier.com/locate/phro Original Research Article National audit of a system for rectal contact brachytherapy Laia Humbert-Vidan a,⇑, Thorsten Sander b, David J Eaton c, Catharine H Clark b,c,d a Department of Medical Physics, St Thomas’ Hospital, London, UK Radiation Dosimetry Group, National Physical Laboratory, Teddington, Middlesex, UK c National Radiotherapy Trials QA (RTTQA) Group, Mount Vernon Hospital, Northwood, Middlesex, UK d Department of Medical Physics, Royal Surrey County Hospital, Guildford, Surrey, UK b a r t i c l e i n f o Article history: Received 12 July 2016 Received in revised form December 2016 Accepted December 2016 Keywords: Contact brachytherapy Electronic brachytherapy Audit a b s t r a c t Background and purpose: Contact brachytherapy is used for the treatment of early rectal cancer An overview of the current status of quality assurance of the rectal contact brachytherapy systems in the UK, based on a national audit, was undertaken in order to assist users in optimising their own practices Material and methods: Four UK centres using the Papillon 50 contact brachytherapy system were audited Measurements included beam quality, output and radiation field size and uniformity Test frequencies and tolerances were reviewed and compared to both existing recommendations and published reviews on other kV and electronic brachytherapy systems External validation of dosimetric measurements was provided by the National Physical Laboratory Results: The maximum host/audit discrepancy in beam quality determination was 6.5%; this resulted in absorbed dose variations of 0.2% The host/audit agreement in absorbed dose determination was within 2.2% The median of the radiation field uniformity measurements was 2.7% and the host/audit agreement in field size was within mm Test tolerances and frequencies were within the national recommendations for kV units Conclusions: The dosimetric characterisation of the Papillon 50 was validated by the audit measurements for all participating centres, thus providing reassurance that the implementation had been performed within the standards stated in previously published audit work and recommendations for kV and electronic brachytherapy units However, optimised and standardised quality assurance testing could be achieved by reducing some methodological differences observed Ó 2017 The Authors Published by Elsevier Ireland Ltd on behalf of European Society of Radiotherapy & Oncology This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/) Introduction Regular dosimetric intercomparison has been undertaken in the UK for the past 30 years [1] During this time audit groups in the UK have been developing and improving audit programmes with the aim of reducing the practice variability between radiotherapy departments [1,2] and maintaining quality standards across the country An independent audit is especially useful when implementing new techniques for which commissioning and quality assurance guidelines or recommendations are not yet in place In 2015 the National Institute of Health and Care Excellence (NICE) issued guidance on safety and efficacy of the rectal contact ⇑ Corresponding author at: Department of Medical Physics, St Thomas’ Hospital, Guy’s and St Thomas’ NHS Foundation Trust, Westminster Bridge Road, London SE1 7EH, UK E-mail addresses: laia.humbert-vidan@nhs.net (L Humbert-Vidan), thorsten sander@npl.co.uk (T Sander), davideaton@nhs.net (D.J Eaton), catharine.clark@nhs net (C.H Clark) brachytherapy technique from a clinical perspective [3] However, as far as we know, there is currently no guidance on equipment quality assurance testing Electronic brachytherapy devices represent a 15% of the kV treatment units in the UK [4] The aim of this audit was to perform a dosimetric intercomparison of the different centres and to provide an overview of the current practice in quality assurance of the systems used for rectal contact brachytherapy in the UK in order to assist current and future users to optimise their own practices as well as to establish a methodology and tolerances for future audits A contact brachytherapy system was released in 2008 for the treatment of early rectal cancer It is used for conservative treatment as an alternative to radical surgery for patients at a higher anaesthetic risk or who are willing to accept a higher recurrence risk in order to avoid a permanent colostomy [5] Contact radiotherapy can also be used as adjuvant radiotherapy to local resection, with 50 Gy usually delivered in fractions, or as a boost to external beam radiotherapy, with 90–110 Gy delivered in fractions [6] http://dx.doi.org/10.1016/j.phro.2016.12.001 2405-6316/Ó 2017 The Authors Published by Elsevier Ireland Ltd on behalf of European Society of Radiotherapy & Oncology This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) 2 L Humbert-Vidan et al / Physics and Imaging in Radiation Oncology (2017) 1–5 10% mm 5% c a b pre-tx = pre-treatment; d = daily; m = monthly; m = 6-monthly; a = annually; c = at commissioning Tolerances stated as per ‘investigation/suspension’ levels Funct = functional Qual = qualitative Applicator factor measurement was not performed by centres where separate calibration factors where input into the Papillon system for the different applicator sizes qual 5% mm 3% mm qual 0.5 mm – a a m a m m m m 6m 6m Radiation field checks Field size Field uniformity a a 3%/10% 3% 10% – 5%/3% 3% 10% 2% 3%/5% n/a 5% – 2%/5% 2% 2% 1% 3%/5% n/a 10% 3–5% 2%/3%b n/a 2% 0.5%–5% pre-tx/a a a – d/m a m m pre-tx/m n/a m – pre-tx/m m m ca pre-tx/m n/a m m pre-tx/m n/aa ma m Dosimetric checks (ionisation chamber) Radiation output Dw (constancy/absolute) Applicator factorsc HVL constancy Linearity Funct Funct Funct Funct Funct Funct pre-tx d/m pre-tx/m pre-tx/m pre-tx/m m Safety checks Interlocks and switches System calibration and applicator factors Warnings and beam status indicator Timer Funct pre-tx pre-tx pre-tx pre-txa Mechanical checks Papillon unit Patient trolley and support frame Int and ext.cameras A B C D da/ma IPEM 81 [13] pre-tx Functb Funct Funct Funct Funct IPEM 81 [13] D C B A Tolerance Centres Eaton [6] Centres Peak tube potential and first half-value layer (HVL1) are the recommended beam quality specifiers for very low energy X-ray beams, such as that produced by the Papillon 50 unit The IPEMB code of practice (CoP) for the determination of absorbed dose for Xrays below 300 kV generating potential [14] recommends scatter free and narrow beam geometry for the HVL measurement Each centre had designed their own custom-built HVL jig (see Table and Fig in Supplementary material) to achieve such measurement conditions; the audit HVL jig was borrowed from centre C A PTW type 23342 0.02 cm3 soft X-ray thin-window secondary standard parallel plate ionisation chamber calibrated in terms of air kerma and a calibrated Scanditronix Wellhofer type Dose electrometer were used All centres used the same ionisation chamber model and all the equipment was calibrated, traceable to the national standard Temperature and pressure were measured with a Digitron handheld thermometer type 2024T and a Greisinger electronic barometer model GTD 1100, respectively Six 99.999% purity aluminium filters were customised for this audit and their thicknesses measured at NPL with a calibrated coordinate measuring machine; the standard deviation of the thickness measurements ranged from approximately 0.002 mmAl to 0.003 mmAl for the thinnest (0.0571 mmAl) and thickest (1.039 mmAl) filters, respectively The audited centres used their own Al filters for their measurements Exposures of 500 MU were performed with increasing levels of attenuation using the aluminium filters Repeat readings were corrected for temperature and pressure and the mean value plotted against the total thickness of added aluminium The HVL value was derived from a second-degree polynomial fit and compared to the host HVL value The effect of host-audit HVL discrepancies on the determination of absorbed dose to water was assessed Frequency 2.1 Beam quality (HVL) Table Summary of QC tests, frequencies and tolerances and comparison to IPEM 81 [13] and to Eaton’s review on electronic brachytherapy [6] Four centres participated in the audit, with Papillon 50 contact brachytherapy systems (Ariane Medical Systems, Ltd, Derby, UK) commissioned between 2009 and 2014 and a workload of 1–30 patients per month A ‘single auditor’ approach with on-site visits was taken as a more consistent and simplified analysis methodology was easier to achieve with centrally organised audits [7–9] Treatment with the Papillon 50 is delivered with a hand-guided X-ray tube that produces a 50 kVp and approximately 2.7 mA beam with dose rates as high as 15 Gy/min Electrons are accelerated towards a rhenium transmission target and photons are produced isotropically The focus-to-surface distance (FSD) of the applicators (29, 32 and 38 mm) varies with applicator diameter (22, 25 and 30 mm, respectively) in order to achieve a collimated beam with a fixed opening of 45° [6,10,11] The audit measurements included beam quality, radiation output, and radiation field size and uniformity A comparison between host and audit measurements was made, with a discussion of the significance of the differences observed The National Physical Laboratory (NPL, Teddington) provided external validation of the procedures during the visit to the first audited centre [8] Most of the dosimetry equipment used was provided by NPL, thus direct traceability for all audit results to the national standard was ensured In addition, constancy checks using a strontium check source were carried out on the ionisation chamber by NPL before the first visit and after the last visit of the audit A review was carried out on the quality assurance programme documentation provided by all centres [12]; this included tolerances and frequencies of tests following their respective ISO 9000 Quality Systems A comparison was made (Table 1) to IPEM 81 recommendations [13] and to a recent review on electronic brachytherapy [6] Eaton [6] Materials and methods L Humbert-Vidan et al / Physics and Imaging in Radiation Oncology (2017) 1–5 2.2 Radiation output All Papillon units were calibrated to deliver 30 Gy for a 3000 MU exposure The absorbed dose to water at the surface of a full-scatter water-equivalent phantom (Dw) was determined by applying the formalism from the IPEM very low energy (0.035–1.0 mmAl) CoP [14,15] The calibration coefficients for the chamber and electrometer were obtained from the respective calibration certificates by NPL, with a quoted uncertainty of 1.2% (k = 2) for the chamber The host’s quoted HVL value was used to approximate the relevant mass energy coefficient ratio in air, [(len/q)w/air]air, and the chamber correction factor, kch, to the nearest values on the reference tables [14–16] The audit measured HVL value was not used to calculate the radiation output in order to exclude any uncertainties or differences due to the HVL jig design By using the host’s HVL value, the only differences in measurement setup/equipment were those related exclusively to the output measurement NPL use a radiation field with circular crosssection and 53 mm diameter whereas this audit used the 30 mm applicator; Perrin et al (2001) [17] have shown that there is no change in kch with field size for the PTW 23442 chamber below a 40 mm diameter field size They also state that the kch is not expected to change with a variation in FSD A poly methyl methacrylate (PMMA, Perspex) output jig was originally designed by centre A (Fig 1) Based on that design, Ariane produced their own polyoxymethylene (POM, Delrin) jig for output and film measurements, which is now provided with the Papillon 50 unit (Fig in Supplementary material) For convenience, audit measurements were carried out in this output jig instead of a full-scatter phantom A comparison was made by centre A between absorbed doses measured in a full scatter phantom and in the output jig to assess the uncertainty introduced by such non full-scatter conditions Centres measured the temperature differently: be it room temperature, chamber temperature or output jig temperature The latter was measured by centre B by means of a custom-built POM jig, which was fitted into the chamber slot within the output jig The effect of these differences on the resulting radiation output was investigated Overall uncertainty in HVL (1.7%) and Dw (2.1%) determination was derived from the root-mean-square of the following estimated uncertainties [12]: measurement reproducibility (1.0% and 0.3% for HVL and Dw, respectively), measurements of temperature (0.2%) and pressure (0.1%), electrometer precision (0.04%), Al filter thickness (1.7%), effect of HVL set up variation on determination of Dw (0.2%) and electrometer (0.2%) and chamber (1.2%) calibrations 2.3 Radiation field size and uniformity For each applicator size two pieces of RTQA Gafchromic film were exposed to Gy and Gy, respectively; dose linearity was assessed with charge readings at increasing MU levels to deter- mine the required MU for doubling the dose level for the second film exposure Films were scanned one week after their exposure using an Epsom type 11000 Pro flatbed scanner (professional mode, reflective scan, 48-bit colour, 96 dpi) Most centres used the jig with the approximately cm thick backscatter block flush against the film rather than using the gap to mark the film orientation Centre B used a thicker custom-built backscatter block (Fig in Supplementary material) The different film exposure methods were compared Film was analysed by the audit team using a bespoke ImageJ macro [18], which included measurements of field size and radiation field uniformity The Gy film was used to measure the field size based on the 50% dose threshold defined by the Gy film The reproducibility of the audit ImageJ macro field size analysis was within pixel (0.25 mm) Radiation field uniformity was defined by the background-corrected ratio of the pixel value (PV) at the four cardinal coordinates (above, below, left and right) to that at the centre of the field [18] Host film analysis methods varied from a uniformity calculation using the ratio (PVmax À PVmin)/ (PVmax + PVmin), to a more comprehensive bespoke Matlab routine that used a 2-channel (red and blue) analysis of the film and produced 2D horizontal and vertical profiles, a surface plot (3D profiles) and a 2D relative dose map Results 3.1 Beam quality (HVL) All audit and host HVL measurements were within 10% of the baseline value established at commissioning by each centre Differences in HVL jig design (see Supplementary material) and measuring equipment resulted in audit/host discrepancies of up to 6.5%, 0.04 mmAl, in the measured HVL When assessing the effect of such HVL discrepancies in the determination of Dw only a 0.2% variation was observed (Fig 2) 3.2 Radiation output The main difference between the CoP recommendations and the host and audit output measurements was the use of the Ariane POM jig instead of measuring at the surface of a full scatter water-equivalent phantom Based on the comparison made by centre A, the averages of the readings with the ionisation chamber in a full scatter block and in the output jig were within 0.4% of each other; this difference was not significant given the 0.8% relative standard deviation for the combined set of measurements and therefore no additional correction factor to the chamber factor, kch, was introduced All absorbed dose to water measurements (both audit and host) were within ±2% of the expected value (30 Gy for a 3000 MU exposure) An agreement of better than 2.2% between auditor and host measurements was observed (Table 2) Strontium constancy checks before and after the audit agreed within 0.13% Differences of up to °C or 0.4 °C were observed post-exposure between room temperature and chamber or jig temperatures, respectively This difference resulted in a 0.2% and 0.1% variation in calculated radiation output, respectively No differences were observed pre-exposure between temperature measurement methods 3.3 Radiation field size and uniformity Fig Original output jig design; it is made of PMMA instead of POM and the chamber is inserted from below Host-audit field size measurements agreement was within mm and both within mm of the specified values Using the ImageJ macro, the median of the measured field uniformity in L Humbert-Vidan et al / Physics and Imaging in Radiation Oncology (2017) 1–5 Fig Graphical representation of the relative differences in HVL measurement between host and audit compared to the resulting differences in absorbed dose to water, Dw, derived from variations in the selection of factors and coefficients for its calculation Table Absorbed dose to water, Dw, determined independently by host and audit for 3000 MU HVL (mmAl) Mass en abs coef ratio kch Dw (Gy) Audit vs host (% dif.) Centre A Audit Host 0.635 1.025 1.06 29.85 30.08 À0.8% Centre B Audit Host 0.537 1.027 1.05 30.10 29.45 +2.2% Centre C Audit Host 0.508 1.027 1.05 30.10 29.49 +2.1% Centre D Audit Host 0.603 1.026 1.06 30.13 29.77 +1.2% terms of the relative difference between the field periphery and the field centre mean pixel values was 2.7% The back section of the output jig that included a cm thick backscatter block was the preference in all centres for film measurements Field size measurements were up to 1% more accurate when positioning the jig such that the backscatter block was flush against the film Average uniformity across the field size was improved by up to 1% with increased backscatter, i.e using centre B’s custom-built backscatter block 3.4 Procedural audit All centres had a well-established and documented Quality System in place and were generally in agreement with the QC testing frequency and tolerances given by the IPEM 81 recommendations for kV units and the practice followed by other electronic brachytherapy users (Table 1) The clinical practice workload varied across centres from less than (centre B) to more than (centre A) patients per week Some correlation between confidence in the machine performance and test frequency or tolerance levels was observed In some cases tolerance levels were also linked to the test methodology accuracy, for instance in film analysis Discussion All measurements, both by the host centres and the audit team, were well within the IPEM 81 recommended tolerances for kV units [13] and the tolerances used by other electronic brachytherapy users [6] The UK CoP [14] recommends narrow beam geometry for HVL measurements A kV regional audit of 70–300 kV in 2008, stated an acceptable audit/host agreement limit of ±3.0% [8,9] However, the rapid dose fall-off of the Papillon 50 kV beam introduces more difficulties in the measurement of the HVL and, as shown by the results from this audit, a larger acceptable agreement limit should be considered at such low energies A compromise is required between radiation scatter to the chamber, minimised with long applicator end to chamber window distances, and exposure times required for a reasonable signal-to-noise ratio Centre B used a custom-built POM applicator to increase the distance; this is in favour of a reduction of natural beam scatter but could be seen as not representative of the clinical scenario Measurements with the chamber in PMMA provide better positioning stability but could introduce worse scatter conditions than in-air measurements Smaller beam collimator diameters in the lead plate, used to reduce scatter, could on the other hand increase electron contamination to the chamber readings Dose rate and depth dose increase with applicator size due to increased scatter contribution [10]; we recommend that an HVL jig be designed to allow investigation of the differences among all clinical field sizes Centre B excluded the machine ramp-up period from the HVL measurements by measuring charge for a fixed period of time while the kV was stable This could have contributed to the host HVL being higher than that measured by the audit team Due to a machine overheating history at centre C, it was decided at the commissioning that HVLs would be measured with exposures of 500 MU; the audit team followed this approach However, all other centres used exposures of 3000 MU, which potentially resulted in a higher signal-to-noise ratio thus reducing the uncertainty in the charge reading The beam profile is flat at the surface of the applicator end; however, due to the inherent rapid dose fall-off of the beam, beam hardness varies across the field size at a distance with the HVL at the periphery being lower than that at the centre If HVL L Humbert-Vidan et al / Physics and Imaging in Radiation Oncology (2017) 1–5 measurements are carried out following the UK CoP [14], the resulting measured HVL could be higher than the mean HVL across the treatment field size We recommend that a comparison be made between the HVLs at the field centre and the periphery with an assessment of the impact on the absorbed dose to water determination The 2008 kV regional audit protocol states an acceptable host/audit radiation output agreement limit of 3.0% [8,9]; the same limit could be applied for the Papillon 50 Different temperature measurement methods did not introduce significant variations in calculated radiation output However, we recommend that temperature be measured as close to the chamber position as possible A custom-built jig fitted in the chamber slot within the output jig provides reproducibility of the measurement technique Even though all centres produced radiation field size and uniformity results within recommended tolerances [13], the large variation between film uniformity analysis methods did not allow for a direct host-audit comparison All Papillon users would benefit from an advanced film analysis providing 3D profile maps As the energy independence of Gafchromic film has been doubted at low energies [11,19], we recommend that film calibration be performed at the same energy range as that of the Papillon radiation beam for its use in dosimetric measurements The dosimetric characterisation of the Papillon 50 was validated with the audit results for all participating centres, thus providing reassurance that the implementation had been performed within the standards stated in previously published audit work and recommendations for kV and electronic brachytherapy units However, it has also highlighted differences across the audited centres, especially for the measurement of beam quality and the analysis of field size and uniformity This audit should be considered as a starting point for the development of national and international guidelines for an optimised and standardised quality assurance testing of the Papillon 50 unit Acknowledgements We would like to thank the staff from the following hospitals for their cooperation and their kind hosting of this audit: Royal Surrey County Hospital, Guildford; Clatterbridge Cancer Centre, Liverpool; Nottingham University Hospital, Nottingham and Castle Hill Hospital, Hull We would also like to thank the National Physical Laboratory, the NCRI Radiotherapy Trials Quality Assurance group and St Thomas’ Hospital for providing the equipment used in this audit We would like to acknowledge Matthew Bolt from Royal Surrey County Hospital for producing the film analysis Image J macro Finally, we would like to acknowledge Ariane Medical Systems Ltd for funding and supporting this audit Appendix A Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.phro.2016.12.001 References [1] Clark CH, Aird EGA, Bolton S, et al Radiotherapy dosimetry audit: three decades of improving standards and accuracy in UK clinical practice and trials Br J Radiol 2015;88:20150251 [2] Eaton DJ, Bolton S, Thomas RAS, Clark CH Interdepartmental dosimetry audits: development of methods and lessons learned J Med Phys 2015;40:183–9 [3] National Institute for Health and Care Excellence Low energy contact X-ray brachytherapy (the Papillon technique) for early stage rectal cancer: Interventional procedure guidance; 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