national audit of a system for rectal contact brachytherapy

Measurements included beam quality, output and radiation field size and uniformity.. Conclusions: The dosimetric characterisation of the Papillon 50 was validated by the audit measuremen

Original Research Article

National audit of a system for rectal contact brachytherapy

Laia Humbert-Vidana,⇑, Thorsten Sanderb, David J Eatonc, Catharine H Clarkb,c,d

a

Department of Medical Physics, St Thomas’ Hospital, London, UK

b 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

a r t i c l e i n f o

Article history:

Received 12 July 2016

Received in revised form 4 December 2016

Accepted 4 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 over-view 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 1 mm Test tolerances and frequencies were within the national recommenda-tions 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 elec-tronic 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 &

licenses/by-nc-nd/4.0/)

1 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

imple-menting 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

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 repre-sent 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 qual-ity 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 tol-erances 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 anaes-thetic 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 3 fractions, or as a boost to external beam radiotherapy, with 90–110 Gy delivered in 3 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.

⇑ 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).

Contents lists available atScienceDirect

Physics and Imaging in Radiation Oncology

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / p h r o

2 Materials and methods

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

methodol-ogy 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

out-put, 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

car-ried out on the quality assurance programme documentation

pro-vided 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]

2.1 Beam quality (HVL)

Peak tube potential and first half-value layer (HVL1) are the

rec-ommended 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

X-rays 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 (seeTable 1andFig 1

in Supplementary material) to achieve such measurement

condi-tions; the audit HVL jig was borrowed from centre C A PTW type

23342 0.02 cm3soft X-ray thin-window secondary standard

paral-lel plate ionisation chamber calibrated in terms of air kerma and a

calibrated Scanditronix Wellhofer type Dose 1 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

hand-held thermometer type 2024T and a Greisinger electronic

barome-ter model GTD 1100, respectively Six 99.999% purity aluminium

filters were customised for this audit and their thicknesses

mea-sured 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

measure-ments 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

dis-crepancies on the determination of absorbed dose to water was

assessed

a /m

Dw

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

cal-ibration 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,

near-est 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

cross-section 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 kchwith 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 2 in Supplementary material) For

conve-nience, audit measurements were carried out in this output jig

instead of a full-scatter phantom A comparison was made by

cen-tre 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

thick-ness (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 1 Gy and 2 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 2 cm thick backscatter block flush against the film rather than using the gap to mark the film orien-tation Centre B used a thicker custom-built backscatter block (Fig 3 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 radia-tion field uniformity The 2 Gy film was used to measure the field size based on the 50% dose threshold defined by the 1 Gy film The reproducibility of the audit ImageJ macro field size analysis was within 1 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 var-ied from a uniformity calculation using the ratio (PVmax 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

3 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 Differ-ences in HVL jig design (seeSupplementary material) and measur-ing 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 cen-tre 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 1°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 Host-audit field size measurements agreement was within

1 mm and both within 1 mm of the specified values Using the ImageJ macro, the median of the measured field uniformity in

Fig 1 Original output jig design; it is made of PMMA instead of POM and the

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 2 cm thick

backscatter block was the preference in all centres for film

mea-surements 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

Sys-tem 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

var-ied across centres from less than 1 (centre B) to more than 4

(cen-tre 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

4 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

brachyther-apy 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 measure-ments Smaller beam collimator diameters in the lead plate, used

to reduce scatter, could on the other hand increase electron con-tamination 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 investi-gation of the differences among all clinical field sizes Centre B excluded the machine ramp-up period from the HVL measure-ments 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 commission-ing 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

Fig 2 Graphical representation of the relative differences in HVL measurement between host and audit compared to the resulting differences in absorbed dose to water, D w , derived from variations in the selection of factors and coefficients for its calculation.

Table 2

Absorbed dose to water, D w , determined independently by host and audit for 3000 MU.

HVL (mmAl) Mass en abs coef ratio k ch D w (Gy) Audit vs host (% dif.)

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

tem-perature 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

uni-formity 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

per-formed 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

How-ever, 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

Tho-mas’ 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, athttp://dx.doi.org/10.1016/j.phro.2016.12.001 References

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