ARTICLE IN PRESS

Applied Radiation and Isotopes 67 (2009) 419–422

Contents lists available at ScienceDirect

Applied Radiation and Isotopes

0969-80

doi:10.1

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journal homepage: www.elsevier.com/locate/apradiso

Dosimetry of the microSelectron-HDR Ir-192 source using PRESAGETM andoptical CT

P. Wai a,�, J. Adamovics b, N. Krstajic a, A. Ismail c, A. Nisbet a,c, S. Doran a

a Department of Physics, School of Electronics & Physical Sciences, University of Surrey, Guildford, Surrey, UKb Heuris Pharma LLC, Skillman & Rider University, Lawrenceville, NJ, USAc Department of Medical Physics, Royal Surrey County Hospital, Guildford, Surrey, UK

43/$ - see front matter & 2008 Elsevier Ltd. A

016/j.apradiso.2008.06.038

esponding author.

ail address: P.Wai@surrey.ac.uk (P. Wai).

a b s t r a c t

Optical CT, using a solid polyurethane (PRESAGETM) radiochromic dosimeter, has been used to evaluate

dose distributions produced by the microSelectron-HDR Ir-192 source. The anisotropy functions

obtained through optical CT are in good agreement with Monte Carlo and previously published results

especially at polar angle above 201. The results indicated an evident potential for using solid polymer

dosimetry as an accurate method for 3-D dosimetry, although refinements to the existing methods are

necessary before the technique can be used clinically.

& 2008 Elsevier Ltd. All rights reserved.

1. Introduction

The microSelectron-HDR Ir-192 source manufactured byNucletron (Nucletron, Neenendaal, The Netherlands) has a half-life of 73.83 days and emits high-energy photons typicallybetween 201 and 884 keV with average energy of 370 keV. Thesource is commonly used in oncology centres worldwide inconjunction with remote afterloaders and its dosimetry has beenextensively studied both experimentally and via Monte Carlosimulation (Nath et al., 1995). However, distances close to thesource are difficult to measure practically, due to the high dosegradients. This makes the microSelectron-HDR an ideal modelsystem for comparing the accuracy and precision of 3-D dosimetryvia optical CT with well-established data, while at the same timedemonstrating the additional information that can be provided bythis new modality.

2. Material and methods

2.1. Dosimetry of microSelectron-HDR Ir-192 source

The Dosimetry of Interstitial Brachytherapy Sources Report(Nath et al., 1995), recently updated (Rivard et al., 2004), wascompiled by Task Group 43 of the Radiation Therapy Committee ofthe American Association of Physicists in Medicine (AAPM) toreview publications on the dosimetry of interstitial brachytherapysources. The report includes recommendations for a dosimetry

ll rights reserved.

protocol together with formalism for dose calculations and adataset for the values of dosimetry parameters. The report definesthe necessary physical quantities for dosimetry of brachytherapysources. One of the parameters is the anisotropy function, F(r, y),defined in Eq. (1). It describes the variation in dose as a function ofradial distance r from the source centre and polar angle y withrespect to the longitudinal source axis.

Fðr; yÞ ¼_Dðr; yÞ_Dðr; y0Þ

GLðr; y0Þ

GLðr; yÞ. (1)

The plane corresponding to (r, y0) is the transverse bisector ofthe source y0 ¼ 901. _Dðr; yÞ is the dose rate at (r, y), GL(r, y) is thegeometry function, which takes account of the non-point-sourcenature of the source. Note that the source is axially symmetric, sothe dose rate, geometry function and anisotropy function are thesame for all azimuthal angles. F(r, y0) is unity by definition, on thetransverse plane but decreases as y approaches 01 or 1801, asencapsulation thickness increases, and as photon energy de-creases.

2.2. Current published results

The microSelectron-HDR used in interstitial brachytherapy hasbeen well documented both theoretically and experimentally.Nath et al. (1993) measured the anisotropy function of a Ir-192source (Best Industries) in 1993 in a solid water phantom using LiFTLD dosimeters; the results were published in the AAPM TG-43(Nath et al., 1995). Other authors used experimental methods suchas ionisation chambers (Mishra et al., 1997), LiF TLD rods(Karaiskos et al., 1998; Kirov et al., 1995; Muller-Runkel andCho, 1994), diodes (Kirov et al., 1995), MOSFETS (Zilio et al., 2006),

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Monte Carlo calculations (Williamson and Li, 1995; Kirov et al.,1995; Daskalov et al., 1998; Karaiskos et al., 1998; Papagianniset al., 2002; Lymperopoulou et al., 2004; Sureka et al., 2006) andradiochromic films (Sureka et al., 2007; Sharma et al., 2004) toobtain the dose distribution.

There are different versions of microSelectron-HDR, the classic/old design and the new design. The classic/old design has anIr-192 core with size 0.6�3.5 mm while the new design is slightlylarger at 0.65�3.6 mm. In this study, the new design was used.

Most published dosimetric results include measurements atdistances larger than 1 cm. Monte Carlo calculations and radio-chromic films were able to provide dose distribution closer than1 cm. For intravascular brachytherapy, the AAPM TG-60 suggestsdosimetric data should be evaluated for distance below 5 mm.Distances less than 5 mm are difficult to measure practically dueto the high dose gradients (Nath et al., 1999).

2.3. Monte Carlo simulations

An MCNP Monte Carlo simulation (Los Alamos NationalLaboratory, MCNP4C) (Briesmeister, 2000)) was conducted tosimulate the equivalent set-up of the Ir-192 implant experimentdescribed in later sections. The source model was based on theMicroSelectron HDR (new design, Nucletron). The new designdimensions were derived from mechanical drawings published byDaskalov (Daskalov et al., 1998) originally supplied by the vendor(Nucletron). For the calculation of anisotropy functions, tally cellswere placed from 01 to 901 at 101 intervals for distances 1 and2 cm from the source centre. To obtain the dose profile at aparticular slice, tally cells were placed along lines situated at 7and 11 mm from the centre of the Ir-192 source and perpendicularto the long axis of the source. By performing the simulation usingboth water and PRESAGETM as the material in which the seed wasplaced and using the elemental composition of PRESAGETM, it wasfound (data not shown) that the two situations yield anisotropyfunctions that differ by only 1.5% or less. Hence, performing anexperimental irradiation in PRESAGETM is expected to give results

Fig. 1. Optical CT experimental: (a) PresageTM dosimeter with catheter in position, (b

loading system with PresageTM dosimeter, (d) irradiated PresageTM dosimeter showing

for the anisotropy function that can be directly compared with thepreviously measured anisotropy function in water and is thussuitable for predicting the dose distribution in tissue.

2.4. Optical CT analysis

Previous polymer gel dosimetry of Ir-192 studies (Deene et al.,2001) have not been totally successful, due to oxygen contamina-tion and other complications. The use of PRESAGETM was expectedto eliminate these problems. The solid polyurethane-basedPRESAGETM (Adamovics and Maryanski, 2004) is a new type ofdosimeter that is transparent, rigid and easily machineable, isstable during irradiation period, and has good post-irradiationstorage properties. It has a linear response at low energies andis insensitive to oxygen, unlike many previous polymer gels (Guoet al., 2006).

A hole, 2 mm in diameter and 2.5 cm in depth was drilled in aPRESAGETM solid dosimeter (batch #181), providing a channel tosecure a source catheter. An Ir-192 cable-driven source of strength1.692 cGy m2 h�1 (4.7 cGy cm2 s�1), equivalent to an activity of153.3 GBq (4.14 ci), was inserted via the prepared catheter andallowed to reside in the channel for a total irradiation time of300 s. This translates to an approximate dose of 13.5 Gy at 1 cmfrom the source centre and 2.7 Gy at 2 cm. Fig. 1 shows variousaspects of the experimental arrangement.

After 12 h, to allow for any post-irradiation colour develop-ment effect (Guo et al., 2006), the optical density at 633 nm wasmeasured in 3-D, via our in-house optical CT system (Krstajic andDoran, 2006). Four hundred projections were acquired with afield-of-view of approximately 6.4�6.4 cm2 and reconstructed byfiltered back-projection into images of matrix size 256�256.A direct calibration of the dose–response function for this batch ofPRESAGETM was not performed, because of the difficulty ofensuring accurate calibration doses using the relevant photonenergy. However, it was verified experimentally using a 6 MV linacthat the dose–response is linear. Thus, it is assumed that theoptical CT data represent a 3-D map of relative dose absorbed by

) flexi-catheter connected to delivery wire, (c) microSelectron-HDR remote after-

colour changes.

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the PRESAGETM. Subsequent analysis of the 3-D data yielded theanisotropy function. Previous experiments (Wai et al., 2006) usinga I-125 interstitial brachytherapy implant suggest that resultswith accuracy of order 5% may be achieved via this technique.Previous studies of the Ir-192 seed suggest that an improvedexperimental determination of the anisotropy function at lowpolar angles would be useful.

0.8

0.9

1.0

1.1

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EBT Film

3. Results and discussions

Fig. 2(a) shows the PRESAGETM sample in position in theoptical CT scanner. Fig. 2(b) shows the raw projection data, whileFigs. 2(c) and (d) give a typical output image through the centre ofthe seed and the corresponding line profile.

Fig. 3 compares the anisotropy functions obtained fromMonte Carlo simulation and experiment. Errors are of order0.015 (1.5%) for polar angles 01oyo901 at 1 cm from the centre ofsource, and 0.019 (1.9%) at 2 cm. The optical CT measuredanisotropy functions were in good agreement with Daskalov(Daskalov et al., 1998) especially at polar angles above 101. As asuitable measure of the success of this study, the results ofDaskalov’s Monte Carlo study over the range 101oyo801 wasused to compare with the optical CT results, the mean value ofjDaskalovðyÞ � OCTðyÞj was 2.1% at 1 cm from source centre andjDaskalovðyÞ �MCNPðyÞj was 0.4%.

0.5

0.6

0.7

0

Polar Angle (°)

Ani

sot

Optical-CTMCNPDaskalov

80604020

Fig. 3. The anisotropy function F(r,y) for r ¼ 1 cm, as found from the optical CT

experiment, MCNP Monte Carlo simulation and the literature.

4. Conclusions

In this study, optical CT with PRESAGETM and MCNP MonteCarlo simulation have been used to determine the anisotropyfunctions of a microSelectron-HDR Ir-192 source commonly usedin HDR brachytherapy.

The 3-D MCNP Monte Carlo simulations were based uponthe AAPM TG-43 recommendations, manufacturer and previously

Arb

itrar

y un

its

0.015

0.010

0.005

0.000–2

a

c

Fig. 2. Optical CT results: (a) photo of irradiated PRESAGETM dosimeter side view inside s

400 projections over 1801; (c) optical CT single slice reconstruction, pixel size—0.25 mm

and artifact correspond to position of drilled hole.

published source geometry, photon energy spectrum and photoncross-sections. For experimental data, the anisotropy functionswere calculated from a 3-D optical CT. The calculated andmeasured anisotropy functions were in good agreement withDaskalov’s results especially at polar angles above 101. For polarangles in the range 301oyo801, the percentage of standarddeviation between results from this study and other authorswas below 2.6% at 1 cm from source centre and below 1.3% at2 cm.

The results of this study indicated an evident potential of theutilisation of PRESAGETM with optical CT as an accurate 3-Dmethod of dosimetry for characterising the anisotropy functionsfor Ir-192 HDR source.

0 –10 0 10 20Length (mm)

b

d

canning glass cell; (b) optical CT projection image of irradiated PRESAGETM, total of2, slice thickness—0.25 mm; (d) line profile across reconstructed image; central dip

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Acknowledgements

The research was supported by United Kingdom Engineeringand Physical Sciences Research Council (EPSRC) research student-ship. We wish to thank Nikola Krstajic, University of Surrey,Guildford, UK, for developing the optical CT system; Abdul Ismailand Christopher Bunton from Royal Surrey Hospital, Guildford,UK, for their expertise and practical support in experiments; andAndy Ma, the Institute of Cancer Research, London, UK, for hisexpert input in Monte Carlo simulations.

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