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S
Dr. S. J. Doran
Department of Physics,
University of Surrey,
Guildford, GU2 7XH, UK
High-resolution measurements of
radiation dose in 3-D using gel
dosimetry
Simon J Doran
Department of Physics
University of Surrey
Acknowledgements
•
Paul Jenneson, Mamdouh Bero, Nik Krstajic
(Physics Dept., University of Surrey)
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Phil Murphy, Mark McJury, Viv Cosgrove (RMH / ICR)
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Mark Oldham (William Beaumont Hospital, Michigan)
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Steve Hepworth (BNFL)
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Dave Bonnet (Maidstone)
Overview of Seminar
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What is gel dosimetry?
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Uses and application areas
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Introduction to MRI and optical methods
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Principal problems
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Research areas
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Report back from conference
What is gel dosimetry and why do it?
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Conventional methods of dosimetry are either single point
(e.g., TLD’s, ion chambers, etc.) or 2-D (film).
Complex radiotherapy treatments (e.g., conformal therapy,
brachytherapy, abutting fields) require 3-D measurements.
Gel dosimetry is a way of achieving this in special test
objects (phantoms) filled with a radiosensitive gel.
Monte Carlo simulation is known to be capable of high
accuracy … but … we still need experimental verification
that delivery occurred as expected.
Steps in a gel dosimetry experiment
1. Prepare heated gel and pour into container or anatomicallyshaped mould. Cool and solidify gel.
2. Irradiate with same protocol as for intended patient.
3. Image with desired modality (MRI, optical, ultrasound, CT, ...)


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MRI: original monomer has long T2,
polymer has short T2.
Optical: original monomer gel is
transparent, polymer gel is cloudy.
Ultrasound: acoustic properties of
gel change with irradiation.
Data from Maryanski et al. Med. Phys. 23(5) 699-705, 1996
Potential uses of gel dosimetry
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Gel dosimetry is not a replacement for routine QA using
ion chambers, etc.
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Machine imperfections (e.g., leakage through MLC leaves)
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Commissioning
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Treatment verification
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Accident prevention
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Special procedures and “one offs”
Application areas for gel dosimetry
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Standard therapy with 3-D planning systems
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Overlapping fields and match lines
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Stereotactic radiotherapy
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IMRT
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Brachytherapy
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Particle therapy (proton, electron, BNCT, etc.)
Physical basis of measurement: (1) Fricke gels
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Fricke solution
Fricke and Hart, 1966: standard “absolute” dosimetry method
Fe(NH4)2(SO4)2.6H2O(aq), HCl(aq), NaCl(aq)
Effect of radiation: Fe2+ + h

Fe3+ + e
Originally detected via UV spectrophotometry
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Detection by NMR
Gore et al. 1984
T1 and T2 reduced by presence of Fe3+
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Fricke gel
Numerous authors 1990-2001
Fricke solution mixed with gelatin to fix dose in space
Physical basis: (2) Diffusion problem in Fricke gels
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The Fe3+(aq) that are used to record dose in a Fricke
gel are not fixed securely enough by the gelatin.
Diffusion occurs on a timescale of a few hours and
causes a blurring of the dose profile.
t=0
[Fe3+]
t=0
t=1 hr
t>0
x
t=16 hr
Physical basis of measurement: (3) Polymer gels
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First proposed by Maryanski et al. in 1993
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Two monomers in a matrix of gelatin
Monomers + h 
long T2
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Polymer
short T2
Simple recipe with 4 constituents:
Acrylamide
N-N'-methylenebisacrylamide
Gelatin
Water
3% (by weight)
3%
5%
89%
Physical basis: (4) Polymer gel mechanism
1. Creation of free radicals by the radiation
H2O + h
HO* + H*
2. Transfer of an OH* radical to one of the co-monomers
Physical basis: (5) Polymer gel mechanism (cont.)
3. Extension of the chain by the encounter of a radical
and a further monomer unit (either of the comonomers). A new longer chain radical is formed.
*
Physical basis: (6) Polymer gel mechanism (cont.)
4. Termination of the chain by the encounter of two radicals. At
the end, a branched and cross-linked structure is formed.
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The polymerisation is limited spatially to a small region around the
site of the incident radiation.
The polymer fragment created is supported in a matrix of gelatin.
Manufacture of polymer gels
N2
N2
• The whole manufacturing process must take place in a sealed
reaction vessel, or nitrogen-filled glove-box to avoid the slightest
contact with air.
• Preparation requires considerable experience as there are a
number of problems that can lead to inconsistent results.
Example of gel dosimetry in routine treatment
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Visualisation of beam penumbra for a 4  4 cm2 field
Target 2 treatment plan
MRI result
Results from the Royal Marsden team (M. McJury, M. Oldham,
M.Leach, S. Webb), Phys. Med. Biol., 43, 1113-1132 (1998)
Example of gel dosimetry in conformal therapy
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Simulated nine-field prostate treatment using NOMOS
MIMIC device
Treatment Plan
Target
organ
MR dose map
Organs
to spare
Results from the Royal Marsden team (M. McJury, M. Oldham,
M.Leach, S. Webb), Phys. Med. Biol., 43, 1113-1132 (1998)
Gel dosimetry in brachytherapy
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No other methods are available for measuring the dose
distributions close to a brachytherapy source.
Data: M Maryanski, Ir-192 seed
Y de Deene et al. PMB 46, 2801 (2001)
Gel dosimetry in vascular brachytherapy
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No other methods are available for measuring the dose
distributions close to a brachytherapy source.
Data: Bonnett et al. DOSGEL 2001
Why look for another method?
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Although MR imaging of the gel can work very well,
there are a number of problems precluding a wide
uptake:

MRI is expensive and cannot currently be used routinely for
radiotherapy QA and planning.

MRI is relatively slow if you really need true 3-D data.

The polymer gel is difficult to make reproducibly.
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Contamination by oxygen causes the polymer gel to fail.

Absolute dosimetry is difficult.

Measurements are temperature and time dependent.
What is optical computed tomography (OCT)?
• As its name suggests, OCT relies on the detection
of radiation in the visible region, rather than X-rays.
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The principles are exactly the same as X-ray CT.
However, the properties of visible light lead to a
number of advantages and disadvantages:
 No ionising radiation
 Equipment is cheap and off-the-shelf (total cost < £10,000)
 We can use optics to manipulate the beam
 Extremely limited range of samples due to strong absorption
and scatter
 Problems of reflection and refraction to contend with
A reminder about X-ray CT
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We are all used to the idea of X-ray CT.
X-rays emitted by the source are attenuated
to varying degrees by the sample (patient).
At each detector a signal is detected that is
proportional to
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e
   ( x ) dx .
Together the signals from all the detectors
form a projection.
Acquisition of a number of projections as
the detector rotates gives a complete
dataset, from which the image is computed
by back-projection.
Physical basis for OCT: (1) Colour-change gel
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Take a standard Fricke gelatin gel and add a metal-ion
indicator sensitive to Fe3+.
Gel changes colour from orange to purple on irradiation.
Attenuation primarily by absorption
1
Change
in optical absorbance
/ cm--1
/ cm
absorbance)
D(optical
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0.4
FXG spectral
dose-response
0.2
0.0
-0.2
350
400
450
500
550
600
650
700
Wavelength / nm
Wavelength / nm
Appearance of gel
post-irradiation
Dose response of gel, with
mercury spectrum inset
Physical basis for OCT: (2) Polymer gel
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Polymer gels attenuate light primarily by scattering.
Above 500 nm, response to dose is approximately linear.
Data: M Maryanski, Ir-192 seed
Data: M Maryanski et al. Phys. Med. Biol. 41, 2705 (1996)
Physical basis for OCT: (3) PRESAGE
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PRESAGE is not a gel but a solid polyurethane.
Active ingredient is a “leuco dye”.
Data: J Adamovics, Heuris Pharma
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Attenuation occurs
primarily by absorption
and is currently optimised
for use with a He-Ne laser
(max absorption at 632 nm)
OCT: Historical perspective
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Colour-change gels introduced in 1991
(Appleby and Leghrouz, Med. Phys. 18, 309-312, 1991)
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“Pencil-beam”, laser-based systems
Typically one plane in ~15 mins. (Tarte et al. Unpublished
Gore et al. Phys. Med. Biol. 41, 2695-2704, 1996;
Kelly et al. Med. Phys. 25(9), 1741-1750, 1998)
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2-D imaging of radiation dose with CCD
(Tarte et al. Med. Phys. 24(9), 1521-1525, 1997)
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Imaging of stacked gels (Gambarini et al. DOSGEL ’99)
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First CCD tomography scanners
(Wolodzko et al., Bero et al., DOSGEL ’99)
Typically 512 planes in ~30 mins., possibly faster still
Two flavours of optical tomography
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Single-beam tomography
(Gore, Maryanski et al., 1996)
Other workers in the field:
Oldham et al. (Michigan)
Jordan et al. (London, Ontario)
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Parallel-beam tomography
(Bero, Doran et al., 1999)
Other workers in the field:
Wolodzko, Appleby et al. (New Jersey)
Jordan et al. (London, Ontario)
Single beam laser scanning of phantom (1)
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Single laser beam moves
across sample in n steps
to give a 1-D projection.
Sample rotates by angle
180/nproj.
Nproj projections are
acquired and
reconstructed by filtered
back-projection.
Scan time typically 15
mins per 2-D slice
Data: Oldham et al. Med. Phys. 30 (4), 623 (2003)
Single beam laser scanning of phantom (2)
Phantom using clear
gelatin and food
colouring
Reconstructed plane
Demonstration of
accuracy of image
attenuation values
Data: Oldham et al. Med. Phys. 30 (4), 623 (2003)
CCD optical CT scanning (1)
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Parallel light beam shines through the sample.
2-D projection is captured in a single shot
Sample is rotated by 180/nproj and procedure repeated. nproj projections
are acquired and reconstructed by filtered back-projection.
Scan time typically 15 mins per 3-D volume
CCD optical CT scanning (2): Potential speed
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Frame-grabber captures 1 frame (768 x 576 pixels) in
50 ms.
Suppose we took ~800 projections (Nyquist
requirement) whilst continually rotating phantom
through 1 revolution.
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We could then potentially acquire a 5123 dataset with
data acquired in 40 s!
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Compare MRI T2 map — up to, say, 5 x 256 x 256 in
about 4 minutes.
However … we are not there yet!!
CCD optical CT scanning (3): Phantom
Data sinogram
Single 2-D slice from
tomography dataset
3-D reconstruction
CCD optical CT scanning (4): Irradiated gel
X-ray tube
10 Gy
57 mm
0 Gy
Schematic
3-D visualisation of
the beam pattern
Single slice from
tomography dataset
Pros and cons of different gels for optical imaging
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Fricke gels
 Easy to make; not oxygen sensitive
 Attenuation by absorption so fast CCD
t=0
t=16 hr
tomography possible

10
Polymer gels
 No diffusion
 Attenuation by scattering, so must use slow
laser tomography

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0 hr after irradiation
1 hr after irradiation
Diffusion simulation
8
Dose / Gy
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Diffusion is a major issue
6
4
2
0
Tricky to make
PRESAGE polyurethane
 Cheap and easy to make; robust solid
 Attenuation by absorption
 Very stable and no diffusion
-10
0
Distance / mm
10
Raman Spectroscopy
Data: Baldock et al. DOSGEL 2001
Ultrasound Measurements
Data: Mather et al. DOSGEL 2001
MAGIC gels insensitive to oxygen
Data: Fong and Gore DOSGEL 2001
Xenon spectroscopy of polymer gels
Data: Gore et al. DOSGEL 2001
Conclusions (1)
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Gel dosimetry has an important role to play in radiotherapy.
A number of clear functions of gel dosimetry have been
established and 3-D application areas identified.
That role is currently limited for a number of reasons:
Technical difficulties with the method
Lack of absolute dosimetry
Difficulty in preparing polymer gels reproducibly
Expense of MRI readout
Lack of support from a major RT company
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At the last major conference (DOSGEL 2001) no clear
consensus at conference about when the technique will
become routine in clinics.
Conclusions (2)
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The best method of radiation dose mapping in 3D that we
currently have is polymer gel MRI.
However, this technique has a number of long-term
problems that will slow its uptake:
High cost of equipment
Difficulties with gel manufacture
High degree of expertise needed
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Optical computed tomography is currently where MRI dose
mapping was 5-10 years ago, but demonstrates a number of
promising features.
Other imaging modalities, such as ultrasound, show some
promise, but require much further development.
Conclusions (3)
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The currently active research areas in this field are:
Understanding in detail how the gels work  physical chemistry
Development of polymer gel that is not spoiled by oxygen
Assessment of the quality of the results obtained via MRI
(Current consensus is ~ 3%)
Development of new types of gel
Development of new imaging modalities
Opening up new application areas
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Finally … the gel dosimetry community is currently very
small. Less than 50 people made the trip to Brisbane for
DOSGEL 2001.
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So … there are lots of opportunities to do exciting research
in this new field.