Measuring dose distribution in steep dose gradients using polymer gel coupled with optical scanning

1
Gel Dosimetry Technique for Measurements in High
Dose Gradients Malcolm P. Heard and Geoffrey S.
Ibbott Department of Radiation Physics The
University of Texas M. D. Anderson Cancer Center,
Houston, Texas
Results
Materials and Methods Gel Preparation A MAGIC
gel containing 5 methacrylic acid was made and
poured into 3 barex canisters. One canister was
used to calibrate the gel response, the second
canister had a 1mm catheter running through the
middle to allow for placement of the source, and
the third canister was used to test the new
technique. The new technique was done as follows
1) Gel poured into a barex canister with a 7 mm
OD catheter running through it 2) The gel was
allowed to set for 8 hours in a refrigerator 3)
The 7 mm catheter was removed from the canister
and replaced with a 1 mm OD barex catheter 4)
The void was filled with MAGIC gel containing no
methacrylic acid, making it insensitive to
radiation. Irradiation The calibration gel was
irradiated using stereotactic radiosurgery beams
to known doses. Four 1.75 cm diameter beams were
used to deliver doses of 3 Gy, 6 Gy, 9 Gy, and 12
Gy. Figure 2 is an image of the calibration gel.
Introduction Polymer gels are made up of water,
gelatin, and acrylic monomers that polymerize
when exposed to radiation. Ionizing radiation
produces free radicals within the gel and these
in turn lead to the formation of polymer
microparticles which remain attached or entangled
with the gelatin. The spatial distribution of the
polymer is representative of the dose
distribution of the radiation (Maryanski et al
1994). Fong et al have developed a polymer gel
called MAGIC. The MAGIC gel is composed of
gelatin, methacrylic acid, ascorbic acid, copper
sulfate, and hydroquinone. The copper sulfate and
ascorbic acid utilize oxygen to make free
radicals which in turn initiate the
polymerization of methacrylic acid (Fong et al
2001). This distinguishes MAGIC gels from
previous polymer gels whose response is inhibited
by oxygen. The amount of methacrylic acid in the
gel determines the response of the gel (Fong et
al 2001). Increasing the amount of methacrylic
acid increases the gel's response to radiation,
conversely decreasing the amount of methacrylic
decreases the response of the gel. The
polymerization of the gels, which represents the
dose distribution, can be imaged using optical
scanning methods. The optical scanner is modeled
after a first generation x-ray CT so that the gel
is scanned in a translate-rotate fashion. The
optical scanner uses a He-Ne laser to scan the
gel. Photodiode detectors are used to measure the
attenuation of the beam as it passes through the
gel. An image is then reconstructed using
filtered back projection to generate a three
dimensional matrix of optical densities that are
proportional to dose.
Figure 4. Dose response curve determined from the
calibration gel irradiated with stereotactic beams
Figure 5. Radial dose function of the 192Ir from
gel measurements using the 1 mm catheter and
Monte Carlo data published by Daskalov et al..
Figure 6. Radial dose function of the 192Ir from
gel measurements using the new technique and
Monte Carlo data published by Daskalov et al..
Measuring dose distribution in steep dose
gradients using polymer gel coupled with optical
scanning is difficult. The dynamic range of
optical CT is limited low-dose regions suffer
from high noise, and increasing the dose causes
imaging artifacts from optically dense regions. A
previous study was done to characterize
interstitial brachytherapy sources using MAGIC
gel. In order to obtain information at larger
distances from
Discussion The gel measurement underestimates
the dose at distances shorter than 7 mm causing a
difference in the radial dose function compared
to Monte Carlo data. This difference is believed
to be partly due to the diffusion of monomers in
close proximity to the source. The high doses
given close to the source cause monomers to
diffuse inwardly as they are consumed in
polymerization. This results in a region within
and adjacent to the high dose region that
underestimates the dose. The gel data also
disagree with the Monte Carlo data at larger
distances ( gt 1 cm) from the source. The gel
measures less dose than is calculated at these
distances. The dose delivered at 1 cm is 3 Gy
which is also the minimum dose used in
determining the calibration curve. The dose
response curve used for the gel in this study
might not be appropriate for doses less than 3
Gy. A threshold dose, or a dose below which the
gel dose not respond, can occur if continual
oxygen contamination occurs or insufficient time
is allowed for oxygen scavenging.
Figure 2. Image of calibration gel irradiated
with stereotactic beams.
The other two gels were irradiated using the
microSelectron-HDR remote afterloading device for
high dose-rate brachytherapy. Irradiation times
were calculated to deliver 12 Gy at 0.5 cm from
the source. Optical Scanning After irradiation
using the HDR source, the catheter was removed
from each gel and the void was filled with gel.
The gels were imaged using an optical CT scanner
(Gore et al 1996) and images were analyzed using
a MATLAB program written for this study. The
calibration gel was also scanned and a dose
response curve was determined. A second order
polynomial fit was applied to the dose response
curve and used to convert the gel response to
dose. The radial dose function was determined for
the 192Ir source and compared to published Monte
Carlo data.
Figure 1. Image of gel irradiated with an
interstitial brachytherapy source. Streaks in the
image are caused by the optically dense region in
the center.
the source it was necessary to deliver
significantly higher doses at those distance
resulting in a region close to the source that
was very optically dense. Figure 1 shows an image
of a gel irradiated with an interstitial
brachytherapy source and the artifact that
results when imaged. The optically dense region
in the center results in streaking artifacts in
the image.
Purpose The purpose of this project was to
develop a method to eliminate imaging artifacts
from steep dose gradients making characterization
of brachytherapy sources possible at larger
distances (gt 1 cm).
References 1) Daskalov, G. M., Loffler, E., and
Williamson, J. F., "Monte Carlo-aided dosimetry
of a new high dose-rate brachytherapy source,"
Med. Phys. 25, 2200-2208 (1998). 2) Fong, P.
M., Keil D. C., Does, M. D., and Gore, J. C.,
Polymer gels for magnetic resonance imaging
of radiation dose distributions at normal room
atmosphere, Phys. Med. Biol. 46, 3105-3113
(2001). 3) Gore, J. C., Ranade, M., Maryanski, M.
J., and Schulz, R. J., Radiation dose
distributions in three dimensions from
tomographic optical density scanning of
polymer gels I. Development of the optical
scanner, Phys. Med. Biol. 41, 2695- 2704
(1996). 4) Maryanski, M. J., et. al., Magnetic
resonance imaging of radiation dose
distributions using a polymer gel dosimeter,
Phys. Med. Biol. 39, 1437-1455 (1994).
Figure 3. Picture of the Optical CT scanner used
in this study
This investigation was supported in part by PHS
grant CA10953 awarded by the NCI, DHHS.