IOP PUBLISHING PHYSICS IN MEDICINE AND BIOLOGY
Phys. Med. Biol. 52 (2007) N241–N248 doi:10.1088/0031-9155/52/10/N03
NOTE
Optimization of MAGIC gel formulation for
three-dimensional radiation therapy dosimetry
J J Luci1,2, H M Whitney1,3 and J C Gore1,2,3
1 Vanderbilt University Institute of Imaging Science, Nashville, TN 37232-2675, USA
2 Department of Radiology and Radiological Sciences, Vanderbilt University, Nashville,
TN 37232-2675, USA
3 Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37232-2675, USA
E-mail: heather.whitney@vanderbilt.edu
Received 14 September 2006, in final form 28 March 2007
Published 24 April 2007
Online at stacks.iop.org/PMB/52/N241
Abstract
Polymer gel dosimetry aims to provide three-dimensional images of radiation
therapy dose distributions in irradiated aqueous gels. The first gels required
manufacture in an oxygen-free environment, but later the MAGIC formulation
was introduced, which could be made in normal atmospheric conditions. Here
we report our studies of the effects of variations in the composition of the
MAGIC gel performed in order to optimize its performance over the useful
dose range of 0 to 20 Gy. A new formulation (termed ‘MAGIC-2’) is comprised
of 87% water, 4% methacrylic acid, 9% gelatin, 17.38 × 10−6 M Cu2+ and a
molar ratio of ascorbic acid to [Cu2+] of 1000:1. MAGIC-2 has a dose–response
slope-to-intercept ratio that is 78% greater than the original formulation and
other more favorable properties.
1. Introduction
Polymer gel dosimeters are comprised of an aqueous matrix (usually gelatin) in which one or
more monomers are dispersed. When exposed to ionizing radiation, polymerization is initiated
by radicals that result from radiolysis. Several bulk properties (e.g., the nuclear magnetic
resonance transverse relaxation rate R2 and optical density) are sensitive to the molecular
weight of the resultant polymer, and measurements of these can be used to determine the
absorbed dose. Employing large containers of the gelatin mixture, it is possible to produce
a 3D dose map using magnetic resonance imaging or optical scanning that may be used to
validate radiation therapy planning or for quality assurance.
The first polymer gel dosimeters (BANG©, or non-commercially, PAG) were based on
the monomers acrylamide and bisacrylamide (Maryanski et al 1993, Baldock et al 1998).
Although effective, these dosimeters required hypoxic conditions in order to prevent molecular
0031-9155/07/100241+08$30.00 © 2007 IOP Publishing Ltd Printed in the UK N241

N242 J J Luci et al
oxygen quenching of the short-lived initiating radicals. This prerequisite dictated that inert
atmosphere glove boxes be used in their preparation, and that container materials be limited to
oxygen-impermeable plastics and glass. Since most clinical radiation physicists had neither
the equipment nor the technical resources to prepare oxygen sensitive formulations, the use of
these gels was hampered.
Previously our laboratory introduced a formulation (Fong et al 2001) that permitted gel
dosimeters to be prepared under normal atmospheric conditions. The new type of dosimeter
was termed methacrylic and ascorbic acid in gelatin initiated by copper, or MAGIC, and is
less toxic than acrylamide-based dosimeters. The polyacrylamide gel dosimeter formulation
was later adapted for preparation in regular atmospheric conditions through the addition of
antioxidants such as tetrakis (hydroxymethyl) phosphonium chloride (De Deene et al 2006,
Venning et al 2005). A summary of the different combinations of formulae can be found
elsewhere (Senden et al 2006).
A recent study (De Deene et al 2006) has compared the PAG, nPAG (a normoxic PAG)
and MAGIC gel formulations for properties such as tissue equivalence, dose sensitivity, spatial
integrity, temperature sensitivity and energy and dose-rate dependence. The authors found
that the methacrylic acid-based gel was superior in terms of dose sensitivity and stability
over time while nPAG performed better in other areas. However, the differences in normoxic
gel dosimeters are due to different chemical reaction schemes and both types deserve more
in-depth study. The utility of MAGIC dosimeters depends heavily on the ability to measure
accurately a significant response to polymerization of some localized property. The precise
dependence of the dose response on the composition of the gels has not been described in
detail. We present here studies designed to investigate the influences of different components
with the aim of optimizing the performance of MAGIC polymer gel dosimeters for practical
applications.
2. Methods
2.1. Gel preparation
The formulation of polymer gels studied here contains the same basic ingredients as
the previous formulation: gelatin (300 bloom, Aldrich; Milwaukee, WI), ascorbic acid
(Mallinckrodt; Paris, KY), CuSO4·5H2O (Aldrich; Milwaukee, WI), methacrylic acid (Sigma;
St. Louis, MO) and HPLC grade distilled water. We omit hydroquinone as it is already present
in the methacrylic acid, added by the manufacturer.
Gels for all experiments were prepared in the following manner: a flask containing water
was placed in an equilibrated water bath at 48 ◦C. The gelatin, ascorbic acid solution (AA)
and copper sulfate solution (Cu2+) were all added and the solution stirred with a magnetic bar
for 2 min. Methacrylic acid (MAA) was then added and the solution stirred for an additional
90 s. The gel was immediately poured into glass test tubes, sealed with screw-cap tops and
centrifuged at 15.4g for 15 s plus ramp time. The gels were taken out of the centrifuge and
placed in a refrigerator for storage overnight, approximately 18 h.
The effects of variations of the gelatin, monomer and copper concentrations were
investigated, as described below.
2.2. Gel irradiation
In each experiment, one gel dosimeter for each concentration variation was reserved
unirradiated, and one was irradiated. Samples to be irradiated were placed in a room