BANG(R) gel (MGS Research, Inc., Guilford, CT) has been evaluated for measu
ring intensity-modulated radiation therapy (IMRT) dose distributions. Treat
ment plans with target doses of 1500 cGy were generated by the Peacock IMRT
system (NOMOS Corp., Sewickley, PA) using test target volumes. The gels we
re enclosed in 13 cm outer diameter cylindrical glass vessels. Dose calibra
tion was conducted using seven smaller (4 cm diameter) cylindrical glass ve
ssels irradiated to 0-1800 cGy in 300 cGy increments. Three-dimensional map
s of the proton relaxation rate R-2 were obtained using a 1.5 T magnetic re
sonance imaging (MRI) system (Siemens Medical Systems, Erlangen, Germany) a
nd correlated with dose. A Hahn spin echo sequence was used with TR=3 s, TE
=20 and 100 ms, NEX=1, using 1X1X3 mm(3) voxels. The MRI measurements were
repeated weekly to identify the gel-aging characteristics. Ionization chamb
er, thermoluminescent dosimetry (TLD), and film dosimetry measurements of t
he IMRT dose distributions were obtained to compare against the gel results
. The other dosimeters were used in a phantom with the same external cross-
section as the gel phantom. The irradiated R-2 values of the large vessels
did not precisely track the smaller vessels, so the ionization chamber meas
urements were used to normalize the gel dose distributions. The point-to-po
int standard deviation of the gel dose measurements was 7.0 cGy. When compa
red with the ionization chamber measurements averaged over the chamber volu
me, 1% agreement was obtained. Comparisons against radiographic film dose d
istribution measurements and the treatment planning dose distribution calcu
lation were used to determine the spatial localization accuracy of the gel
and MRI. Spatial localization was better than 2 mm, and the dose was accura
tely determined by the gel both within and outside the target. The TLD chip
s were placed throughout the phantom to determine gel measurement precision
in high- and low-dose regions. A multidimensional dose comparison tool tha
t simultaneously examines the dose difference and distance-to-agreement was
used to evaluate the gel in both low-and high-dose,gradient regions. When
3% and 3 mm criteria were used for the comparisons, more than 90% of the TL
D measurements agreed with the gel, with the worst of 309 TLD chip measurem
ents disagreeing by 40% of the criteria. All four MRI measurement session g
el-measured dose distributions were compared to evaluate the time behavior
of the gel. The low-dose regions were evaluated by comparison with TLD meas
urements at selected points, while high-dose regions were evaluated by dire
ctly comparing measured dose distributions. Tests using the multidimensiona
l comparison tool showed detectable degradation beyond one week postirradia
tion, but all low-dose measurements passed relative to the test criteria an
d the dose distributions showed few regions that failed. (C) 1999 American
Association of Physicists in Medicine. [S0094-2405(99)02508-0].