Evaluation of polymer gels and MRI as a 3-D dosimeter for intensity-modulated radiation therapy

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].