Imaging dose: calibration of polymer gel dosimeters for use in targeted radionuclide therapy

There is a lack of standardised methodology to perform dose calculations for targeted radionuclide therapy and at present no method exists to objectively evaluate the various approaches employed. The aim of the work described here was to investigate the practicality and accuracy of calibrating polymer gel dosimeters such that dose measurements resulting from complex activity distributions can be verified. 12 vials of the polymer gel dosimeter, MAGIC, were uniformly mixed with varying concentrations of P-32 such that absorbed doses ranged from 0 to 30 Gy after a period of 360 hrs before being imaged on a magnetic resonance scanner. In addition 9 vials were prepared and irradiated using an external 6 MV X-ray beam. Magnetic resonance transverse relaxation time, T/sub 2/, maps were obtained using a multi-echo spin echo sequence and converted to R/sub 2/ maps (where T/sub 2/ = 1/R/sub 2/). Absorbed doses for P-32 irradiated gel were calculated according to the Medical Internal Radiation Dose schema and EGSnrc Monte Carlo simulations. Here the energy deposited in cylinders representing those of the irradiated vials was scored. Effects from oxygen contamination were present in the internally irradiated vials. An increased sensitivity to O/sub 2/ was seen, thought to be a result of the longer irradiation period. However, below the region of contamination dose response appeared homogenous and accuracy of calibration was comparable for both methods. The largest errors in calculated dose originated from the initial activity measurements, and were approximately 10%. Measured R/sub 2/ values ranged from 5-35 s/sup -1/ with an average standard deviation of 1%. A clear relationship between R/sub 2/ and dose was observed, with a 20% increased dose-response for internally irradiated gels. Curve fits to the calibration data followed a y=ax/sup b/+c relationship. The linear relation coefficients for internally and externally irradiated gels were 0.994 and 0.983 respectively. With the ability to accurately calibrate internally dosed polymer gels, this technology shows promise as a means to evaluate dosimetry methods, particularly in cases of non-uniform uptake of a radionuclide.