Optimisation of electron paramagnetic resonance dosimetry for unsealed source radiotherapy

Electron paramagnetic resonance (EPR) is an in vitro method which can be used to measure the absorbed dose delivered to bone by unsealed source radiotherapy. The objectives of the present work were (1) to optimise the radiation-induced EPR signal in crystalline bone mineral (hydroxyapatite, HA) and in human bone; and (2) to investigate the effects of heating and preservation of samples obtained at autopsy. Samples of human bone, fixed in formalin or alcohol, and HA were dried in a desicator. EPR spectra were measured at several microwave powers (1) in controls; (2) after irradiation with cobalt-60 radiation or, (3) after heating, to simulate potential thermal effects of sawing. Unirradiated bone showed a strong, broad (width = 20 Gauss) signal with a g-value of approximately 2.00. Unirradiated HA showed no peaks in its spectrum. Irradiated bone showed a peak near the naturally-occurring background signal. Irradiated HA possessed a signal (signal width = 10 Gauss) near g=2.00. For both materials, the intensity of the radiation-induced signal increased linearly with dose. This signal was maximised at a microwave power of approximately 2mW. HA heated to temperatures above 200{degrees}C developed a signal at g=2.000 and a second peak of equal magnitude at a slightlymore » higher g-value. Subsequent irradiation of the heated samples caused the peak at g=2.000 to intensify. Heated bone possessed and EPR spectrum with a broad signal and a g-value greater than the radiation-induced signal. EPR dosimetry is well-suited for dosimetric studies of bone-seeking radiopharmaceuticals over the range of 0.01Gy to 100Gy. Optimal EPR power and controlled handling of specimens are required for accurate dosimetry in bone. This work was funded by the Medical Research Council of Canada and Amersham.« less