RADIATION-DOSIMETRY IN HUMAN BONE USING ELECTRON-PARAMAGNETIC-RESONANCE

Accurate measurements of dose in bone are required in order to improve the dosimetry of systemic radiotherapy for osseous metastases. Bone i s an integrating dosimeter which records the radiation history of the skeleton. During irradiation, electrons become trapped in the crystall ine component of bone mineral (hydroxyapatite). The traps are very sta ble; at room temperature, emptying of the traps occurs with a half-lif e of many years. The population of trapped unpaired electrons is propo rtional to the radiation dose administered to the bone and can be meas ured in excised Bone samples using electron paramagnetic resonance (EP R). EPR spectra of synthetic hydroxyapatite, irradiated with Go-60, we re obtained at room temperature and at 77 K. At room temperature, the radiation-induced signal, with a g-value of 2.001 +/- 0.001, increased linearly with absorbed dose above a lower threshold of 3 Gy, up to do ses of 200 Gy. In contrast with pure hydroxyapatite, EPR spectra of ex cised human bone showed a broad 'native' signal, due to the organic co mponent of bone, which masks the dosimetrically important signal. This native signal is highly variable from sample to sample and precludes the use of EPR as an absolute dosimetry technique. However, after subt raction of the background signal, irradiated human bone showed a linea r response with a lower limit of measurement similar to that of synthe tic hydroxyapatite. Bone is an in vivo linear dosimeter which can be e xploited to develop accurate estimates of the radiation dose delivered during systemic radiotherapy and teletherapy. However, improved sensi tivity of the EPR dosimetry technique is necessary before it can be ap plied reliably in clinical situations.