Response of lithium formate EPR dosimeters at photon energies relevant to the dosimetry of brachytherapy

Purpose: To investigate experimentally the energy dependence of the detector response of lithium formate EPR dosimeters for photon energies below 1 MeV relative to that at Co-60 energies. High energy photon beams are used in calibrating dosimeters for use in brachytherapy since the absorbed dose to water can be determined with high accuracy in such beams using calibrated ion chambers and standard dosimetry protocols. In addition to any differences in mass-energy absorption properties between water and detector, variations in radiation yield (detector response) with radiation quality, caused by differences in the density of ionization in the energy imparted (LET), may exist. Knowledge of an eventual deviation in detector response with photon energy is important for attaining high accuracy in measured brachytherapy dose distributions. Methods: Lithium formate EPR dosimeters were irradiated to known levels of air kerma in 25-250 kV x-ray beams and in Cs-137 and Co-60 beams at the Swedish Secondary Standards Dosimetry Laboratory. Conversions from air kerma free in air into values of mean absorbed dose to the detectors were made using EGSnrc MC simulations and x-ray energy spectra measured or calculated for the actual beams. The signals from the detectors were measured using EPR spectrometry. Detector response (the EPR signal per mean absorbed dose to the detector) relative to that for Co-60 was determined for each beam quality. Results: Significant decreases in the relative response ranging from 5% to 6% were seen for x-ray beams at tube voltages <= 180 kV. No significant reduction in the relative response was seen for Cs-137 and 250 kV x rays. Conclusions: When calibrated in Co-60 or MV photon beams, corrections for the photon energy dependence of detector response are needed to achieve the highest accuracy when using lithium formate EPR dosimeters for measuring absorbed doses around brachytherapy sources emitting photons in the energy range of 20-150 keV such as 169Yb and electronic sources. c 2010 American Association of Physicists in Medicine. [DOI: 10.1118/1.3475938]