Evaluation of a lithium formate EPR dosimetry system for dose measurements around 192Ir brachytherapy sources

A dosimetry system using lithium formate monohydrate (HCO2Li x H2O) as detector material and electron paramagnetic resonance (EPR) spectroscopy for readout has been used to measure absorbed dose distributions around clinical 192Ir sources. Cylindrical tablets with diameter of 4.5 mm, height of 4.8 mm, and density of 1.26 g/cm3 were manufactured. Homogeneity test and calibration of the dosimeters were performed in a 6 MV photon beam. 192Ir irradiations were performed in a PMMA phantom using two different source models, the GammaMed Plus HDR and the microSelectron PDR-v1 model. Measured absorbed doses to water in the PMMA phantom were converted to the corresponding absorbed doses to water in water phantoms of dimensions used by the treatment planning systems (TPSs) using correction factors explicitly derived for this experiment. Experimentally determined absorbed doses agreed with the absorbed doses to water calculated by the TPS to within +/-2.9%. Relative standard uncertainties in the experimentally determined absorbed doses were estimated to be within the range of 1.7%-1.3% depending on the radial distance from the source, the type of source (HDR or PDR), and the particular absorbed doses used. This work shows that a lithium formate dosimetry system is well suited for measurements of absorbed dose to water around clinical HDR and PDR 192Ir sources. Being less energy dependent than the commonly used thermoluminescent lithium fluoride (LiF) dosimeters, lithium formate monohydrate dosimeters are well suited to measure absorbed doses in situations where the energy dependence cannot easily be accounted for such as in multiple-source irradiations to verify treatment plans. Their wide dynamic range and linear dose response over the dose interval of 0.2-1000 Gy make them suitable for measurements on sources of the strengths used in clinical applications. The dosimeter size needs, however, to be reduced for application to single-source dosimetry.