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

Abstract

A dosimetry system using lithium formate monohydrate ͑HCO 2 Li· H 2 O͒ as detector material and electron paramagnetic resonance ͑EPR͒ spectroscopy for readout has been used to measure absorbed dose distributions around clinical 192 Ir sources. Cylindrical tablets with diameter of 4.5 mm, height of 4.8 mm, and density of 1.26 g / cm 3 were manufactured. Homogeneity test and calibration of the dosimeters were performed in a 6 MV photon beam.192 Ir 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 ͑TPSs͒ 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 Ϯ2.9%. 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 PDR͒, 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 192 Ir sources. Being less energy dependent than the commonly used thermoluminescent lithium fluoride ͑LiF͒ 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.

Influence of phantom material and dimensions on experimental dosimetry

“…31 Experimental phantoms are usually cubic and commonly used TPSs use as references a water cylinder of diameter and height 40 cm ͑Varian for the GammaMed Plus and 12i sources [20][21][22] ͒ or a water sphere of diameter 30 cm ͑Nucletron for all their sources 17,32-34 ͒. Results…”

A system for remote dosimetry audit of 3D-CRT, IMRT and VMAT based on lithium formate dosimetry

“…A BRUKER EleXsys E500 was used with a BRUKER standard resonator (4102ST) for all readouts. Details about the dosimeter manufacturing and handling can be found in earlier publications [10][11][12].…”

“…Lithium formate has previously been used for verification of IMRT treatments [10] and for measurement around 192 Ir brachytherapy sources [11]. The LET-dependence of response for photon energies < 150 keV relative to 60 Co is known [12] and imply that the lithium formate system is a good candidate for use also in brachytherapy audits.…”

EPR imaging of dose distributions aiming at applications in radiation therapy

“…Electron paramagnetic resonance, EPR, has long been used to map dose distributions in radiation therapy as for instance experimental verification of dose distributions around brachytherapy sources using alanine/agarose gel and alanine film (1) or lithium formate dosimeter tablets (2). An example of application in external beam therapy has been for verification measurements of intensity modulated radiotherapy (IMRT) dose plans, using lithium formate tablets (3).…”