Eli O. Hole

EPR dosimetric properties of formates.

As a part of a program to develop an electron paramagnetic resonance (EPR) dosimeter suited for clinical use (doses in the cGy range), polycrystalline samples of lithium formate monohydrate (HCO2Li.H2O), magnesium formate dihydrate (C2H2O4Mg.2H2O), and calcium formate (C2H2O4Ca) have been examined. L-Alanine was included for comparison and reference. Samples were irradiated with 60Co gamma-rays and 60-220 kV X-rays. The dosimeter response was assessed using the peak-to-peak amplitude of the first-derivative EPR spectrum. Dose-response curves for the 60Co gamma-irradiated samples were constructed, and the dependences of the response on the photon energy, microwave power, and modulation amplitude were studied. Stability of the irradiation products upon storage (signal fading) was also investigated. Lithium formate monohydrate is by far the best candidate of the tested formates, suitable for measuring doses down to approximately 0.1 Gy. Lithium formate monohydrate is more sensitive than alanine by a factor of 5.6-6.8 in the tested photon energy range, it exhibits no zero-dose signal and shows a linear dose response in the dose range from 0.2 to 1000 Gy. Its EPR signal was found unchanged in shape and intensity 1 week after irradiation to 10 Gy. Various less favorable properties rendered the other formates generally unsuitable, although calcium formate exhibits some interesting EPR dosimetric properties.

Alanine Radicals, Part 3: Properties of the Components Contributing to the EPR Spectrum of X-Irradiated Alanine Dosimeters

Abstract Malinen, E., Heydari, M. Z., Sagstuen, E. and Hole, E. O. Alanine Radicals, Part 3: Properties of the Components Contributing to the EPR Spectrum of X-Irradiated Alanine Dosimeters. Radiat. Res. 159, 23–32 (2003). The amino acid l-α-alanine has attracted considerable interest for use in radiation dosimetry and has been formally accepted as a secondary standard for high-dose and transfer dosimetry. Recent results have shown that the alanine EPR spectrum consists of contributions from three different radicals. A set of benchmark spectra describing the essential spectral features of these three radical components was used for reconstructions of the experimental spectra. In the present work, these basis spectra have been used to investigate the differential effects of variations in radiation doses and microwave power, as well as the dependence upon temperature annealing and UV illumination. The results presented here, based solely on relatively low-energy (60–80 keV) X rays, indicate that the three components behave very similarly with respect to radiation dose at room temperature. However, with respect to the thermal annealing/fading behavior and microwave power saturation properties, the three species behave significantly differently. It is concluded that even if it is now realized that three different radicals contribute to the composite EPR alanine spectrum, this has a minor impact on the established protocols for present-day applications (high-dose) of EPR/alanine dosimetry. However, some care should be exercised when e.g. constructing calibration curves, since fading and power saturation behavior may vary over the dose range in question. New results from UV-illumination experiments suggest a possible procedure for experimental spectral separation of the EPR signals due to the three radicals.

The solid-state radiation chemistry of simple amino acids, revisited.

Abstract Sagstuen, E., Sanderud, A. and Hole, E. O., The Solid-State Radiation Chemistry of Simple Amino Acids, Revisited. Radiat. Res. 162, 112–119 (2004). The solid-state radiation-induced free radical formation in simple amino acids like α-glycine (gly) and l-α-alanine (ala) has been the subject of investigations by EPR spectroscopy since the late 1950s. The EPR spectra from crystals of gly and ala generally are very complex due to the simultaneous trapping of several free radicals regardless of irradiation and observation temperatures. Untangling these complex spectra is necessary for understanding the mechanisms for the solid-state radiation chemistry of amino acids. Recently, radical formation in gly and ala after room-temperature irradiation has been reinvestigated in our laboratories using X-, K- and Q-band EPR and ENDOR spectroscopy, combined with the ENDOR-induced EPR (EIE) techniques as well as single-crystal and powder EPR and ENDOR spectrum simulations. Several new radical products have been detected and characterized, most prominently the gly species H2N − C·H − COOH and the ala species H3+N − C·(CH3) − COO− and H2N − C·(CH3) − COOH. A short description of these radicals is given, and an overview of the solid-state radiation chemistry of the simple amino acids is presented, based on a review of the literature combined with these recent experimental results.

An investigation of the photon energy dependence of the EPR alanine dosimetry system

The electron paramagnetic resonance (EPR) alanine dosimetry system is based on EPR measurements of radicals formed in alanine by ionizing radiation. The system has been studied to determine its energy dependence for photons in the 10-30 MV region relative to those of 60Co and to find out if the system would be suitable for dosimetry comparisons. The irradiations were carried out at the National Research Council, Ottawa, Canada and the doses ranged from 8 to 54 Gy. The EPR measurements were performed at the University of Oslo, Norway. The ratio of the slope of the alanine reading versus dose-to-water curve for a certain linac photon beam quality and the corresponding slope for a reference 60Co gamma-radiation gives an experimental measure of the relative dose-to-water response of the EPR alanine dosimetry system. For calculating the linear regression coefficients of these alanine reading versus dose curves, the method of weighted least squares was used. This method is assumed to produce more accurate regression coefficients when applied to EPR dosimetry than the common method of standard least squares. The overall uncertainty on the ratio of slopes was between 0.5 and 0.6% for all three linac energies. The relative response for all the linac beams compared to cobalt was less than unity: by about 0.5% for the 20 and 30 MV points but by more than 1% for the 10 MV point. The given standard uncertainties negate concluding that there is any significant internal variation in the measured response as a function of beam quality between the three linac energies. Thus, we calculated the average dose response for all three energies and found that the alanine response is 0.8% (+/-0.5%) lower for high energy x-rays than for 60Co gamma-rays. This result indicates a small energy dependence in the alanine response for the high-energy photons relative to 60Co which may be significant. This result is specific to our dosimetry system (alanine with 20% polyethylene binder pressed into a particular shape) including its waterproofing sleeve of PMMA (2 mm thick); however, we expect that this result may apply to other similar detectors.

Electron paramagnetic resonance (EPR) dosimetry using lithium formate in radiotherapy: comparison with thermoluminescence (TL) dosimetry using lithium fluoride rods

Solid-state radiation dosimetry by electron paramagnetic resonance (EPR) spectroscopy and thermoluminescence (TL) was utilized for the determination of absorbed doses in the range of 0.5-2.5 Gy. The dosimeter materials used were lithium formate and lithium fluoride (TLD-100 rods) for EPR dosimetry and TL dosimetry, respectively. 60Co gamma-rays and 4, 6, 10 and 15 MV x-rays were employed. The main objectives were to compare the variation in dosimeter reading of the respective dosimetry systems and to determine the photon energy dependence of the two dosimeter materials. The EPR dosimeter sensitivity was constant over the dose range in question, while the TL sensitivity increased by more than 5% from 0.5 to 2.5 Gy, thus displaying a supralinear dose response. The average relative standard deviation in the dosimeter reading per dose was 3.0% and 1.2% for the EPR and TL procedures, respectively. For EPR dosimeters, the relative standard deviation declined significantly from 4.3% to 1.1% over the dose range in question. The dose-to-water energy response for the megavoltage x-ray beams relative to 60Co gamma-rays was in the range of 0.990-0.979 and 0.984-0.962 for lithium formate and lithium fluoride, respectively. The results show that EPR dosimetry with lithium formate provides dose estimates with a precision comparable to that of TL dosimetry (using lithium fluoride) for doses above 2 Gy, and that lithium formate is slightly less dependent on megavoltage photon beam energy than lithium fluoride.

Interphantom and interscanner variations for Hounsfield units—establishment of reference values for HU in a commercial QA phantom

In computer tomography (CT) diagnostics, the measured Hounsfield units (HU) are used to characterize tissue and are in that respect compared to nominal HU values found in the radiological literature. Quality assurance (QA) phantoms are commercially available with a variety of tissue substitutes and materials to test the HU values in CT. It is however recognized from CT physics that the HU for a given material is energy dependent and may vary substantially between scanners. The aim of this study is to analyze the characteristics of a commonly used QA phantom, the Catphan 500/600 (The Phantom Laboratory, NY). Four CT phantoms were scanned on one CT scanner to examine possible interphantom variations in HU values. Secondly, one selected phantom was scanned at three kVp levels on eight different CT scanners. The interphantom variations in HU values were small, in the range 2-5 HU. The interscanner variations were however substantial, in the range 7-56 HU depending on energy and material. Varying the x-ray energy produced a shift in the measured HU of up to 79 HU on one scanner. Reference HU values for the eight sensitometric test materials in Catphan are provided for eight CT scanner models from four vendors. The reference HU values are provided for 80, 120 and 140 kVp. Our results suggest that scanner-independent threshold levels for HU should be used only with extreme caution. Tissue characterization can be used provided that a scanner-specific data set for normal and abnormal is determined.

Alanine Radicals, Part 4: Relative Amounts of Radical Species in Alanine Dosimeters after Exposure to 6–19 MeV Electrons and 10 kV–15 MV Photons

Abstract Malinen, E., Hult, E. A., Hole, E. O. and Sagstuen, E. Alanine Radicals, Part 4: Relative Amounts of Radical Species in Alanine Dosimeters after Exposure to 6–19 MeV Electrons and 10 kV–15 MV Photons. Radiat. Res. 159, 149–153 (2003). The amino acid l-α-alanine can be used for high-precision dosimetry over a wide dose range, using EPR spectroscopy for monitoring radical concentrations. It is important, however, to understand the underlying composition of the observed EPR spectrum. In previous work, it was shown that the EPR signal from irradiated alanine consists of at least three different radical species, with the relative importance of each of these being almost independent of absorbed dose. However, it was not known whether the relative importance of each radical is independent of the radiation quality responsible for the EPR signal. In the present work, the relative contributions of the different radical species to the total EPR signal from alanine dosimeters irradiated with 6–19 MeV electrons and 10 kV–15 MV photons at a dose of 10 Gy were examined. By spectrum reconstruction using benchmark spectra generated from a simulation procedure, the relative amounts of the three different radical species were shown to be virtually independent of these radiation beam qualities.

The energy dependence of lithium formate and alanine EPR dosimeters for medium energy x rays

PURPOSE To perform a systematic investigation of the energy dependence of alanine and lilthium formate EPR dosimeters for medium energy x rays. METHODS Lithium formate and alanine EPR dosimeters were exposed to eight different x-ray beam qualities, with nominal potentials ranging from 50 to 200 kV. Following ionometry based on standards of absorbed dose to water, the dosimeters were given two different doses of approximately 3 and 6 Gy for each radiation quality, with three dosimeters for each dose. A reference series was also irradiated to three different dose levels at a 60Co unit. The dose to water energy response, that is, the dosimeter reading per absorbed dose to water relative to that for 60Co gamma-rays, was estimated for each beam quality. In addition, the energy response was calculated by Monte Carlo simulations and compared to the experimental energy response. RESULTS The experimental energy response estimates ranged from 0.89 to 0.94 and from 0.68 to 0.90 for lithium formate and alanine, respectively. The uncertainties in the experimental energy response estimates were typically 3%. The relative effectiveness, that is, the ratio of the experimental energy response to that following Monte Carlo simulations was, on average, 0.96 and 0.94 for lithium formate and alanine, respectively. CONCLUSIONS This work shows that lithium formate dosimeters are less dependent on x-ray energy than alanine. Furthermore, as the relative effectiveness for both lithium formate and alanine were systematically less than unity, the yield of radiation-induced radicals is decreased following x-irradiation compared to irradiation with 60Co y-rays.

Radiation dose measurements with alanine/agarose gel and thin alanine films around a 192Ir brachytherapy source, using ESR spectroscopy

Alanine/agarose gel and alanine films in stacks have been used for measurements of absorbed dose around an HDR 192Ir source in a vaginal cylinder-applicator, with and without a 180 degrees tungsten shield. The gel and the films were analysed by means of ESR spectroscopy and calibrated against an ion chamber in a 4 MV photon beam to obtain absolute dose values. The gel serves as both dosimeter and phantom material, and the thin (130 microm) films are used to achieve an improved spatial resolution in the dose estimations. Experimental values were compared with Monte Carlo simulations using two different codes. Results from the measurements generally agree with the simulations to within 5%, for both the alanine/agarose gel and the alanine films.

A simple method for estimating dose uncertainty in ESR/alanine dosimetry

Abstract ESR/alanine dosimetry is widely used as a reference, transfer and routine standard dosimetry system. In order to use it as a reliable standard, or in every situation where a high degree of accuracy is needed, the calculation of the absolute dose uncertainty is required. Even though general guides for estimating dose uncertainties are available, the actual calculation may be rather complicated. In this paper, an analytical expression for the dose uncertainty for the ESR/alanine dosimetry system is presented, and shown to be rather simple to use. The treatment is valid for doses below ca. 103 Gy. An alternative procedure should be followed if the calibration dose uncertainties are very varying, e.g. when the calibration dose range is wide. This procedure, the so-called effective variance method [Orear, J. (1982) Am. J. Phys. 50, 912], is described. Both methods are supplied with one numerical example. Suggestions on obtaining calibration curve parameters and their uncertainties, are emphasized.