Sara Olsson

New materials for ESR dosimetry

New materials for electron spin resonance (ESR) dosimetry have been investigated with the aim to find systems more sensitive than L-alanine accepted as a standard for high dose determinations. Among the investigated systems ammonium tartrate, 2-methylalanine, salts of formic acids and dithionates have been found to be more sensitive than alanine by a factor 2-10. The lower limit applies to tissue equivalent materials, while much higher sensitivities were obtained with formates and dithionates containing heavier atoms. The increased sensitivity was mainly attributed to suitable ESR properties of the room temperature stable radicals as regards spectral shape (narrow lines, little or no hyperfine structure) and microwave saturation properties (short relaxation times). The radical structures have when necessary been clarified by ENDOR spectroscopy, while the saturation properties have been screened by pulsed ESR measurements.

Development of ammonium tartrate as an ESR dosimeter material for clinical purposes

The crystalline substance ammonium tartrate is investigated with respect to its suitability as a clinical dosimetry material. The properties investigated are: the radical stability for absorbed doses relevant for clinical use, the improvement in sensitivity when the crystals are deuterated and the linear electron transfer (LET)-dependence. After photon irradiation to an absorbed dose of 20 Gy, the signal increases rapidly during the first 6 h. After this period, the changes are more moderate and can be corrected for. The signal-to-noise ratio of irradiated ammonium tartrate is twice the corresponding value of alanine. By deuterating the crystals, the sensitivity can be further improved by a factor of 1.4. As expected, the signal decreases with increasing LET of the radiation, but no changes in the spectrum shape was observed.

Ammonium tartrate as an ESR dosimeter material

ESR dosimetry with L-α-alanine as a dosimeter material is a well known and frequently used method for measuring high absorbed doses, for example at sterilisation and food irradiation. The increased sensitivity of modem spectrometers and an increased knowledge of the radiochemical properties of alanine have lowered the detection limit of ESR/alanine dosimetry to the dose range relevant for radiation therapy. The aim of this thesis is to contribute to an extended use of ESR dosimetry in radiation therapy, including development of an alanine-based gel dosimeter and investigation of alternative dosimeter materials.The dosimeter material is often a polycrystalline powder, which can be mixed with a binder and formed to solid dosimeters in the shape of tablets, rods or films. It can also be distributed in a gel, to serve as both dosimeter material and phantom material. Thus, problems caused by the detector displacing the medium are avoided. The gel developed in this thesis is based on polycrystalline alanine, distributed in an agarose gel. It was tested for calibrated measurements in a brachytherapy situation. With further development, the alanine/agarose gel was found to have potential as a tool for verification of treatment plans, also at complicated dose distributions. One problem to be solved before the gel can be used at an arbitrary irradiation geometry is the low sensitivity of the gel. At present, the sensitivity puts a limit on the spatial resolution at low doses because of the high demands on measurement precision in radiation therapy.The low sensitivity of the alanine/agarose gel, and also the need for a high precision at low doses together with a small dosimeter size when using traditional solid dosimeters, have raised the need for alternative, more sensitive, dosimeter materials than alanine. Such a material should form only one radiation induced radical, giving an ESR spectrum with few and narrow lines for easy evaluation. It should also be possible to increase the microwave power and the field modulation amplitude without severe distortion of the spectrum shape. Other important criteria are a low effective atomic number, a signal that is stable over time and, preferably, a linear dose response.Two compounds are presented; ammonium tartrate and 2-methylalanine. Both are more sensitive than alanine at low settings of microwave power and modulation amplitude, and fulfil the criteria of a low effective atomic number and a linear dose response. Ammonium tartrate is saturated already at low microwave power levels whereas the modulation amplitude can be increased to very high levels without saturation. However, the spectrum shape is then distorted. The signal-changes over time are rapid during the first hours after irradiation, but are then stabilised and can be corrected for.The other investigated substance, 2-methylalanine, is more similar to alanine regarding signal stability over time and dependency of microwave power and field modulation amplitude. However, irradiation of 2-methylalanine results in only one detectable radical, where the alanine spectrum is composed of at least two radicals. The less complicated spectrum of 2-methylalanine makes it more safe to increase the microwave power and the modulation amplitude to their saturation levels.

Lithium formate EPR dosimetry for verifications of planned dose distributions prior to intensity-modulated radiation therapy

The objective of the present investigation was to evaluate lithium formate electron paramagnetic resonance (EPR) dosimetry for measurement of dose distributions in phantoms prior to intensity-modulated radiation therapy (IMRT). Lithium formate monohydrate tablets were carefully prepared, and blind tests were performed in clinically relevant situations in order to determine the precision and accuracy of the method. Further experiments confirmed that within the accuracy of the current method, the dosimeter response was independent of beam energies and dose rates used for IMRT treatments. The method was applied to IMRT treatment plans, and the dose determinations were compared to ionization chamber measurements. The experiments showed that absorbed doses above 3 Gy could be measured with an uncertainty of less than 2.5% of the dose (coverage factor kappa = 1.96). Measurement time was about 15 min using a well-calibrated dosimeter batch. The conclusion drawn from the investigation was that lithium formate EPR dosimetry is a promising new tool for absorbed dose measurements in external beam radiation therapy, especially for doses above 3 Gy.

Ammonium Formate, a Compound for Sensitive EPR Dosimetry

Abstract Gustafsson, H., Olsson, S., Lund, A. and Lund, E. Ammonium Formate, a Compound for Sensitive EPR Dosimetry. Radiat. Res. 161, 464–470 (2004). Alanine EPR dosimetry has been applied successfully when measuring intermediate and high radiation doses. Although the performance of alanine dosimetry is being improved, the sensitivity of the material is too low for a fast and simple low- dose determination. Here we present the results using ammonium formate as an EPR dosimeter material. Ammonium formate is seven times more sensitive than alanine, using spectrometer settings optimized for the latter. Deuterated ammonium formate is found to be more than eight times more sensitive than alanine. Analysis of signal stability with time shows that the ammonium formate signal is stable by 5 min after irradiation and that no change in signal intensity is found during 8 days. The atomic composition of ammonium formate is closer to that of tissue than alanine, and thus the energy dependence is smaller than that of alanine at photon energies below 200 keV. Power saturation studies indicate that the energy transfer between the spins and the lattice is fast in ammonium formate, which gives the possibility of using high microwave power without saturation to further increase the sensitivity. These results suggest that ammonium formate has some important properties required of an EPR dosimeter for applications in dosimetry in the dose range typical for radiation therapy.

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.

EPR Dosimetric Properties of 2-Methylalanine: EPR, ENDOR and FT-EPR Investigations

Abstract Olsson, S., Sagstuen, E., Bonora, M. and Lund, A. EPR Dosimetric Properties of 2-Methylalanine: EPR, ENDOR and FT-EPR Investigations. Radiat. Res. 157, 113–121 (2002). To find an EPR dosimeter material that is sensitive enough for clinical use, the substance 2-methylalanine (2MA) with the chemical structure (CH3)2C(NH3+)COO− was tested for its sensitivity to ionizing radiation, dose response, and radical stability over time. At equal and moderate settings of microwave power and modulation amplitude, 2MA was found to be 70% more sensitive than l-α-alanine, which is the most common EPR dosimeter material today. The dose response is linear, at least in the dose range of interest (0.5–100 Gy), and the time-dependent variations in signal intensity are very small and may be corrected for easily. The energy dependence of the stopping power and energy absorption was calculated and was found to be similar to that of alanine. The dependence of the signal intensity on microwave power and modulation amplitude was investigated, and the optimal settings were found to be 25 mW (Bruker ER 4102ST) and 12 gauss, respectively. Single crystals of 2MA were analyzed using ENDOR and ENDOR-induced EPR to identify the radiation-induced radicals that formed. Only one radical, in which the amino group is detached from the original molecule, was identified. This radical is obviously dominating and is apparently the only one relevant for dosimetry purposes. The complete set of coupling parameters for three hyperfine couplings is reported. The power saturation properties and spectral line width are ruled by the relaxation times T1 and T2. To determine the relaxation times of 2MA, pulsed EPR experiments were performed on single crystals. Two different values of T1 were obtained, one in the range 1–3 , shown to be of importance for the dosimetry properties, and another that is strongly anisotropic with a value between 10 and 35 that does not seem to affect the saturation behavior. T2 was estimated to be of the order of 200–300 ns.

Dose response and fading characteristics of an alanineAgarose gel

Abstract The dose response of an alanineagarose gel, analyzed by ESR spectrometry, and the stability of the radiation-induced free radicals have been investigated. The stability of the ESR signal is higher for dosimeter samples analyzed at 77 K than for dried samples, analyzed at room-temperature. The dose response is linear to within ±2% in the absorbed dose interval 2–100 Gy. The variations in spectral line shape were analyzed at temperatures between 77 and 270 K. The experimental ESR spectrum at 77 K was compared with a simulated spectrum of polycrystals of L-α-alanine.

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

The aim of this work was to develop and test a remote end-to-end audit system using lithium formate EPR dosimeters. Four clinics were included in a pilot study, absorbed doses determined in the PTV agreed with TPS calculated doses within ±5% for 3D-CRT and ±7% (k=1) for IMRT/VMAT dose plans.