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Development of sensitive EPR dosimetry methods
Linköping University, Department of Medicine and Health Sciences. Linköping University, Faculty of Health Sciences.
2008 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Electron paramagnetic resonance (EPR) dosimetry using the well established dosimeter material alanine is a generally accepted dosimetric method for measurements of high absorbed doses. Alanine EPR dosimetry is however not sensitive enough for high precision measurements of low (< 5 Gy) absorbed doses using reasonably measurement times and small dosimeters. It has therefore not been possible to fully exploit the benefits of EPR dosimetry for applications in radiation therapy.

The aim of this thesis was to show that sensitive EPR dosimetry is a competitive method for applications in radiation therapy fulfilling the requirements of measurement precision. Our strategy for reaching this goal was to search for new, more sensitive, EPR dosimeter materials fulfilling the criteria of being tissue equivalent, having a high radical yield and having a narrow EPR spectrum suitable for dosimetry. The best materials were found among formates and dithionates. Doping with small amounts of metal ions and recrystallisation in D2O were tested to further increase the sensitivity. Four promising candidate materials were tested regarding radical stability and dose response and among them lithium formate was chosen for dosimetry in radiation therapy applications.

A high precision EPR dosimetry method was developed using lithium formate. The method included the development of a production method for EPR dosimeters with very homogenous shape, mass and composition. A read-out process was developed with maximal measurement precision for reasonably short measurement times. The method also included a dosimeter quality control before actual dose measurements. Measurement accuracy was controlled for every new dosimeter batch.

This high precision lithium formate EPR dosimetry method was evaluated for pretreatment verifications of intensity modulated radiation therapy (IMRT) treatment plans. The precision and accuracy was shown to be sufficient (< 5 %) for measurements of doses above 1.5 Gy using one single dosimeter and a measurement time of 15 minutes. The described evaluation is therefore a demonstration of the improved precision at low dose determinations that is available with our sensitive EPR dosimeter materials.

While the EPR signal intensity is proportional to absorbed dose, the signal shape is in some cases dependent on the radiation quality. A new method is presented for simultaneous measurements of beam LET (linear energy transfer) and absorbed dose in heavy charged particle beams using potassium dithionate EPR dosimetry. The study shows that when irradiating a dosimeter with 35 MeV carbon ions, the ratio of the signal amplitudes from two radicals in potassium dithionate vary along the track indicating a dependence on linear energy transfer, LET. Potassium dithionate may therefore be a promising EPR dosimeter material for simultaneous measurements of absorbed dose and LET in heavy charged particle radiation fields.

Place, publisher, year, edition, pages
Institutionen för medicin och hälsa , 2008.
Series
Linköping University Medical Dissertations, ISSN 0345-0082 ; 1044
Keyword [en]
Electron paramagnetic resonance (EPR), Formic acids, pharmacology, Lithium, Nickel, Radiometry, Rhodium
National Category
Medicinal Chemistry
Identifiers
URN: urn:nbn:se:liu:diva-11099ISBN: 978-91-7393-975-1 (print)OAI: oai:DiVA.org:liu-11099DiVA: diva2:17549
Public defence
2008-03-13, Eken, Campus US, Linköpings universitet, Linköping, 09:00 (English)
Opponent
Supervisors
Available from: 2008-03-03 Created: 2008-03-03 Last updated: 2009-08-21
List of papers
1. Ammonium formate, a compound for sensitive EPR dosimetry
Open this publication in new window or tab >>Ammonium formate, a compound for sensitive EPR dosimetry
2004 (English)In: Radiation Research, ISSN 0033-7587, Vol. 161, no 4, 464-470 p.Article in journal (Refereed) Published
Abstract [en]

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.

National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:liu:diva-12959 (URN)10.1667/RR3142 (DOI)
Available from: 2008-03-03 Created: 2008-03-03
2. Radiation-induced radicals in lithium formate monohydrate (LiHCO2·H2O). EPR and ENDOR studies of X-irradiated crystal and polycrystalline samples
Open this publication in new window or tab >>Radiation-induced radicals in lithium formate monohydrate (LiHCO2·H2O). EPR and ENDOR studies of X-irradiated crystal and polycrystalline samples
Show others...
2004 (English)In: PCCP : physical chemistry chemical physics, ISSN 1463-9076, Vol. 6, 3017-3022 p.Article in journal (Refereed) Published
Abstract [en]

Single crystals and polycrystalline samples of lithium formate monohydrate (HCO2Li·H2O) were X-irradiated at 295 K and studied using X-band EPR, ENDOR, and ENDOR-induced EPR (EIE) spectroscopy at 200 or 295 K. Two different radical species were observed. The overall dominating species is the CO2 radical trapped in the crystal matrix at an orientation not very different from that of the parent CO2 fragment in the unirradiated matrix. The g- and 13C hyperfine coupling tensors of the CO2 radical were determined. The large linewidth (about 1.5 mT) of the polycrystalline EPR spectrum is due to extensive hyperfine couplings with lithium ions and protons in the environment. Four lithium couplings and four proton couplings associated with the CO2 radical were measured, and all couplings were assigned to specific matrix nuclei. The spectra yield evidence for a second radical in low relative abundance. One small lithium hyperfine interaction detected was ascribed to this radical. Spectral simulations of the EPR and ENDOR spectra support the conclusions made.

National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:liu:diva-12960 (URN)10.1039/b402846e (DOI)
Available from: 2008-03-03 Created: 2008-03-03 Last updated: 2009-06-08
3. Enhanced sensitivity of lithium dithionates doped with rhodium and nickel for EPR dosimetry
Open this publication in new window or tab >>Enhanced sensitivity of lithium dithionates doped with rhodium and nickel for EPR dosimetry
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2005 (English)In: Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, ISSN 1386-1425, Vol. 62, no 1-3, 614-620 p.Article in journal (Refereed) Published
Abstract [en]

Electron paramagnetic resonance (EPR) studies of X-irradiated lithium dithionate, Li2S2O6·2H2O, doped with Ni and Rh have shown that these impurities enhance the yield of radicals formed by X-irradiation at room temperature. The signal in the doped samples, measured peak-to-peak of the single EPR derivative line attributed to the SO3 anion was about 3–4 times that of the pure lithium dithionate and more than 10 times stronger than the alanine signal. These impurities also shortened the spin-lattice relaxation time, T1, which gives the possibility to measure the doped samples at a higher microwave power. This implies that sensitivity could be further enhanced in the already sensitive EPR dosimeter material lithium dithionate.

Keyword
EPR dosimetry; Free radicals; Radiation; Radiation yield; Spin-lattice relaxation; Effect of dopants
National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:liu:diva-12961 (URN)10.1016/j.saa.2005.01.024 (DOI)
Available from: 2008-03-03 Created: 2008-03-03 Last updated: 2009-05-15
4. Development of nickel-doped lithium formate as potential EPR dosimeter for low dose determination
Open this publication in new window or tab >>Development of nickel-doped lithium formate as potential EPR dosimeter for low dose determination
2007 (English)In: Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, ISSN 1386-1425, Vol. 67, no 5, 1370-1373 p.Article in journal (Refereed) Published
Abstract [en]

EPR dosimetry employing l-α-alanine has been in vogue during the past few years, due to its tissue equivalence and linear dose response. However, l-α-alanine dosimetry has been improved during the past years, the sensitivity of this material is still too low for clinical applications. Polycrystalline lithium formate doped with NiCl2 was therefore examined for radiation response in the dose range of clinical interest (<5 Gy) using CW EPR and pulse EPR techniques. At equal and moderate settings of microwave power and modulation amplitude lithium formate doped with 1.6 wt% of NiCl2 was almost four times more sensitive compared to l-α-alanine, which is the most common EPR dosimeter standard. It was shown that the nickel-doped lithium formate has an excellent radiation response with a low limit of the measurable dose, and a linear dose response in the range 1–5 Gy. The relaxation and power saturation studies showed that high microwave power can be applied during measurements to improve the sensitivity of this material as an EPR dosimeter. These results show that lithium formate doped with Ni(II) exhibits promising properties required for further development of an EPR dosimeter in the dose range typical for clinical dosimetry.

Keyword
EPR, EPR dosimetry, Lithium formate
National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:liu:diva-12962 (URN)10.1016/j.saa.2006.10.026 (DOI)
Available from: 2008-03-03 Created: 2008-03-03 Last updated: 2009-04-15
5. Lithium formate EPR dosimetry for verifications of planned dose distrubutions prior to intensity modulated radiation therapy
Open this publication in new window or tab >>Lithium formate EPR dosimetry for verifications of planned dose distrubutions prior to intensity modulated radiation therapy
2008 (English)In: Physics in Medicine and Biology, ISSN 0031-9155, Vol. 53, no 17, 4667-4682 p.Article in journal (Refereed) Published
Abstract [en]

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 k = 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.

National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:liu:diva-12963 (URN)10.1088/0031-9155/53/17/014 (DOI)
Available from: 2008-03-03 Created: 2008-03-03 Last updated: 2009-04-28
6. Potassium dithionate EPR dosimetry for determination of absorbed dose and LET distributions in different radiation qualities
Open this publication in new window or tab >>Potassium dithionate EPR dosimetry for determination of absorbed dose and LET distributions in different radiation qualities
2011 (English)In: Radiation Measurements, ISSN 1350-4487, Vol. 46, no 9, 936-940 p.Article in journal (Refereed) Published
Abstract [en]

With an increasing interest in using protons and light ions for radiation therapy there is a need for possibilities to simultaneously determine both absorbed dose (D) and linear energy transfer, LET, (LΔ). Potassium dithionate (K2S2O6) tablets were irradiated in a conventional 6 MV linear accelerator photon beam and a N7+ beam (E = 33.5 MeV/u) respectively. The EPR spectrum of irradiated potassium dithionate is a narrow doublet consisting of two signals, R1 and R2, with different microwave power saturation properties. On the basis of identification in related substances by EPR and ENDOR, these two signals are assigned to two non-equivalent SO3 – radicals. Our experiments showed that the ratios of these two lines (R1/R2) were clearly connected to beam LET. Irrespective of the mechanistic details this investigation suggests a new method for measurement of absorbed dose and beam LET by using potassium dithionate EPR dosimetry.

Place, publisher, year, edition, pages
Elsevier, 2011
Keyword
EPR dosimetry; Linear energy transfer; LET; Charged particles
National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:liu:diva-12964 (URN)10.1016/j.radmeas.2011.03.006 (DOI)000296039300039 ()
Note
Funding agencies|Swedish Cancer Society| 4276-B05-07XBC |Available from: 2008-03-03 Created: 2008-03-03 Last updated: 2012-01-24

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