The Third International Intercomparison on EPR Tooth Dosimetry: part 2, final analysis.

The objective of the Third International Intercomparison on EPR Tooth Dosimetry was to evaluate laboratories performing tooth enamel dosimetry <300 mGy. Final analysis of results included a correlation analysis between features of laboratory dose reconstruction protocols and dosimetry performance. Applicability of electron paramagnetic resonance (EPR) tooth dosimetry at low dose was shown at two applied dose levels of 79 and 176 mGy. Most (9 of 12) laboratories reported the dose to be within 50 mGy of the delivered dose of 79 mGy, and 10 of 12 laboratories reported the dose to be within 100 mGy of the delivered dose of 176 mGy. At the high-dose tested (704 mGy) agreement within 25% of the delivered dose was found in 10 laboratories. Features of EPR dose reconstruction protocols that affect dosimetry performance were found to be magnetic field modulation amplitude in EPR spectrum recording, EPR signal model in spectrum deconvolution and duration of latency period for tooth enamel samples after preparation.

EPR properties of intact and deproteinated dentin.

The effect of sample preparation on dentin electron paramagnetic resonance (EPR) spectra was investigated. The analysis was performed on dentin samples prepared by pure mechanical treatment or by an alkaline deproteination method. It was observed that (1) mechanical treatment induces both stable and transient signals, depending on the specific mechanical operation applied; and (2) sodium hydroxide deproteination removes the native signal, but at the same time introduces new confounding signals in the EPR spectrum. Also, it increases the radiation sensitivity of dentin. These findings suggest that attention must be paid to dentin sample preparation.

Improvements in CVD diamond properties for radiotherapy dosimetry.

The goal of this work was to compare the behaviour of a chemical vapour deposited (CVD) diamond sample, grown at the University of Florence using a local procedure, with that of a commercial CVD diamond. The comparison was performed exposing both systems to 25 MV photons and measuring the current response during irradiation. Properties of dosimetric interest such as stability of response, dose rate dependence and rise time were investigated. After a preliminary study, which evidenced better performances of the commercial device with respect to the local CVD diamond, the latter was irradiated with a high fluence of fast neutrons. As a result of the neutron treatment, the quality of the CVD home-made diamond has been improved to match with that of the commercial dosemeter.

Use of commercial alanine and TL dosemeters for dosimetry intercomparisons among Italian radiotherapy centres.

In the implementation of a large-scale dosimetry intercomparison one of the main constraints is the availability of large number of dosemeters of the highest quality. Therefore, ISS tested the possibility of using commercially available dosemeters, alanine pellets and thermoluminescence (TL) dosimetry chips, for transfer dosimetry within the Italian intercomparison programme. In this work the characterisation of commercial alanine and TL dosemeters along with the ISS dose assessment procedure used in the Italian intercomparison are reported. Results demonstrate the feasibility of the ISS approach to transfer dosimetry since it is possible to measure 10 Gy with a combined uncertainty of 1% (1sigma) and 1 Gy with a combined uncertainty of 1.7% (1sigma) with alanine and TL dosemeters, respectively.

On measuring the output of an IORT mobile dedicated accelerator.

In the present work, dose-to-water values derived from Fricke (operated by the Italian Primary Standard Dosimetry Laboratory) and alanine (operated by the Istituto Superiore di Sanità, Italy) measurements in IORT electron beams characterised by a high dose per pulse were compared to show the extent of equivalence of the two dosimetry systems. This study demonstrates agreement (within 2%) of the two dosimetry systems for plane-base IORT applicators, but in the case of small diameter (40 mm) and bevelled (22.5 degrees ) applicators, Fricke dosemeters underestimated doses by 2.4%. For base bevelled IORT applicators (22.5 degrees or more) with small diameter ( approximately 40 mm or less) reduced dimensions of the dosemeter are needed. Under these measurement conditions, the alanine dosemeter gives better accuracy in beam output determination compared with the Fricke dosemeter used.

Comparison of EPR occupational lifetime external dose assessments for Mayak nuclear workers and film badge dose data.

The Mayak worker cohort is one of the major sources of information on health risks due to protracted exposures to plutonium and external ionizing radiation. Electron paramagnetic resonance (EPR) measurements in tooth enamel in combination with personal dose monitoring can help to improve external dose assessment for this cohort. Here, the occupational lifetime external exposure was evaluated individually for 44 nuclear workers of three plants of the Mayak Production Association by EPR measurements of absorbed doses in collected tooth enamel samples. Analysis included consideration of individual background doses in enamel and dose conversion coefficients specific for photon spectra at selected work areas. As a control, background doses were assessed for various age groups by EPR measurements on teeth from non-occupationally exposed Ozyorsk residents. Differences in occupational lifetime doses estimated from the film badges and from enamel for the Mayak workers were found to depend on the type of film badge and the selected plant. For those who worked at the radiochemical processing plant and who were monitored with IFK film badges, the dose was on average 570 mGy larger than estimated from the EPR measurements. However, the average difference was found to be only -4 and 6 mGy for those who were monitored with IFKU film badges and worked at the reactor and the isotope production plant respectively. The discrepancies observed in the dose estimates are attributed to a bias in film badge evaluation.

The 3rd international intercomparison on EPR tooth dosimetry: Part 1, general analysis.

The objective of the 3rd International Intercomparison on Electron Paramagnetic Resonance (EPR) Tooth Dosimetry was the evaluation of laboratories performing tooth enamel dosimetry below 300 mGy. Participants had to reconstruct the absorbed dose in tooth enamel from 11 molars, which were cut into two halves. One half of each tooth was irradiated in a 60Co beam to doses in the ranges of 30-100 mGy (5 samples), 100-300 mGy (5 samples), and 300-900 mGy (1 sample). Fourteen international laboratories participated in this intercomparison programme. A first analysis of the results and an overview of the essential features of methods applied in different laboratories are presented. The relative standard deviation of results of all methods was better than 27% for applied doses in the range of 79-704 mGy. In the analysis of the unirradiated tooth halves 8% of the samples were identified as outliers with additional absorbed dose above background dose.

Use of alanine for dosimetry intercomparisons among Italian radiotherapy centers.

A pilot program of postal dosimetry intercomparison among 16 Italian Radiotherapy Centers was performed using the ISS Alanine/EPR dosimetry as a transfer system. Dosimeters were irradiated at 10 Gy with high-energy photon beams, both in reference condition in a water phantom and in an anthropomorphic phantom during the simulated treatment of rectum cancer. Intercomparison design along with alanine performances analyzing the different contributions to the combined uncertainty in dose assessment are reported. Main results of the pilot intercomparison, terminated in 2002, are also presented.

Features of EPR dosimetry with CaSO4:Dy phosphor.

The electron paramagnetic resonance(EPR) technique was used to study the paramagnetic centers induced by ionizing radiation in thermoluminescent CaSO4:Dy. Two different radiation induced centers, detectable in the (1-10(7)) Gy dose range and attributed to (SO3)- and Ca-vacancy center, (VCa)2-, respectively, can be used for dosimetry purpose. The (VCa)2- center is detectable only upon sample annealing at temperatures higher than about 180 degrees C. A reliable dose assessment can be performed in the (1-10(7)) Gy dose range and likely over 10(7) Gy. The sensitivity decreases with thermal cycles, while the yield of both centers increases with accumulated dose (pumping effect), at a greater extent for (VCa)2- center. (VCa)2- center is stable with time, while (SO3)- center shows a dose-dependent fading. Among the applications that can be imaged for this dosimeter, very high dose assessment capability is of particular importance.

Diamond detector versus silicon diode and ion chamber in photon beams of different energy and field size.

The aim of this work was to test the suitability of a PTW diamond detector for nonreference condition dosimetry in photon beams of different energy (6 and 25 MV) and field size (from 2.6 cm x 2.6 cm to 10 cm x 10 cm). Diamond behavior was compared to that of a Scanditronix p-type silicon diode and a Scanditronix RK ionization chamber. Measurements included output factors (OF). percentage depth doses (PDD) and dose profiles. OFs measured with diamond detector agreed within 1% with those measured with diode and RK chamber. Only at 25 MV, for the smallest field size, RK chamber underestimated OFs due to averaging effects in a pointed shaped beam profile. Agreement was found between PDDs measured with diamond detector and RK chamber for both 6 MV and 25 MV photons and down to 5 cm x 5 cm field size. For the 2.6 cm x 2.6 cm field size, at 25 MV, RK chamber underestimated doses at shallow depth and the difference progressively went to zero in the distal region. PDD curves measured with silicon diode and diamond detector agreed well for the 25 MV beam at all the field sizes. Conversely, the nontissue equivalence of silicon led, for the 6 MV beam, to a slight overestimation of the diode doses in the distal region, at all the field sizes. Penumbra and field width measurements gave values in agreement for all the detectors but with a systematic overestimate by RK measurements. The results obtained confirm that ion chamber is not a suitable detector when high spatial resolution is required. On the other hand, the small differences in the studied parameters, between diamond and silicon systems, do not lead to a significant advantage in the use of diamond detector for routine clinical dosimetry.

Preliminary results on a dedicated silicon diode detector for proton dosimetry.

The present work reports preliminary measurements on the behaviour of a new p-type stereotactic silicon diode. Hi-pSi, produced by Scanditronix and dedicated to proton dosimetry. Diode response was investigated in low-energy proton beams (26.7 MeV and 12 MeV nominal energy), mainly with attention to stability, linearity, dose rate and energy dependence of the detector response. Three different Hi-pSi diodes of the same type were investigated. The diode response was linear with dose and the standard deviation of repeated readings was less than 2.5%. A marked dependence on dose rate was observed for one of the diodes (a response increase of 47% in the 0.7-11 Gy x min(-1) range). After the dose rate and water to silicon mass collision stopping power ratio correction of the diode response in the depth dose measurements, the difference, at the Bragg peak, with respect to the reference chamber was about 4%, ascribed to poor knowledge of the materials in front of the sensitive volume. The diode response was also nearly independent of linear energy transfer (LET) in the 9.6-21.5 MeV effective energy range.

Alanine response to proton beams in the 1.6-6.1 MeV energy range.

Alanine response to low-energy protons was studied with alanine dosemeters of 2 mm thickness, irradiated with proton beams of energy in the 1.6-6.1 MeV range. The detector's range-averaged relative effectiveness to 60Co radiation ranged from 0.61 to 0.65. For fluence values up to 5 x 10(10) protons x cm(-2), the alanine response was linear.

Monte Carlo calculation and experimental verification of the photon energy response of tooth enamel in a head-sized plexiglas phantom.

The use of electron paramagnetic resonance (EPR) tooth dosimetry for calculation of organ doses requires conversion of the measured absorbed dose in enamel. Before deriving conversion factors from simulation calculations with a realistic anthropomorphic human phantom, in the current study a simplified phantom was chosen to compare EPR measurement and Monte Carlo calculation. The dose response of tooth enamel of molars at various positions inside a cylindrical Plexiglas phantom of head-size was calculated hy Monte Carlo modelling in parallel photon beams of X rays of 63 keV equivalent energy and 60Co gamma rays (1.25 Mev). For X ray exposure, preliminary results of EPR dosimetry with tooth enamel samples prepared from molars irradiated in the phantom were in agreement with calculation. The mean value of the ratio of the measured to the calculated dose was 0.93 +/- 0.08.

A correction method for diamond detector signal dependence with proton energy.

Small dosimeters as solid state detectors can be useful for the dosimetric characterization and periodic quality control of radiotherapy proton beams. The calibration of solid state detectors for proton beams is not a solved problem especially for ophthalmologic proton beams, where these detectors present a LET-dependent signal. In this work a PTW diamond detector has been selected because of its good signal reproducibility (0.3%) and stable response with accumulated dose. A method that takes into account the LET dependence of the diamond detector signal, at 62 MeV proton beam, is here proposed. In particular an empirical correction factor, kDD(Eo) (Rres), has been determined as a function of the residual range quality index, to correct the diamond detector signal for a proton beam of incident effective energy E0= 62 MeV. A dedicated software allows us to use the diamond detector as an on-line reference dosimeter, where an ionization chamber may be difficult to use, or for periodic quality control procedures. The article also reports a comparison between the signal dependence on proton energy of silicon, diamond, and radiochromic film detectors.

Characteristics of silicon and diamond detectors in a 60 MeV proton beam.

The calibration factor variation for a PTW natural diamond detector and a Scanditronix p-type stereotactic silicon diode (designed for use in photon beams) was studied in the 10-59 MeV range. Irradiations were performed in a water phantom with the 60 MeV ocular therapy beam at the CCO (UK). The diamond detector showed a sensitivity increase with energy, underestimating the dose by about 18% at the Bragg peak, by 7% at the centre and by 17% at the distal end of the SOBP region. The silicon diode did not show any significant sensitivity change with energy. However, a decrease in response of 24% was observed for an accumulated dose of 300 Gy.

An investigation of the operating characteristics of two PTW diamond detectors in photon and electron beams.

The dosimetric properties of two PTW Riga diamond detectors type 60003 were studied in high-energy photon and electron therapy beam. Properties under study were current-voltage characteristic, polarization effect, time stability of response, dose response, dose-rate dependence, temperature stability, and beam quality dependence of the sensitivity factor. Differences were shown between the two detectors for most of the previous properties. Also, the observed behavior was, to some extent, different from what was reported in the PTW technical specifications. The necessity to characterize each diamond detector individually was addressed.

Mechanically induced EPR signals in tooth enamel.

Sample preparation of tooth enamel for electron paramagnetic resonance (EPR) dosimetry usually involves mechanical operations. The present study shows that mechanical operations performed without water cooling generate a paramagnetic center inducing a stable isotropic EPR signal with g-value of 2.00320 and linewidth of about 0.1 mT. Using EPR spectrum simulation, the similarity between the mechanically induced signal and the signal generated when the enamel is heated in air at a temperature above 600 degrees C was investigated. Results indicate that the mechanically induced signal is related to sample temperature increase during mechanical friction.

Curved optics for x-ray phase contrast imaging by synchrotron radiation.

Conventional radiographic techniques have strong limitations when low-absorption contrast samples are imaged. Phase contrast radiography has been shown to produce high-quality images of soft tissues. In this technique the recorded intensity patterns are related to gradients in the refractive index of the sample. A critical point of this new technique is the need to employ crystal analysers, which results in an appreciable reduction in the beam intensity and consequently in rather long exposure times. In this paper the use of focused beams is suggested to overcome this aspect. Biological samples with small structures and low absorption variations were imaged using both flat and curved monochromator crystals, demonstrating that the use of curved optics leads to a decrease in the exposure time with only a limited degradation of the spatial resolution. This opens up the possibility of using the phase contrast technique with laboratory sources.

Dosimetric characterization of silicon and diamond detectors in low-energy proton beams.

The dosimetric behaviour of a Scanditronix p-type silicon diode and a PTW natural diamond detector was studied in low-energy proton beams in the 8.3-21.5 MeV range. The properties investigated were linearity, reproducibility, dose rate dependence, energy and linear energy transfer (LET) dependence. The influence of detector thickness on the results of depth dose measurements was also demonstrated. A Markus parallel plate ionization chamber was used for reference dosimetry. Silicon diode and diamond detectors showed linearity at therapeutic dose level, reproducibility better than 1% (1sigma) and sensitivity variation with dose rate and proton energy.

Dental radiography: tooth enamel EPR dose assessment from Rando phantom measurements.

Electron paramagnetic resonance dosimetry of tooth enamel is now established as a suitable method for individual dose reconstruction following radiation accidents. The accuracy of the method is limited by some confounding factors, among which is the dose received due to medical x-ray irradiation. In the present paper the EPR response of tooth enamel to endoral examination was experimentally evaluated using an anthropomorphic phantom. The dose to enamel for a single exposure of a typical dental examination performed with a new x-ray generation unit working at 65 kVp gave rise to a CO2- signal of intensity similar to that induced by a dose of about 2 mGy of 60Co. EPR measurements were performed on the entire tooth with no attempt to separate buccal and lingual components. Also the dose to enamel for an orthopantomography exam was estimated. It was derived from TLD measurements as equivalent to 0.2 mGy of 60Co. In view of application to risk assessment analysis, in the present work the value for the ratio of the reference dose at the phantom surface measured with TLD to the dose at the tooth measured with EPR was determined.