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.

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.

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.

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.

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.

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.

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.

Gamma irradiation effects on poly(DL-lactictide-co-glycolide) microspheres.

Gamma radiation treatment plays an increasingly important role in the sterilization/sanitization of pharmaceutical products. However, irradiation may affect the stability of the product and thus its safety of use. We investigated the influence of ionizing radiation on modified release microparticulate drug delivery systems made of two types of polylactide-co-glycolide copolymers (PLG): RG 503 and RG 503H; these polymers have identical molecular weights but different chemical structures. The effect of gamma radiation on polymer stability of the raw polymers (P) and related microspheres (Ms) was evaluated. Samples were irradiated at different irradiation doses (5, 15 and 25 kGy) using 60Co as radiation source. The microspheres were prepared using the spray drying technique. Degradation of PLG and related microspheres was evaluated during six months in terms of average molecular weight (Mw) loss by gel permeation chromatography (GPC) and variation in glass transition temperature (Tg) using differential calorimetry (DSC). The presence of free radicals in the product was tested by electron paramagnetic resonance (EPR). Both P and Ms showed a trend in decreasing their Mw at time 0 as a function of irradiation dose. For RG503 the decay in Mw is always negligible for doses below 15 kGy while it is about 10% for 25 kGy. After 150 days Mw decay was 25% in the microspheres and 20% in the raw polymer. It was not possible to evaluate the radiation effect, at different storage times, for RG503H because this polymer resulted to be unstable even in the regular storage conditions without being irradiated. The concentration of radiation-induced free radicals was higher in RG 503H (both P and Ms) and they were more stable than the free radicals species observed in the case of polymer RG 503. Alterations and/or production of new radicals were observed on exposure of RG 503H microspheres to the light. Radiolytic degradation of RG 503 under vacuum is characterized by a prevalence of the chain scission events leading to a decrease of Mw. Some crosslinking can occur mainly in the post irradiation stage through the decay and coupling of the hydrogen abstraction radicals. A hydroperoxydative cycle, whose mechanism is suggested, is generated in the presence of oxygen.

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.

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.

Alanine dosimetry of proton therapy beams.

An analysis of some of the properties of the ESR-alanine dosimetry in therapeutic proton beams is reported. Thin alanine-based detectors (1 and 2 mm thick pellets and 0.25 mm thick films) were tested in a clinical 62 MeV proton beam. The response of the alanine detectors in unmodulated and modulated proton beams was studied in tissue equivalent phantoms. The dose assessed by alanine was compared to the dose provided by a Markus parallel plate ionization chamber, used for reference dosimetry. Experiments in the 5-250 Gy dose range showed linearity of alanine dose response and no significant differences in the alanine response per unit dose to gammas and protons. Agreement within the experimental uncertainties was found between the alanine and the Markus chamber depth dose curves, including the Bragg peak region.

Radiochromic film dosimetry of a low energy proton beam.

In this work some dosimetric characteristics of MD-55-2 GafChromic films were studied in a low energy proton beam (21.5 MeV) directly in a water phantom. The nonlinearity of the optical density was quantified by a factor P(lin). A correction factor P(en), that accounts for optical density dependence on the energy, was empirically determined. The effects of detector thickness in depth dose measurements and of the film orientation with respect to beam direction were investigated. The results show that the MD-55-2 films provide dose measurements with the films positioned perpendicularly to the proton beam. A dosimetric formalizm is proposed to determine the dose to water at depth d, with films oriented perpendicularly to the beam axis. This formalism uses a calibration factor of the radiochromic film determined directly on the proton beam at a reference depth in water, and the P(lin) factor, that takes into account the nonlinearity of the calibration curve and the P(en) factor that, in turn takes into account the change of proton beam energy in water. The MD-55-2 films with their high spatial resolution and the quasiwater equivalent material are attractive, positioned perpendicularly along the beam axis, for the absolute dose determination of very small beam sizes and modulated proton beams.

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.

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.

Alanine/EPR dosimetry in brachytherapy.

The paper reports the experimental procedure adopted to determine the absorbed dose rate in water per reference air kerma rate, D(Kr), (d, theta), along the transverse bisector axis of a 137Cs brachytherapy source. The dose rate measurements have been carried out at difference distances, d, from the source using alanine dosemeters in a water phantom. The reference air kerma rate, Kr, was determined adopting a 'direct procedure' that uses a spherical ionization chamber in air. The dose rate constant of the source examined was D(Kr) (1, pi/2) = 0.99 +/- 0.03 cGy h(-1) (microGy h(-1))(-1). The values of the radial dose function along the transverse axis, g(d), determined with an uncertainty of 3.4% (1sigma), were found to be in good agreement with the results reported in the literature. The uncertainty in dose rate value has been estimated as 2.8% (1sigma) for distances from the source up to 7 cm. Kr has been determined with 1.2% (1sigma) uncertainty. So D(r) (d, pi/2) values were determined with 3% (1sigma) uncertainty.

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.

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.

Effectiveness of chemical etching for background electron paramagnetic resonance signal reduction in tooth enamel.

A method of preparing enamel samples for electron paramagnetic resonance retrospective dosimetry that leads to a reduction of the non-radiation induced signals (background) is proposed. The method is effective in removing residual dentin and in reducing surface paramagnetic centers both native and induced by the sample preparation procedure. The main steps in the method are the use of longwave length ultraviolet light for distinguishing dentin from enamel and of chemical etching with phosphoric acid for removal of surface paramagnetic centers. Several acid concentrations and etching times were tested and the optimization of the method is illustrated in regard to both sample mass loss and background signal amplitude reduction. The best acid concentration was determined to be 42%, leading to a 30% reduction in the background electron paramagnetic resonance signal amplitude per unit mass.