C.E. de Almeida

Intercomparison of calibration procedures for HDR sources in Brazil

The lack of well established dosimetry protocols for HDR sources is a point of great concern regarding the uniformity of procedures within a particular country and worldwide. The main objective of this paper is to report the results from ten institutions of an intercomparison of calibration procedures for 192Ir HDR sources currently in use in Brazil. The treatment irradiator of one institution was calibrated by a reference system and used by all participants with their own measuring electrometers and ionization chambers under the same experimental conditions. Two methods were used: the calibration jig and the well-type ionization chamber. Each participant was allowed to use their own method and formalism. The results of this exercise were very positive since this was the first time in Brazil that a group of users gathered to share their experience and openly discuss the physical concepts behind the calibration procedures. The results were all within +/-3.0%, except one case where -4.6% was observed and later identified as a problem with the Nk value for x-rays. Though the magnitude of the deviations found was generally acceptable considering the diversity of formalisms currently in use, a proposal is now being prepared to be adopted as a national protocol. The identification of the institutions was left out for the sake of confidentiality.

Experimental derivation of wall correction factors for ionization chambers used in high dose rate 192Ir source calibration.

At present there are no specific primary standards for 192Ir high dose rate sources used in brachytherapy. Traceability to primary standards is guaranteed through the method recommended by the AAPM that derives the air kerma calibration factor for the 192Ir gamma rays as the average of the air kerma calibration factors for x-rays and 137Cs gamma-rays or the Maréchal et al. method that uses the energy-weighted air kerma calibration factors for 250 kV x rays and 60Co gamma rays as the air kerma calibration factor for the 192Ir gamma rays. In order to use these methods, it is necessary to use the same buildup cap for all energies and the appropriate wall correction factor for each chamber. This work describes experimental work used to derive the A(W) for four different ionization chambers and different buildup cap materials for the three energies involved in the Maréchal et al. method. The A(W) for the two most common ionization chambers used in hospitals, the Farmer NE 2571 and PTW N30001 is 0.995 and 0.997, respectively, for 250 kV x rays, 0.982 and 0.985 for 192Ir gamma rays, and 0.979 and 0.991 for 60Co gamma rays, all for a PMMA build-up cap of 0.550 gm cm(-2). A comparison between the experimental values and Monte Carlo calculations shows an agreement better than 0.9%. Availability of the A(W) correction factors for all commercial chambers allows users of the in-air calibration jig, provided by the manufacturer, to alternatively use the Maréchal et al. method. Calibration laboratories may also used this method for calibration of a well-type ionization chamber with a comparable accuracy to the AAPM method.

Monte Carlo calculations of the ionization chamber wall correction factors for 192Ir and 60Co gamma rays and 250 kV x-rays for use in calibration of 192Ir HDR brachytherapy sources

As in the method for the calibration of 192Ir high-dose-rate (HDR) brachytherapy sources, the ionization chamber wall correction factor A(w), is needed for 192Ir and 60Co gamma rays and 250 kV x-rays. This factor takes into account the variation in chamber response due to the attenuation of the photon beam in the chamber wall and build-up cap and the contribution of scattered photons. Monte Carlo calculations were performed using the EGS4 code system with the PRESTA algorithm, to calculate the A(w) factor for 51 commercial ionization chambers and build-up caps exposed to the typical energy spectrum of 192Ir and 60Co gamma rays and 250 kV x-rays. The calculated A(w) correction factors for 192Ir and 60Co sources and 250 kV x-rays agree very well to within 0.1% with published experimental data (the statistical uncertainty is less than 0.1% of the calculated correction factor value). For the 192Ir sources, A(w) varies from 0.973 to 0.993 and for the 250 kV x-rays the minimum value of A(w) for all chambers studied is 0.983. The calculated A(w) correction factors can be used to calculate the air kerma calibration factor of HDR brachytherapy sources, when interpolative methods are considered, contributing to the reduction in the overall uncertainties in the calibration procedure.

8 and 10 MeV Electron Beams Small Field-Size Dosimetric Parameters Through the Fricke Xylenol Gel Dosimeter

When small field sizes are recommended in radiotherapy, the dosimeter must have an adequate spatial resolution in order to determine the absorbed dose at the region of interest. The study of electron small field size is important since its dosimetry is not commonly performed in the clinical routine. It was verified that the Fricke Xylenol Gel (FXG) chemical dosimeter, with an effective atomic number of 7.75 and density of 1.05 g/cm3, presents adequate spatial resolution for absorbed dose distribution measurements, when small field sizes (square and circular) for 8 and 10 MeV electron beams are considered. The absorbed dose values are proportional to the absorbance spectrophotometric measurements that are proportional to the concentration of Fe+3 and the xylenol orange (XO) dye complex produced in the gel. The FXG behavior, for small field sizes irradiated with electron beams, was compared with those obtained using a small ionization chamber (IC). In this study, dosimetric parameters, such as beam profile, output factor, and percentage depth dose were evaluated. Since the dosimeter results showed no significant differences and the IC is considered the standard reference dosimeter by radiotherapy protocols, the FXG was validated for dosimetric parameter measurements to small field-size electron-beam irradiations.

Perturbation corrections for flat and thimble-type cylindrical standard ionization chambers for 60Co gamma rays: Monte Carlo calculations

The use of an ionization chamber for absorbed dose determinations in a medium requires one to take into account perturbation corrections due to the presence of the chamber cavity in the medium. Evaluation of these corrections for perturbation and their variation with depth in the medium has been performed for a flat cylindrical and a cylindrical (thimble-type) ionization chamber placed in a graphite phantom irradiated by a 60Co gamma beam using Monte Carlo calculations (EGS4 system with correlated sampling variance reduction technique). The results of these calculations agree with published experimental and theoretical data to better than 0.18%, with a statistical uncertainty of less than 0.17%.

The radiation metrology network related to the field of mammography: implementation and uncertainty analysis of the calibration system

It is recognized by the international guidelines that it is necessary to offer calibration services for mammography beams in order to improve the quality of clinical diagnosis. Major efforts have been made by several laboratories in order to establish an appropriate and traceable calibration infrastructure and to provide the basis for a quality control programme in mammography. The contribution of the radiation metrology network to the users of mammography is reviewed in this work. Also steps required for the implementation of a mammography calibration system using a constant potential x-ray and a clinical mammography x-ray machine are presented. The various qualities of mammography radiation discussed in this work are in accordance with the IEC 61674 and the AAPM recommendations. They are at present available at several primary standard dosimetry laboratories (PSDLs), namely the PTB, NIST and BEV and a few secondary standard dosimetry laboratories (SSDLs) such as at the University of Wisconsin and at the JAEAs SSDL. We discuss the uncertainties involved in all steps of the calibration chain in accord with the ISO recommendations.

Perturbation correction of a cylindrical thimble-type chamber in a graphite phantom for 60Co gamma rays

An experimental determination has been made of the perturbation correction to be applied to a graphite cylindrical (thimble-type) ionisation chamber used for the measurement of absorbed dose in graphite. The results show that the magnitude of this correction increases with depth and depends on the size of the cavity. The knowledge of this correction makes possible the use of such a chamber as a standard dose in graphite for 60Co gamma rays.

Determination of absorbed dose in water at the reference point D(r{sub 0},{theta}{sub 0}) for an {sup 192}Ir HDR brachytherapy source using a Fricke system

A ring-shaped Fricke device was developed to measure the absolute dose on the transverse bisector of a Ir192 high dose rate (HDR) source at 1cm from its center in water, D(r0,θ0). It consists of a polymethylmethacrylate (PMMA) rod (axial axis) with a cylindrical cavity at its center to insert the Ir192 radioactive source. A ring cavity around the source with 1.5mm thickness and 5mm height is centered at 1cm from the central axis of the source. This ring cavity is etched in a disk shaped base with 2.65cm diameter and 0.90cm thickness. The cavity has a wall around it 0.25cm thick. This ring is filled with Fricke solution, sealed, and the whole assembly is immersed in water during irradiations. The device takes advantage of the cylindrical geometry to measure D(r0,θ0). Irradiations were performed with a Nucletron microselectron HDR unit loaded with an Ir192 Alpha Omega radioactive source. A Spectronic® 1001 spectrophotometer was used to measure the optical absorbance using a 1mL quartz cuvette with 1.00cm light pathlength. The PENELOPE Monte Carlo code (MC) was utilized to simulate the Fricke device and the Ir192 Alpha Omega source in detail to calculate the perturbation introduced by the PMMA material. A NIST traceable calibrated well type ionization chamber was used to determine the air-kerma strength, and a published dose-rate constant was used to determine the dose rate at the reference point. The time to deliver 30.00Gy to the reference point was calculated. This absorbed dose was then compared to the absorbed dose measured by the Fricke solution. Based on MC simulation, the PMMA of the Fricke device increases the D(r0,θ0) by 2.0%. Applying the corresponding correction factor, the D(r0,θ0) value assessed with the Fricke device agrees within 2.0% with the expected value with a total combined uncertainty of 3.43% (k=1). The Fricke device provides a promising method towards calibration of brachytherapy radiation sources in terms of D(r0,θ0) and audit HDR source calibrations.

The influence of urbanization on natural radiation levels in anomalous areas.

This paper verifies the effects of urbanization on the original levels of environmental gamma radiation in an anomalous area. The fieldwork was done in the city of Guarapari, on the seacoast of the state of Espírito Santo, Brazil. This place was chosen because the whole region is naturally rich in monazite sand, which contains thorium and uranium. A similar study was also carried out in the 1960s. The measurements were done using an inorganic NaI scintillometer detector with Cu+Pb filters coupled to a scaler. The methodology of measurements used was similar to the one applied in the fieldwork carried out in the 1960s. In addition, a simulation was performed in the laboratory, to determine the attenuation of the materials used in the urbanization of the region. The results of this work clearly show that there was a reduction in the levels of external radiation in the streets and squares of Guarapari. It was concluded that the reduction was due to attenuation by the materials used in the urbanization.

A graphite transmission ionization chamber

A pancake-type transmission chamber made of high-purity graphite and open to the atmosphere has been designed and constructed at the Secondary Standard Dosimetry Laboratory (SSDL-Rio de Janeiro). Tests performed on the chamber following the International Electrotechnical Commission recommendations indicate that its performance characteristics are comparable to those expected from a secondary standard ionization chamber.