Extrapolation Chamber Research Articles

Dosimetry of indigenously developed 177Lu patch source for surface brachytherapy—Experimental and Monte Carlo methods

This paper describes the evaluation of dosimetry characteristics of an in-house developed 177Lu skin patch source for treatment of non-melanoma skin cancer. A 177Lu skin patch source based on Nafion-115 membrane backbone containing 3.46 ± 0.01 mCi of activity was used. Activity measurement of the patch source was based on gamma ray spectrometry using a HPGe detector. The efficiencies of the HPGe detector were fitted using an orthogonal polynomial function. The absorbed dose rate to water at 5 μm depth in water was determined using an extrapolation chamber, EBT3 Gafchromic film and compared with Monte Carlo methods. The correction factors such as Bragg-Gray stopping power ratio of water-to-air and chamber wall material being different from water, needed to be applied on measurements for establishing the dose rate at 5 μm depth, were calculated using the Monte Carlo method. Absorbed dose rate at 5 μm depth in water (surface dose rate) measured using an extrapolation chamber and EBT3 Gafchromic film were 9.9 ± 0.7 and 8.2 ± 0.1 Gy h−1 mCi−1 respectively for the source activity of 3.46 ± 0.01 mCi. The surface dose rate calculated using the Monte Carlo method was 8.7 ± 0.2 Gy h−1 mCi−1, which agrees reasonably well with measurement. The measured dose rate per mCi offers scope for ascertaining treatment time required to deliver the dose for propitious therapeutic outcome. Additionally, on-axis depth dose and lateral dose profiles at 5 μm and 1 mm depth in water phantom were also calculated using the Monte Carlo method.

Surface dose rate from a flat 106Ru/106Rh episcleral plaque measured with a planar windowless extrapolation chamber and un-laminated EBT3 film

In this work, a planar extrapolation chamber without an entrance window was used to measure surface dose rate to water from a flat 106Ru/106Rh plaque manufactured by Eckert & Ziegler BEBIG. Measurements with the flat source were performed as part of a larger aim to develop a convex windowless extrapolation chamber for determining surface dose rate from curved 106Ru/106Rh plaques. Monte Carlo corrections for backscatter and divergence in the chamber were generated with the EGSnrc cavity user code. Surface dose rate values from the extrapolation chamber were compared with un-laminated EBT3 film results based on a 60Co calibration. To determine surface dose using radiochromic film, an additional Monte Carlo correction was needed to account for phantom scatter and volume-averaging within the active layer for the on-contact measurement setup. Additionally, extrapolation chamber results for the flat plaque were compared with scintillation detector measurements performed by the manufacturer. The average surface dose rate from the extrapolation chamber was found to be 1.5% lower than the film measurement result and 8.0% lower than the manufacturer-provided source calibration. Based on the results, it was concluded that a windowless extrapolation chamber can be used as a primary measurement device to determine surface dose rate from a 106Ru/106Rh episcleral plaque, and radiochromic film measurements can also be carried out to serve as an accurate comparison.

Design, fabrication and characterization of a windowless extrapolation chamber for low-energy X-rays: Experimental and Monte Carlo results

Extrapolation chamber is a special parallel-plate ionization chamber, particularly suggested for low penetrating radiations. Its sensitive volume is varied through a change of the inter-electrode distance, ideally satisfying the Bragg-Gray cavity theory. A windowless extrapolation chamber was designed and constructed for low-energy X-rays, produced at the tube voltage of 70–100 kV. It consists of the following elements and innovation. The electrodes of the chamber are made of a homogenous block of graphite instead of a coating graphite layer to reduce the leakage current. Also, the entrance window (first layer of front window) has been eliminated to decrease the beam perturbation. To the best of our knowledge, this type of chamber has not been used in low energy X-rays before. The performance of the chamber was investigated in terms of its saturation curve, polarity effect, leakage current, linearity of response and short-term stability. It was also simulated using Monte Carlo method and in the worst case, the relative ionization current obtained in experiment differs about 4.5% with that in the simulation result. Moreover, the influence of chamber components on its response was examined. All of the results showed the feasibility of its use for detection of 70–100 kV X-rays.

Surface Dose Rate from a106Ru Episcleral Plaque Measured with a Convex Windowless Extrapolation Chamber and EBT3 Film

Surface Dose Rate from a106Ru Episcleral Plaque Measured with a Convex Windowless Extrapolation Chamber and EBT3 Film

Modelling the absorbed dose rate of the beta standard BSS2 147Pm source

The dosimetric measurements of 147Pm beta radiation beams have limitations due to their low energy, low dose rate, great dispersion and attenuation in air or tissue. In this work, the Monte Carlo model was developed for a 147Pm absorbed dose rate determination. This model consists of an extrapolation chamber and a 147Pm beta radiation source. Moreover, the absorbed dose rate was determined by experimental measurements and the MCNP Monte Carlo code was used. The relative number of particles that crossed the radioactive source window and the particles that crossed the extrapolation chamber entrance window were determined. The source fluence spectrum was also determined. The results of the simulation and the experimental calculations are in agreement with the absorbed dose rate from the PTB calibration certificate. The results obtained are considered acceptable, and they agree within the uncertainties. The difference between the experimental result and that from the Monte Carlo model, compared to that from the calibration certificate, was only 0.8% in both cases.

Determination of correction factors in beta radiation beams using Monte Carlo method

The absorbed dose rate is the main characterization quantity for beta radiation. The extrapolation chamber is considered the primary standard instrument. To determine absorbed dose rates in beta radiation beams, it is necessary to establish several correction factors. In this work, the correction factors for the backscatter due to the collecting electrode and to the guard ring, and the correction factor for Bremsstrahlung in beta secondary standard radiation beams are presented. For this purpose, the Monte Carlo method was applied. The results obtained are considered acceptable, and they agree within the uncertainties. The differences between the backscatter factors determined by the Monte Carlo method and those of the ISO standard were 0.6%, 0.9% and 2.04% for 90Sr/90Y, 85Kr and 147Pm sources respectively. The differences between the Bremsstrahlung factors determined by the Monte Carlo method and those of the ISO were 0.25%, 0.6% and 1% for 90Sr/90Y, 85Kr and 147Pm sources respectively.

Alpha radiation dosimetry using Fluorescent Nuclear Track Detectors

To answer the need for better tools for alpha radiation radiobiology and microdosimetry research, a novel irradiation setup based on a honeycomb collimator, in combination with Fluorescent Nuclear Track Detectors (FNTD) for alpha radiation dosimetry and spectroscopy, was introduced. FNTDs are a novel type of small, crystalline detector that can visualize individual alpha particles and simultaneously measure their location, velocity direction and energy with good accuracy. The performance of FNTDs for alpha radiation dosimetry was evaluated for the first time and the results were compared to extrapolation chamber measurements and simulations. The surface dose rate to water of the irradiation setup for two different honeycomb collimators, measured using FNTDs, agreed with the extrapolation chamber measurements within 6%. The simulations underestimated the surface dose rate to water for the first collimator and overestimated the dose for the second collimator, indicating the sensitivity to manufacturing errors in the collimators of this irradiation setup. The dose homogeneity in the setup was measured using radiochromic film and showed variations of less than 5%, making this setup, in combination with the rich information obtained regarding the spatial, angular and energy distributions of the alpha particles, obtained using the FNTDs, ideal for microdosimetry and radiobiology experiments. The accuracy and ease-of-use of FNTDs in addition to the surface or absorbed dose and fluence of the radiation field indicate that these detectors are prime candidates for research applications in the field of alpha radionuclide therapy.

Testing an extrapolation chamber in computed tomography standard beams

The computed tomography (CT) is responsible for the highest dose values to the patients. Therefore, the radiation doses in this procedure must be accurate. However, there is no primary standard system for this kind of radiation beam yet. In order to search for a CT primary standard, an extrapolation ionization chamber built at the Calibration Laboratory (LCI) of the Instituto de Pesquisas Energéticas e Nucleares (IPEN), was tested in this work. The results showed to be within the international recommended limits.

Re-evaluation of the correction factors for the GROVEX

The GROVEX (GROssVolumige EXtrapolationskammer, large-volume extrapolation chamber) is the primary standard for the dosimetry of low-dose-rate interstitial brachytherapy at the Physikalisch-Technische Bundesanstalt (PTB). In the course of setup modifications and re-measuring of several dimensions, the correction factors have been re-evaluated in this work. The correction factors for scatter and attenuation have been recalculated using the Monte Carlo software package EGSnrc, and a new expression has been found for the divergence correction. The obtained results decrease the measured reference air kerma rate by approximately 0.9% for the representative example of a seed of type Bebig I25.S16C. This lies within the expanded uncertainty (k = 2).

Determination of transmission factors in beta radiation beams

In beta emitters, in order to evaluate the absorbed dose rate at different tissue depths, it is necessary to determine the transmission factors. In this work, the transmission factors determined in beta secondary standard radiation beams are presented. For this purpose, an extrapolation chamber was used. The results obtained were considered acceptable, and they are within the uncertainties in comparison with the values provided by the source calibration certificate. The maximum differences between the results obtained in this work and those from the calibration certificate were 3.3%, 3.8% and 5.9% for 90Sr/90Y, 85Kr and 147Pm sources respectively.

Windowless extrapolation chamber measurement of surface dose rate from a 90Sr/90Y ophthalmic applicator

In this study, a planar extrapolation chamber without an entrance window was tested through measurements of a 90Sr/90Y ophthalmic applicator that had been previously calibrated at the National Institute of Standards and Technology (NIST). This work was performed as part of a larger aim to determine surface dose rate from curved 106Ru/106Rh plaques using a convex windowless extrapolation chamber. Capacitance measurements were carried out to assess the initial air gap width between the source face and collecting electrode. The EGSnrc DOSRZnrc Monte Carlo user code was used to determine correction factors for backscatter, divergence, and attenuation caused by the aluminized Mylar® entrance window in the NIST extrapolation chamber. Measured surface dose rate values were compared with the expected result from the NIST source calibration, and the average observed difference was +4.1% with a standard deviation of ±0.6%. The dose rate from each measurement trial was within the estimated combined uncertainty in the reference source calibration of ±7.0% (k = 2) given by NIST. The experiment results suggest that an entrance window is not needed to achieve accurate dose rate measurements for a beta-emitting source with a planar extrapolation chamber, as long as the collecting electrode diameter is small compared to the source diameter. Therefore, a convex extrapolation chamber without an entrance window is proposed for the calibration of concave 106Ru/106Rh episcleral plaques.

Variance reduction technique in a beta radiation beam using an extrapolation chamber

This paper aims to show how the variance reduction technique “Geometry splitting/Russian roulette” improves the statistical error and reduces uncertainties in the determination of the absorbed dose rate in tissue using an extrapolation chamber for beta radiation. The results show that the use of this technique can increase the number of events in the chamber cavity leading to a closer approximation of simulation result with the physical problem. There was a good agreement among the experimental measurements, the certificate of manufacture and the simulation results of the absorbed dose rate values and uncertainties. The absorbed dose rate variation coefficient using the variance reduction technique “Geometry splitting/Russian roulette” was 2.85%.

Windowless Extrapolation Chamber and Radiochromic Film Measurements of a Flat 106Ru Eye Plaque

Windowless Extrapolation Chamber and Radiochromic Film Measurements of a Flat 106Ru Eye Plaque

Development of a primary standard for absorbed dose from unsealed radionuclide solutions

Currently, the determination of the internal absorbed dose to tissue from an administered radionuclide solution relies on Monte Carlo (MC) calculations based on published nuclear decay data, such as emission probabilities and energies. In order to validate these methods with measurements, it is necessary to achieve the required traceability of the internal absorbed dose measurements of a radionuclide solution to a primary standard of absorbed dose. The purpose of this work was to develop a suitable primary standard. A comparison between measurements and calculations of absorbed dose allows the validation of the internal radiation dose assessment methods.The absorbed dose from an yttrium-90 chloride (90YCl) solution was measured with an extrapolation chamber. A phantom was developed at the National Physical Laboratory (NPL), the UK’s National Measurement Institute, to position the extrapolation chamber as closely as possible to the surface of the solution. The performance of the extrapolation chamber was characterised and a full uncertainty budget for the absorbed dose determination was obtained. Absorbed dose to air in the collecting volume of the chamber was converted to absorbed dose at the centre of the radionuclide solution by applying a MC calculated correction factor. This allowed a direct comparison of the analytically calculated and experimentally determined absorbed dose of an 90YCl solution.The relative standard uncertainty in the measurement of absorbed dose at the centre of an 90YCl solution with the extrapolation chamber was found to be 1.6% (k = 1). The calculated 90Y absorbed doses from published medical internal radiation dose (MIRD) and radiation dose assessment resource (RADAR) data agreed with measurements to within 1.5% and 1.4%, respectively. This study has shown that it is feasible to use an extrapolation chamber for performing primary standard absorbed dose measurements of an unsealed radionuclide solution. Internal radiation dose assessment methods based on MIRD and RADAR data for 90Y have been validated with experimental absorbed dose determination and they agree within the stated expanded uncertainty (k = 2).

Extrapolation Ionization Chamber Dosimetry of Fluorescent X-Ray Energies from 4.5 to 19.6 keV.

Characteristic X rays of energies less than approximately 20 keV are of interest in radiobiology and radiation oncology. There is evidence that these low-energy photons produce higher relative biological effectiveness (RBE) and lower oxygen enhancement ratio (OER) relative to higher energies. Lower energy X rays also offer the advantage of healthy tissue sparing beyond the target treatment depth. Electronic brachytherapy systems that can deliver characteristic and bremsstrahlung X rays of varying energy are in clinical use as well as under development. We performed low-energy extrapolation ionization chamber dosimetry using two methods: 1. the exposure-to-dose method; and 2. the Burlin theory method combined with the extrapolation chamber method of Klevenhagen. We investigated fluorescent X rays emitted from seven metals: titanium (Ti, Z = 22); chromium (Cr, Z = 24); iron (Fe, Z = 26); cobalt (Co, Z = 27); copper (Cu, Z = 29); zinc (Zn, Z = 30); and molybdenum (Mo, Z = 42). X rays were produced by irradiation of the metals with a 55 kVp, 45 mA silver anode spectrum. The data obtained were air kerma rate (cGy/min), and radiation dose rate (cGy/min) in phosphate-buffered saline (PBS) solution and water. Air kerma rates ranged from 3.55 ± 0.10 to 14.36 ± 0.39 cGy/min. Dose rates ranged from 3.85 ± 0.10 to 16.96 ± 0.46 cGy/min in PBS and 3.59 ± 0.10 to 16.06 ± 0.43 cGy/min in water. Dose-rate energy dependence of both models was examined by taking a ratio of measured to Monte Carlo calculated dose rates. Dosimetry method 1 exhibited a linear relationship across all energies with a slope of 0.0127 keV(-1) and R(2) of 0.9276. Method 2 exhibited a linear relationship across all energies with a slope of 0.0467 keV(-1) and R(2) of 0.9933. Method 1 or 2 may be used as a relative dosimetry system to derive dose rates to water by using a second reference ion chamber with a NIST-traceable calibration for the molybdenum spectrum.

Determination of true null electrode spacing of an extrapolation chamber for x-ray dosimetry

An accurate determination of the true null distance between electrodes, that defines the sensitive volume of an extrapolation chamber, is critical for the establishment of a primary measurement method for absorbed dose in tissue. In this work, a critical analysis of two methodologies for determining the true null electrode spacing of a 23392 Böhm model PTW extrapolation chamber was done.

WE‐DE‐201‐05: Evaluation of a Windowless Extrapolation Chamber Design and Monte Carlo Based Corrections for the Calibration of Ophthalmic Applicators

Purpose:To test the validity of a windowless extrapolation chamber used to measure surface dose rate from planar ophthalmic applicators and to compare different Monte Carlo based codes for deriving correction factors.Methods:Dose rate measurements were performed using a windowless, planar extrapolation chamber with a 90Sr/90Y Tracerlab RA‐1 ophthalmic applicator previously calibrated at the National Institute of Standards and Technology (NIST). Capacitance measurements were performed to estimate the initial air gap width between the source face and collecting electrode. Current was measured as a function of air gap, and Bragg‐Gray cavity theory was used to calculate the absorbed dose rate to water. To determine correction factors for backscatter, divergence, and attenuation from the Mylar entrance window found in the NIST extrapolation chamber, both EGSnrc Monte Carlo user code and Monte Carlo N‐Particle Transport Code (MCNP) were utilized. Simulation results were compared with experimental current readings from the windowless extrapolation chamber as a function of air gap. Additionally, measured dose rate values were compared with the expected result from the NIST source calibration to test the validity of the windowless chamber design.Results:Better agreement was seen between EGSnrc simulated dose results and experimental current readings at very small air gaps (<100 µm) for the windowless extrapolation chamber, while MCNP results demonstrated divergence at these small gap widths. Three separate dose rate measurements were performed with the RA‐1 applicator. The average observed difference from the expected result based on the NIST calibration was −1.88% with a statistical standard deviation of 0.39% (k=1).Conclusion:EGSnrc user code will be used during future work to derive correction factors for extrapolation chamber measurements. Additionally, experiment results suggest that an entrance window is not needed in order for an extrapolation chamber to provide accurate dose rate measurements for a planar ophthalmic applicator.

SU-G-201-05: Comparison of Different Methods for Output Verification of Eleckta Nucletron's Valencia Skin Applicators

Purpose: The provided output factors for Elekta Nucletron's skin applicators are based on Monte Carlo simulations. These outputs have not been independently verified, and there is no recognized method for output verification of the vendor's applicators. The purpose of this work is to validate the outputs provided by the vendor experimentally. Methods: Using a Flexitron Ir-192 HDR unit, three experimental methods were employed to determine dose with the 30 mm diameter Valencia applicator: first a gradient method using extrapolation ionization chamber (Far West Technology, EIC-1) measurements in solid water phantom at 3 mm SCD was used. The dose was derived based on first principles. Secondly a combination of a parallel plate chamber (Exradin A-10) and the EIC-1 was used to determine air kerma at 3 mm SCD. The air kerma was converted to dose to water in line with TG-61 formalism by using a muen ratio and a scatter factor measured with the skin applicators. Similarly a combination of the A-10 parallel plate chamber and gafchromic film (EBT 3) was also used. The Nk factor for the A-10 chamber was obtained through linear interpolation between ADCL supplied Nk factors for Cs-137 and M250. Results: EIC-1 measurements in solid water defined the outputs factor at 3 mm as 0.1343 cGy/U hr. The combination of A-10/ EIC-1 and A-10/EBT3 lead to output factors of 0.1383 and 0.1568 cGy/U hr, respectively. For comparison the output recommended by the vendor is 0.1659 cGy/U hr. Conclusion: All determined dose rates were lower than the vendor supplied values. The observed discrepancy between extrapolation chamber and film methods can be ascribed to extracameral gradient effects that may not be fully accounted for by the former method.

SU‐F‐T‐64: An Alternative Approach to Determining the Reference Air‐Kerma Rate from Extrapolation Chamber Measurements

Purpose:Since 2008 the Physikalisch‐Technische Bundesanstalt (PTB) has been offering the calibration of 125I‐brachytherapy sources in terms of the reference air‐kerma rate (RAKR). The primary standard is a large air‐filled parallel‐plate extrapolation chamber. The measurement principle is based on the fact that the air‐kerma rate is proportional to the increment of ionization per increment of chamber volume at chamber depths greater than the range of secondary electrons originating from the electrode x0.Methods:Two methods for deriving the RAKR from the measured ionization charges are: (1) to determine the RAKR from the slope of the linear fit to the so‐called 'extrapolation curve’, the measured ionization charges Q vs. plate separations x or (2) to differentiate Q(x) and to derive the RAKR by a linear extrapolation towards zero plate separation. For both methods, correcting the measured data for all known influencing effects before the evaluation method is applied is a precondition. However, the discrepancy of their results is larger than the uncertainty given for the determination of the RAKR with both methods.Results:A new approach to derive the RAKR from the measurements is investigated as an alternative. The method was developed from the ground up, based on radiation transport theory. A conversion factor C(x1, x2) is applied to the difference of charges measured at the two plate separations x1 and x2. This factor is composed of quotients of three air‐kerma values calculated for different plate separations in the chamber: the air kerma Ka(0) for plate separation zero, and the mean air kermas at the plate separations x1 and x2, respectively. The RAKR determined with method (1) yields 4.877 µGy/h, and with method (2) 4.596 µGy/h. The application of the alternative approach results in 4.810 µGy/h.Conclusion:The alternative method shall be established in the future.

Comparison of Hp(0.07, 0°) for 90Sr/90Y Beta Radiation with CaF2:Dy TLD-200 Dosimeters.

The results for the comparison of Hp(0.07,0°) in 90Sr/90Y, carried out by the SSDL-ININ (pilot) and CPHR, are presented. Four calibration curves (CCs) are constructed with 32 TLD-200 dosimeters irradiated on a PMMA phantom. The CCs, TLD's Response RTLD (nC) vs Hp(0.07,0°), are in the range0.2-100 mSv. The SSDL-ININrate Hp(0.07, 0°) value is (43.85±0.57) µSv s-1 obtained with a primary extrapolation chamber serial 040. These same dosimeters are irradiated at the CPHR with a rate of Hp(0.07, 0°)=(14.94±0.36) µSv s-1, also calibrated with another primary standard extrapolation chamber serial 105. After irradiation at CPHR, the dosimeters are returnedto the SSDL-ININ for reading, and then irradiated again to construct the fourth CC (CC4). The comparison is based on a modification of the standard ANSI/HPS N13.11-2009, where the reference values are [Hp]ININ,ref. For the values of [Hp]CPHR the statistics bias B=0.002; standard deviation, S=0.099; and tolerance level L=0.099 are determined. The homoscedasticity test is performed for the variances associated to the mean performance quotientB.