Standard Ionization Chamber Research Articles

SU‐GG‐I‐66: Comparison of Use of Cs‐137 Versus Scattered X‐Ray Beam for Calibration of Diagnostic Survey Meters

Purpose: To determine the effect of the standard calibration of survey meters using Cs‐137 compared to the clinical measurement of radiation exposure of scattered radiation from a typical diagnostic x‐ray beam. Method and Materials: A phantom consisting of 8 inches of Lucite was used as a source of scattered radiation. A standard pancake ion chamber with a volume of 150 cc that had been recently calibrated was used as a reference. Three different survey meters (Victoreen Model 470 A Panorama, Keithley Model 36150, and Fluke 451) used in various applications of radiation surveys were placed at the same distance from the center of the scattering material as the ion chamber. Each of the test survey meters had been calibrated recently with a Cs‐ 137 source. X‐ray exposures were made as a function of kVp and additional filtration (Al and Cu). Measurements were made in the integral and rate mode. Results: There was no significant deviation noted in the rate mode of the test meters compared to the 100 cc chamber. No significant systematic deviation was noted for exposures obtained with no additional filtration and the use of external filtration of Al and Cu to simulate the types of filtration encountered in modern x‐ray and angiographic systems. Some systematic differences were noted for the integrate mode, but this appeared to be a function of the device and not the x‐ray spectrum. Conclusion: The use of Cs‐137 for calibrating survey meters used in diagnostic imaging radiation survey measurement provides an accurate calibration for diagnostic survey meters. Conversely, an x‐ray beam with scatter could be employed to calibrate survey meters for diagnostic applications.

WE‐E‐332‐01: A Unified Dose Indicator for Tomographic Acquisition Modalities

Introduction : Tomographic techniques in diagnostic and interventional radiology or radiation therapy are in rapid development. The CTDI formalism that is currently used to assess dose has been challenged by several groups. The goal of this contribution is to evaluate the required scan length to reach equilibrium on various MDCT units and estimate the average dose delivered within a slice when using systems having a cone beam geometry.Method and Materials: Measurements were performed on three MDCT systems (GEMS 8, 16 and 64‐row), a flat panel fluoroscopy (Allura — Philips), an IGRT (Synergy, Elekta ltd) and on a tomotherapy (TomoTherapy Inc.) using two standard CTDI phantoms and a home made phantoms (PMMA cylinder of 30 cm filled with water) with a conventional small volume ion chamber (0.6 cc Farmer type) and a standard pencil ion chamber. Dose profiles were also recorded at various positions within the slice to study the impact of scatter as a function of the distance between the centre of the phantom and its periphery. Results: For CT units a theoretical length of 400 mm is required to reach the equilibrium at the center of the phantom. The measurements show that a dose plateau was reached after 420 mm. The average dose within a slice measured in our home made phantom for standard abdominal CT (120 kV, 210 effective mAs) was 15.3 mGy; 26.9 mGy for the fluoro‐CT (122kV, 274mAs, 20.7s); 39.0mGy for IGRT system (120kV, 219mAs, 27×27cm2 field size, one rotation) and 13.1 mGy for the Tomotherapy system (3.5 MV, pitch=0.8). Conclusion: The CTDI concept should be replaced by a more generic methodology, such as the one presented here, that could be used in all cone beam geometries. Measurements should be done using water equivalent phantoms.

SU‐GG‐T‐138: Dosimetric Characteristics of 2D Ion Chamber Array Matrix for Verification Radiotherapy Treatments

Purpose: To study the dosimetric characteristics of 2D ion chamber array matrix for verification of Radiotherapy treatment plans. Method and Materials: The ionization chamber array consists of 1020 single air‐vented plane‐parallel plate ion chambers arranged in 32 × 32 matrix. Each chamber is of 4.5 mm diameter, 5 mm height and 0.08 cc of sensitive volume. The measurements were performed in Clinac DHX linear accelerator with 6 MV and 18 MV photons in a solid water phantom. The absolute dose and output factors were estimated for 6 and 18 MV photons using matrix device. The detector reproducibility, dose linearity and dose rate effect was studied. The performance of 2D array, in measuring clinical dose maps, was also investigated. Dose profiles of physical and enhanced dynamic wedge fields were analyzed. Furthermore, various IMRT fluence patterns generated from treatment planning system (TPS) were measured and compared. Results: The matrix device gives the absolute dose values based on estimated calibration factor. The reproducibility of measurements was good and the output and the output factor values were in good agreement with the standard ion chamber measurements. The system response was found to be linear in the dose range of 2–500 cGy and the response of the detector was found to be independent of dose rate. The 2D verification of intensity map at different depths for physical and enhanced dynamic wedges were in good agreement. The calculated and measured IMRT fluence patterns were found to be in good agreement (γ ⩽1:96 %, criteria 3 % &3 mm). Conclusion: On the basis of broad range of tests performed in this study, we conclude that the 2D array matrix is dosimetrically accurate and it can be a useful device for QA and verification of clinical radiotherapy beams.

Design and implementation of a head‐and‐neck phantom for system audit and verification of intensity‐modulated radiation therapy

The head and neck is a challenging anatomic site for intensity‐modulated radiation therapy (IMRT), requiring thorough testing of planning and treatment delivery systems. Ideally, the phantoms used should be anatomically realistic, have radiologic properties identical to those of the tissues concerned, and allow for the use of a variety of devices to verify dose and dose distribution in any target or normal‐tissue structure.A phantom that approaches the foregoing characteristics has been designed and built; its specific purpose is verification for IMRT treatments in the head‐and‐neck region. This semi‐anatomic phantom, HANK, is constructed of Perspex (Imperial Chemical Industries, London, U.K.) and provides for the insertion of heterogeneities simulating air cavities in a range of fixed positions. Chamber inserts are manufactured to incorporate either a standard thimble ionization chamber (0.125 cm3: PTW, Freiburg, Germany) or a smaller PinPoint chamber (0.015 cm3: PTW), and measurements can be made with either chamber in a range of positions throughout the phantom. Coronal films can also be acquired within the phantom, and additional solid blocks of Perspex allow for transverse films to be acquired within the head region.Initial studies using simple conventional head‐and‐neck plans established the reproducibility of the phantom and the measurement devices to within the setup uncertainty of ±0.5 mm. Subsequent verification of 9 clinical head‐and‐neck IMRT plans demonstrated the efficacy of the phantom in making a range of patient‐specific dose measurements in regions of dosimetric and clinical interest. Agreement between measured values and those predicted by the Pinnacle3 treatment planning system (Philips Medical Systems, Andover, MA) was found to be generally good, with a mean error on the calculated dose to each point of +0.2% (range: −4.3% to +2.2%;n=9) for the primary planning target volume (PTV), −0.1% (range: −1.5% to +2.0%;n=8) for the nodal PTV, and +0.0% (range: −1.8% to +4.3%;n=9) for the spinal cord. The suitability of the phantom for measuring combined dose distributions using radiographic film was also evaluated.The phantom has proved to be a valuable tool in the development and implementation of clinical head‐and‐neck IMRT, allowing for accurate verification of absolute dose and dose distributions in regions of clinical and dosimetric interest.PACS numbers: 87.53.‐j, 87.53.Xd, 87.56.Fc

Development of a primary standard for calibration of 64Cu activity measurement systems

A 64Cu national primary standard, was developed by the National Institute for Ionising Radiation Metrology (INMRI) of the ENEA (ENEA-INMRI) using the CIEMAT/NIST method of 4 πβ liquid scintillation spectrometry with 3H-standard efficiency tracing. Relatively short 64Cu half-life is required for the work to be performed at the production site. It was produced at the Scanditronix MC40 Cyclotron of the Joint Research Centre (JRC) of the European Commission (Ispra, Italy) through the 64Zn(d,2p) 64Cu reaction. Significant efforts were made to identify and quantify the impurities of 61Cu and 65Zn in the mother solution, which were activated through the 64Zn(d, αn) 61Cu and 64Zn(d,p) 65Zn reactions, respectively. To this purpose, a new procedure for the determination of pure β-emitter impurities by the CIEMAT/NIST method has been applied. A transfer standard portable well-type ionisation chamber was also calibrated with minimum uncertainty.

Medical beam monitor—Pre-clinical evaluation and future applications

Future medical ion beam applications for cancer therapy which are based on scanning technology will require advanced beam diagnostics equipment. For a precise analysis of beam parameters we want to resolve time structures in the range of microseconds to nanoseconds. A prototype of an advanced beam monitor was developed by the University of Applied Sciences Wiener Neustadt and its research subsidiary Fotec in co-operation with CERN RD42, Ohio State University and the Jožef Stefan Institute in Ljubljana. The detector is based on polycrystalline Chemical Vapor Deposition (pCVD) diamond substrates and is equipped with readout electronics up to 2 GHz analog bandwidth. In this paper we present the design of the pCVD-detector system and results of tests performed in various particle accelerator based facilities. Measurements performed in clinical high energy photon beams agreed within 1.2% with results obtained by standard ionization chambers.

SU‐FF‐I‐29: Optimizing Radiation Dose and Image Quality in a Multi‐Detector Computed Tomography

Purpose: The study was carried out to find optimal scanning parameters for an MDCT at Ramathibodi hospital. Method and materials: Two types of phantom were employed to assess image quality and standard ionization chamber was used to measure radiation dose. Tube current, slice thickness, pixel size and pitch were varied from routine techniques to evaluate factors affecting image quality (spatial resolution, contrast detectability and image noise) and dose (CTDIair) with aim of optimization. Results: From our study showed that, optimization of radiation dose and image quality could be achieved by proper selection of scanning parameters. Reduction of tube current could be made 22 – 28%, pitch could be doubled from 0.75 to 1.5 (50% dose reduction) in slice thickness less than 7.5 mm without any significant effects on noise and contrast. In examinations that do not require high spatial resolution, larger pixel size could be made and could result in 15 – 17% dose reduction. Conclusions: We concluded from our study that, optimal scanning parameters were 115, 105, 150 and 170 mA for head (above/below posterior fossa) and abdomen (pre‐ and post‐ contrast) protocols, respectively. Slice thickness should be made thinnest possible before the penumbra dose penalty becomes significant. Pitch and pixel size should be selected based on clinical requirements.

TU‐D‐M100F‐07: Simplified Clinical Quality Assurance for Helical Tomotherapy

Purpose: To evaluate a Web‐based Quality Assurance tool for maintaining the clinical integrity of the Tomotherapy machine. Method and Materials: This tool takes advantage of the output of the two distinct ion chamber systems on the Tomotherapy machine. One is the standard sealed ion chamber that resides in the head of a machine near the primary collimators and is used to calculate Monitor Units. The other is a set of pressurized Xenon detectors that are used for imaging. Both systems monitor ionizations independently and are rigidly attached so that their geometry is constant regardless of the orientation of the beam on the gantry which makes possible testing for both static and rotational modes. The collected data can be reviewed via a Web page and a downloadable Report in PDF format is available within seconds after collection of the data. Results: As an example, one performs a 200 second rotational delivery with the couch removed from the beam. The data are analyzed in both an integrated and pulse‐by pulse methodology to determine consistency between the two chamber systems, changes in output and changes in energy. The collected information is evaluated relative to a standard delivery created during machine commissioning. Comparison to standard ion chamber measurements in phantom under static conditions indicate that the analyses are consistent for calibration and more sensitive to energy changes than percentage depth dose measurements made in phantom. Additional information about the machine's performance is identified easily. Conclusion: New technology offers an opportunity to perform standard Quality Assurance tests in a new manner. The simple test described here is more sensitive and performed more quickly than traditional methodology. Other tests are possible. Conflict of Interest: Three authors are employees of Tomotherapy, Inc.

SU-FF-T-333: Orthovoltage Dosimetry: Percent Depth Dose Multiplicity From Two Energies

Purpose: To produce a multiplicity of depth dose curves on a two‐kVp orthovoltage system by appropriate combination of the 75 kVp and 225 kVp x‐ray beams. Method and Materials: Using tabulated percent depth dose (PDD) curves at 75 and 225 kVp for five rectangular applicators (4×6, 6×8, 8×10, 10×15, and 15×20 cm2) for our Philips RT‐250 unit, we calculated the mixed‐beam PDDs for each applicator using w 75 ⋅ PDD 75 + w 225 ⋅ PDD 225 , where w denotes the beam component weight, the subscript indicates the component kVp, and w 75 +w 225 =1 . By specifying a dose D0 at the surface and an isodose level at a particular depth, we calculated the required beam weights; incorporating the tissue dose rates then allows us to calculate the beam‐on times for the 75 and 225 kVp beams, respectively. All single‐ and mixed‐energy PDDs were verified in 20×20 cm2 blocks of solid water (Gammex RMI, Middleton, WI) using a field standard electrometer and ion chamber set. The ion chamber fit snugly at the centre of a 2.0 cm thick block. The depth doses were measured at solid water depths of 1.0 to 15.0 cm keeping the source‐to‐surface distance constant at 50.0 cm, and using 10.0 cm of backscatter material. The PDD values were calculated by scaling the measurements relative to the tabulated or theoretical values at a depth of 1.0 cm. Results: The ion chamber measurements at 75 and 225 kVp were within 2 percentage points of the tabulated values for all rectangular applicators. Similarly, the mixed‐energy PDD curves for f 75={0.25, 0.5, 1.0} were also within 2 percentage points of the theoretical expectations. Conclusion: A firm theoretical formalism to create PDDs combining the 75 kVp and 225 kVp beams was formulated and verified experimentally using ion chamber measurements. Our mixed‐energy methodology allows one to synthesize a multiplicity of PDDs between any two orthovoltage energies.

SU‐FF‐T‐218: Evaluation of the Exradin #a19 Ion Chamber for Reference Dosimetry in Megavoltage Photon Beams

Purpose: The Exradin #a19 is a new design of reference‐class 0.6cc ion chamber that externally resembles the NE2571 Farmer chamber but internally is very similar to the well‐known Exradin #a12. This project involved the characterization of a single version of this chamber in high energy x‐rays. Method and Materials: The Exradin #a19 was compared against primary and reference standard ion chamber chambers at the National Research Council of Canada in a range of megavoltage photon beams (6–25 MV). Investigations looked at chamber settling, ion recombination and polarity, and determination of experimental kQ factors. Results: Chamber settling — the chamber response stabilizes very quickly (within 5 minutes), even after a large change in the polarizing voltage.i) The polarity correction was measured for 6 MV and 25 MV beams and found to be small (within 0.15% of unity).ii) The recombination correction showed a linear variation with the dose‐per‐pulse. The correction was similar to that of other Farmer‐type chambers.iii) Beam quality conversion factors were found to be closer to that of the NE2571 than the #a12, which is not consistent with the guidance given in TG‐51. However, with only a single chamber under investigation one cannot draw any definitive conclusions.Conclusion: An initial investigation of the Exradin #a19 Farmer‐type ion chamber suggests that it is fit for purpose for the calibration of megavoltage photon beams. Further work is in progress to evaluate this chamber in kilovoltage x‐ray and high‐energy electron beams.

Automatic Patient Centering for MDCT: Effect on Radiation Dose

The purpose of this study was to determine with phantom and patient imaging the effect of an automatic patient-centering technique on the radiation dose associated with MDCT. A 32-cm CT dose index (CTDI) phantom was scanned with 64-MDCT in three positions: gantry isocenter and 30 and 60 mm below the isocenter of the scanner gantry. In each position, surface, peripheral, and volume CTDIs were estimated with a standard 10-cm pencil ionization chamber. The institutional review board approved the study with 63 patients (36 men, 27 women; mean age, 51 years; age range, 22-83 years) undergoing chest (n = 18) or abdominal (n = 45) CT using the z-axis automatic exposure control technique. Each patient was positioned according to the region being scanned and then was centered in the gantry. Before scanning of a patient, automatic centering software was used to estimate patient off-centering and percentage of dose reduction with optimum recentering. Data were analyzed with linear correlation and the Student's t test. Peripheral and surface CTDIs increased approximately 12-18% with 30-mm off-center distance and 41-49% with 60-mm off-center distance. Approximately 95% (60/63) of patients were not positioned accurately in the gantry isocenter. The mean radiation dose saving with automatic centering of all patients was 13.0% +/- 0.9% (range, 2.6-29.9%). There was strong correlation between off-center distance and percentage of surface CTDI reduction with recentering of patients in the gantry isocenter (r2 = 0.85, p < 0.0001). Surfaces doses can be reduced if radiologic technologists can better center patients within the CT gantry. Automatic centering technique can help in optimum patient centering and result in as much as 30% reduction in surface dose.

Investigation of the Air Kerma Response of Spherical Ionization Chambers for Unfolded Pulse Height Distributions of 60Co and 137Cs using the EGS4 Monte Carlo Code

Data required for the determination of the absolute air kerma rate for (60)Co and (137)Cs gamma-rays using spherical cavity chambers were calculated using the EGS4 Monte Carlo system. Mass energy-absorption coefficient ratio and the stopping power ratio were calculated for a 10 cm(3) primary standard graphite-walled ionization chamber from the unfolded energy pulse height distributions of (60)Co and (137)Cs sources. Wall correction factors and non-uniformity correction factors for two graphite and one air equivalent plastic walled ionization chambers were also calculated with EGS4 code. The wall correction factors were compared with those determined by an experimental extrapolation method. To check the accuracy of the calculations the results were compared with those obtained from other primary standard laboratories such as NIST and NRCC. For a 10 cm(3) graphite ionization chamber, the mass energy-absorption coefficient ratios were 0.99917 for (60)Co and 1.0004 for (137)Cs. The values differed by 0.02-0.05 % for (60)Co and 0.11 % for (137)Cs from those of two laboratories. The stopping power ratios were 0.99984 for (60)Co and 1.0087 for (137)Cs. Comparison with NIST values showed differences of 0.06 % for (60)Co and 0.04 % for (137)Cs. The wall correction factors were obtained and they were different by 0.6-1.1 % for (60)Co and (137)Cs compared to the experimental linear extrapolation method. These values were compared with Monte Carlo derived values from other laboratories. The non-uniformity correction factors were also calculated and they differed from unity, the traditional value used in most standard national metrology laboratories.

Development of dosimetry using detectors of diagnostic digital radiography systems

Dosimetry using an imaging plate (IP) of computed radiography (CR) systems was developed for quality control of output of the x-ray equipment. Sensitivity index, or the S number, of the CR systems was used for estimating exposure dose under the routine condition: exposure dose from 1.0 to 1.0 x 10(2) microC kg(-1), tube voltages from 50 to 120 kV, and added filtration from 0 to 4.0 mm Al. The IP was calibrated by using a 6 cc ionization chamber having traceability to the National Standard Ionization Chamber. The uncertainty concerning the fading effect was suppressed less than 1.9% by reading the latent image 4 min+/-5 s after irradiation at the room temperature 25.9+/-1.0 degrees C. The S number decreased linearly on the logarithmic graph regardless of the beam quality as exposure dose increased. The relationship between the exposure dose (E) and the S number was fitted by the equation E=a' X S(-b). The coefficient a' decreased when the added filtration and the tube voltage were increased. The coefficient b was 0.977+/-0.007 in all beam qualities. The dosimetry using the IP and the equation can estimate the exposure dose in a range from 9.0 x 10(-2) to 5.0 microC kg(-1) within an uncertainty of +/-5% required by the Japanese Industry Standard. This dose range partially included the doses under routine condition. The doses between 1.0 and 1.0 x 10(2) microC kg(-1) under the routine condition can be shifted to the 5% region by using an absorber. The IP dosimetry is applicable to the quality control of the CR systems.

A Multiple acquisition sequence for IMRT verification with a 2D ion chamber array

This investigation deals with the implementation of a multiple acquisition (MA) sequence for radiotherapy beam verification with a 2-dimensional (2D) ion chamber array. The acquisition is carried out through multiple beam deliveries with the 2D detector array in different positions. The MA sequence is performed through remote-controlled movements of the treatment couch. In this work, the MA dose map of an intensity-modulated radiotherapy (IMRT) modulated beam is presented and compared to standard acquisition mode data and ion chamber measurements. The implementation of MA sequence increases the number of measurement points and therefore the efficiency of the detection system and the final imaging resolution. Results from this investigation show that the imaging resolution of the system used in standard acquisition mode can be increased up to 4 times. The procedure described in this work can be automated, including couch movements in the radiotherapy plan sent to the treatment workstation. Furthermore, this specific solution could be successfully applied to matrices of detector with a different construction design.

A new scintillating fiber dosimeter using a single optical fiber and a CCD camera

Radiotherapy treatments become more and more accurate, using very small irradiation fields and complex dose depositions. So small dosimeters for real time and in vivo dosimetry, suitable for photons as well as for electrons beams are highly desired. In this context, a scintillating fiber dosimeter (SFD) has been developed by the Laboratoire de Physique Corpusculaire de Caen (LPC Caen), France, in collaboration with one of the French regional center for cancer treatment Centre Regional de lutte contre le cancer F. Baclesse (CRLCC F. Baclesse), Caen, France, and the ELDIM Company, Herouville, France. This plastic dosimeter is water equivalent, and it is suitable for photons as well as for electrons beams without correction. It is a real time dosimeter, with an excellent signal to noise ratio, and a spatial resolution of about a few millimeters. The aim of this study was to reduce the size of the scintillator in order to improve the spatial resolution of this dosimeter. So, a new light collection device has been developed to reduce the length of the scintillator from 1 cm to 1 mm without loss in the signal to noise ratio. The accuracy of this improved prototype has been tested by comparison with standard ionization chambers and the difference between the two devices never exceeded one percent for photon and for electron irradiation beams. A first set of commercial SFD is under completion at ELDIM and it will be soon clinically tested in several French centers for cancer treatment.

SU‐FF‐T‐352: Output Verification and Clinical Implementation of Leipzig Applicators for Treating Small Skin Lesions by HDR‐Brachytherapy Using Multiple Dosimetry Systems

Purpose: The purpose of this paper is to provide a guide to commissioning the Nucletron Lepzig Applicators in a clinical setup for treating small surface lesions using HDR brachytherapy. Method and Materials: Recently we have acquired a set of six Leipzig Applicators (3 Horizontal applicators with inner diameters 1 cm, 2 cm and 3 cm and 3 Vertical with same diameter sizes) for our microselectron‐HDR V2 afterloader. Initially we have measured dose rates for 3 Horizontal applicators using Standard Imaging Exradin A10 parallel plate ion chamber and films (gafchromic and EDR2). Since the sensitive area of this ion chamber is comparable to the effective field area for the smaller 1 cm diameter applicators we felt the necessity to verify the dose rates using gafchromic films as well as EDR2 film. We have also used MOSFET and a micro parallel plate chamber (Exradin A14P, provided by Standard Imaging) and small cylindrical chambers (Exradin A1SL,and A14SL) for independent verification of our measurements. Results: To implement these applicators for clinical use we measured surface dose rates and depth dose rates for each applicators using variety of dosimetry. We subsequently developed an Excel Spreadsheet program to compute dwell time for a given prescription and to print a plan report documentation for the treatment. Conclusion: In this presentation we compare our data with published data and describe the details of clinical implementation of these applicators.

A comparison of Australian and Canadian calibration coefficients for air kerma and absorbed dose to water for60Co γ radiation

Australian and Canadian calibration coefficients for air kerma and absorbed dose to water for 60Co gamma radiation have been compared using transfer standard ionization chambers of types NE 2561 and NE 2611A. Whilst the primary standards of air kerma are similar, both being thick-walled graphite cavity chambers but employing different methods to evaluate the Awall correction, the primary standards of absorbed dose to water are quite different. The Australian standard is based on measurements made with a graphite calorimeter, whereas the Canadian standard uses a sealed water calorimeter. The comparison result, expressed as a ratio of calibration coefficients R=N(ARPANSA)/N(NRC), is 1.0006 with a combined standard uncertainty of 0.35% for the air kerma standards and 1.0052 with a combined standard uncertainty of 0.47% for the absorbed dose to water standards. This demonstrates the agreement of the Australian and Canadian radiation dosimetry standards. The results are also consistent with independent comparisons of each laboratory with the BIPM reference standards. A 'trilateral' analysis confirms the present determination of the relationship between the standards, within the 0.09% random component of the combined standard uncertainty for the three comparisons.

Development of a Primary Standard for Calibration of [18F]FDG Activity Measurement Systems

The 18F national primary standard was developed by the INMRI-ENEA using the 4πβ Liquid Scintillation Spectrometry Method with 3H-Standard Efficiency Tracing. Measurements were performed at JRCIspra under a scientific collaboration between the Institute for Health and Consumer Production, the Amersham Health and the National Institute for Occupational Safety and Prevention (ISPESL). The goal of the work was to calibrate, with minimum uncertainty, the INMRI-ENEA transfer standard portable well-type ionisation chamber as well as other JRC-Ispra and Amersham Health reference Ionising Chambers used for FDG activity measurement.

Calibration of the Capintec CRC-712M dose calibrator for 18F

Primary standardisation was performed on a solution of 18F using the 4 πβ– γ coincidence counting efficiency-tracing extrapolation method with 60Co used as a tracer nuclide. The result was used to calibrate the ANSTO secondary standard ionisation chamber which is used to disseminate Australian activity standards for gamma emitters. Using the secondary activity standard for 18F, the Capintec CRC-712M dose calibrator at the Australian National Medical Cyclotron (NMC) Positron Emission Tomography (PET) Quality Control (QC) Section was calibrated. The dial setting number recommended by the manufacturer for the measurement of the activity of 18F is 439. In this work, the dial setting numbers for the activity measurement of the solution of 18F in Wheaton vials were experimentally determined to be 443±12, 446±12, 459±11, 473±15 for 0.1, 1, 4.5 and 9 ml solution volumes, respectively. The uncertainties given above are expanded uncertainties ( k = 2 ) giving an estimated level of confidence of 95%. The activities determined using the manufacturer recommended setting number 439 are 0.8%, 1.4%, 4.0% and 6.5% higher than the standardised activities, respectively. It is recommended that a single dial setting number of 459 determined for 4.5 ml is used for 0.1–9 ml solution in Wheaton vials in order to simplify the operation procedure. With this setting the expended uncertainty ( k = 2 ) in the activity readout from the Capintec dose calibrator would be less than 6.2%.

Spectral discrimination of Čerenkov radiation in scintillating dosimeters

Radiation therapy accelerators require highly accurate dose deposition and the output must be monitored frequently and regularly. Ionization chambers are the primary tool for this control, but their size, their high voltage needed, and the correction needed for electrons make them unsuitable for use during patient treatment. We have developed a small (1-mm-diam and 1-mm-long active part), flexible, and water-equivalent dosimeter. It is suitable for photon and electron beams without corrections, and performs on line dose measurements. This detector is based on only one scintillating fiber and a CCD camera. A new signal processing is used to remove the effect of Cerenkov radiation background, which only requires a preliminary calibration. Central-axis depth-dose distribution comparisons have been achieved with standard ionization chambers, over a range from 8 to 25 MV photons and from 6 to 21 MeV electrons in order to validate this calibration. Results show a very good agreement, with less than 1% difference between the two detectors.