Energy and rate dependence of diagnostic x-ray exposure meters

Variations in x-ray exposure measurements among a variety of contemporary diagnostic exposure meters are investigated. Variations may result from systematic errors due to calibration, beam-quality dependence and exposure-rate dependence. It is concluded that the majority of general purpose diagnostic meters will agree to within 10% of each other if exposure rates are below 1.3 mC kg-1S-1 of air (5 R s-1) and beam qualities are typical for general purpose radiology, excluding mammography. For exposure rates comparable to those in barium enema radiography the variations can range up to 25% or more. Variations up to 40% were observed among general purpose exposure meters at mammographic beam qualities. In the mammographic range, mammographic (thin window) exposure meters varied by no more than 2%.     

Quality assurance for computed-tomography simulators and the computed-tomography-simulation process: report of the AAPM Radiation Therapy Committee Task Group No. 66

This document presents recommendations of the American Association of Physicists in Medicine (AAPM) for quality assurance of computed-tomography- (CT) simulators and CT-simulation process. This report was prepared by Task Group No. 66 of the AAPM Radiation Therapy Committee. It was approved by the Radiation Therapy Committee and by the AAPM Science Council.     

Performance evaluation of computed radiography systems

Recommended methods to test the performance of computed radiography (CR) digital radiographic systems have been recently developed by the AAPM Task Group No. 10. Included are tests for dark noise, uniformity, exposure response, laser beam function, spatial resolution, low-contrast resolution, spatial accuracy, erasure thoroughness, and throughput. The recommendations may be used for acceptance testing of new CR devices as well as routine performance evaluation checks of devices in clinical use. The purpose of this short communication is to provide a tabular summary of the tests recommended by the AAPM Task Group, delineate the technical aspects of the tests, suggest quantitative measures of the performance results, and recommend uniform quantitative criteria for the satisfactory performance of CR devices. The applicability of the acceptance criteria is verified by tests performed on CR systems in clinical use at five different institutions. This paper further clarifies the recommendations with respect to the beam filtration to be used for exposure calibration of the system, and the calibration of automatic exposure control systems.     

Reference dosimetry in clinical high-energy photon beams: comparison of the AAPM TG-51 and AAPM TG-21 dosimetry protocols

Task Group 51 (TG-51) of the Radiation Therapy Committee of the American Association of Physicists in Medicine (AAPM) has recently developed a new protocol for the calibration of high-energy photon and electron beams used in radiation therapy. The formalism and the dosimetry procedures recommended in this protocol are based on the use of an ionization chamber calibrated in terms of absorbed dose-to-water in a standards laboratory's 60Co gamma ray beam. This is different from the recommendations given in the AAPM TG-21 protocol, which are based on an exposure calibration factor of an ionization chamber in a 60Co beam. The purpose of this work is to compare the determination of absorbed dose-to-water in reference conditions in high-energy photon beams following the recommendations given in the two dosimetry protocols. This is realized by performing calibrations of photon beams with nominal accelerating potential of 6, 18 and 25 MV, generated by an Elekta MLCi and SL25 series linear accelerator. Two widely used Farmer-type ionization chambers having different composition, PTW 30001 (PMMA wall) and NE 2571 (graphite wall), were used for this study. Ratios of AAPM TG-51 to AAPM TG-21 doses to water are found to be 1.008, 1.007 and 1.009 at 6, 18 and 25 MV, respectively when the PTW chamber is used. The corresponding results for the NE chamber are 1.009, 1.010 and 1.013. The uncertainties for the ratios of the absorbed dose determined by the two protocols are estimated to be about 1.5%. A detailed analysis of the reasons for the discrepancies is made which includes comparing the formalisms, correction factors and quantities in the two protocols, as well as the influence of the implementation of the different standards for chamber calibration. The latter has been found to have a considerable influence on the differences in clinical dosimetry, even larger than the adoption of the new data and recommended procedures, as most intrinsic differences cancel out due to the adoption of the new formalism.     

The effect of differences in data base on the determination of absorbed dose in high-energy photon beams using the American Association of Physicists in Medicine protocol

Exposure rates were adjusted at the National Institute of Standards and Technology (NIST) on January 1, 1986 to take into account more recent values for some physical parameters, mainly in electron stopping power ratios. Exposure calibration factors for 60Co gamma rays Nx will therefore be lowered by 1.1%. Consequently, absorbed dose determinations in high-energy photon beams will be reduced by the same amount if the values for these physical parameters remain unchanged in the American Association of Physicists in Medicine (AAPM) protocol. If the same data base as used at NIST is applied in the AAPM protocol, then Ngas/Nx values, water-air stopping power ratios, and Pwall values will be different. The overall change in absorbed dose determinations using a consistent set of data will be a reduction of 0.8% for 60Co gamma rays and 1.5% for a 20-MV x-ray beam compared to the values before January 1, 1986. Since the net effect is small when different sets of data are applied, the new NIST exposure calibration factors may be used in combination with the AAPM protocol without significant error.     

Clinical reference dosimetry: comparison between AAPM TG-21 and TG-51 protocols

We compare the results of absorbed dose determined at reference conditions according to the AAPM TG-21 dose calibration protocol and the new AAPM TG-51 protocol. The AAPM TG-21 protocol for absorbed dose calibration is based on ionization chambers having exposure calibration factors for 60Co gamma rays, N(x). The new AAPM TG-51 dosimetry protocol for absorbed dose calibration is based on ionization chambers having 60Co absorbed dose-to-water calibration factor, N60Co(D,w). This study shows that the dose changes are within 1% for a cobalt beam, 0.5% for photon energies of 6 and 18 MV, and 2%-3% for electron beams with energies of 6 to 20 MeV. The chamber primary calibration factors, Nx and N60Co(D,w), are traceable to the Canadian primary standards laboratory (NRCC). We also present estimated dose changes between the two protocols when calibration factors are traceable to NIST in the United States.     

Dosimetric considerations for patients with HIP prostheses undergoing pelvic irradiation. Report of the AAPM Radiation Therapy Committee Task Group 63

This document is the report of a task group of the Radiation Therapy Committee of the AAPM and has been prepared primarily to advise hospital physicists involved in external beam treatment of patients with pelvic malignancies who have high atomic number (Z) hip prostheses. The purpose of the report is to make the radiation oncology community aware of the problems arising from the presence of these devices in the radiation beam, to quantify the dose perturbations they cause, and, finally, to provide recommendations for treatment planning and delivery. Some of the data and recommendations are also applicable to patients having implanted high-Z prosthetic devices such as pins, humeral head replacements. The scientific understanding and methodology of clinical dosimetry for these situations is still incomplete. This report is intended to reflect the current state of scientific understanding and technical methodology in clinical dosimetry for radiation oncology patients with high-Z hip prostheses.     

Dosimetric prerequisites for routine clinical use of photon emitting brachytherapy sources with average energy higher than 50 kev

This paper presents the recommendations of the American Association of Physicists in Medicine (AAPM) and the European Society for Therapeutic Radiology and Oncology (ESTRO) on the dosimetric parameters to be characterized, and dosimetric studies to be performed to obtain them, for brachytherapy sources with average energy higher than 50 keV that are intended for routine clinical use. In addition, this document makes recommendations on procedures to be used to maintain vendor source strength calibration accuracy. These recommendations reflect the guidance of the AAPM and the ESTRO for its members, and may also be used as guidance to vendors and regulatory agencies in developing good manufacturing practices for sources used in routine clinical treatments.     

Third-party brachytherapy source calibrations and physicist responsibilities: report of the AAPM Low Energy Brachytherapy Source Calibration Working Group

The AAPM Low Energy Brachytherapy Source Calibration Working Group was formed to investigate and recommend quality control and quality assurance procedures for brachytherapy sources prior to clinical use. Compiling and clarifying recommendations established by previous AAPM Task Groups 40, 56, and 64 were among the working group's charges, which also included the role of third-party handlers to perform loading and assay of sources. This document presents the findings of the working group on the responsibilities of the institutional medical physicist and a clarification of the existing AAPM recommendations in the assay of brachytherapy sources. Responsibility for the performance and attestation of source assays rests with the institutional medical physicist, who must use calibration equipment appropriate for each source type used at the institution. Such equipment and calibration procedures shall ensure secondary traceability to a national standard. For each multi-source implant, 10% of the sources or ten sources, whichever is greater, are to be assayed. Procedures for presterilized source packaging are outlined. The mean source strength of the assayed sources must agree with the manufacturer's stated strength to within 3%, or action must be taken to resolve the difference. Third party assays do not absolve the institutional physicist from the responsibility to perform the institutional measurement and attest to the strength of the implanted sources. The AAPM leaves it to the discretion of the institutional medical physicist whether the manufacturer's or institutional physicist's measured value should be used in performing dosimetry calculations.     

Supplement to the 2004 update of the AAPM Task Group No. 43 Report

Since publication of the 2004 update to the American Association of Physicists in Medicine (AAPM) Task Group No. 43 Report (TG-43U1), several new low-energy photon-emitting brachytherapy sources have become available. Many of these sources have satisfied the AAPM prerequisites for routine clinical use as of January 10, 2005, and are posted on the Joint AAPM/RPC Brachytherapy Seed Registry. Consequently, the AAPM has prepared this supplement to the 2004 AAPM TG-43 update. This paper presents the AAPM-approved consensus datasets for these sources, and includes the following 125I sources: Amersham model 6733, Draximage model LS-1, Implant Sciences model 3500, IBt model 1251L, IsoAid model IAI-125A, Mentor model SL-125/ SH-125, and SourceTech Medical model STM1251. The Best Medical model 2335 103Pd source is also included. While the methodology used to determine these data sets is identical to that published in the AAPM TG-43U1 report, additional information and discussion are presented here on some questions that arose since the publication of the TG-43U1 report. Specifically, details of interpolation and extrapolation methods are described further, new methodologies are recommended, and example calculations are provided. Despite these changes, additions, and clarifications, the overall methodology, the procedures for developing consensus data sets, and the dose calculation formalism largely remain the same as in the TG-43U1 report. Thus, the AAPM recommends that the consensus data sets and resultant source-specific dose-rate distributions included in this supplement be adopted by all end users for clinical treatment planning of low-energy photon-emitting brachytherapy sources. Adoption of these recommendations may result in changes to patient dose calculations, and these changes should be carefully evaluated and reviewed with the radiation oncologist prior to implementation of the current protocol.     

A novel technique to suppress grid line artifacts

It has been established that coarse strip density, air-interspace grid systems can suppress scatter in general radiography and in mammography more effectively than conventional high strip density grids. However, such systems have never gained clinical acceptance due to the large distance the grid needs to move to suppress gridline artifacts and due to their corresponding bulk. We present a novel technique for suppressing grid lines using an x-ray exposure wave form with a soft start and soft stop. The wave form is achieved by varying the x-ray tube current during the exposure. We derive the conditions that the time dependence of the x-ray exposure output needs to meet to suppress gridline artifacts with only a modest grid movement. The technique allows for the design of compact coarse strip density grid systems. We present experimental results that demonstrate the feasibility of the technique.     

Effects of radiation quality on the calibration of kerma-area product meters in x-ray beams

The calibration coefficients of kerma-area product meters significantly depend on the energy spectrum of the x-ray beam. This effect was examined by measuring the calibration coefficients for several radiation qualities in the range generally used in diagnostic x-ray imaging. The intention was to determine the calibration coefficients for other radiation qualities by interpolation between the measured values, relative to one or more suitable parameters. The x-ray tube voltage, total filtration and half-value thickness were examined as possible specifiers of the energy distribution. No single parameter provided an interpolation of calibration coefficients with the accuracy recommended by the ICRU and IAEA, except for a narrow range of radiation qualities. At least two of the parameters are needed to reliably specify the radiation quality for the interpolation of calibration coefficients.     

A tandem calibration method for kerma-area product meters

For measurements of the kerma-area product (KAP) in diagnostic x-ray imaging, a method for calibrating field KAP meters with a reference KAP meter is presented. In this tandem calibration method, the field KAP chamber is positioned similarly as in measurements with patients. The reference KAP chamber is placed at a specified distance and used in the x-ray beam simultaneously with the field KAP chamber. The tandem method provides a feasible and practical way for calibrating field KAP meters of any type in their clinical position. Accurate measurements of the irradiation geometry are not required, but comprehensive calibration for the reference KAP meter is needed.     

Stopping-power and mass energy-absorption coefficient ratios for Solid Water

The AAPM Task Group 21 protocol provides tables of ratios of average restricted stopping powers and ratios of mean energy-absorption coefficients for different materials. These values were based on the work of Cunningham and Schulz. We have calculated these quantities for Solid Water (manufactured by RMI), using the same x-ray spectra and method as that used by Cunningham and Schulz. These values should be useful to people who are using Solid Water for high-energy photon calibration.     

Kilovoltage x-ray dosimetry--an experimental comparison between different dosimetry protocols

Kilovoltage dosimetry protocols by the IAEA (TRS-277 and TRS-398), DIN (6809), IPEMB (with addendum), AAPM (TG-61) and NCS (report 10) were compared experimentally in four clinical beams. The beams had acceleration potentials of 30, 80, 120 and 200 kV, with half-value layers ranging from 0.6 mm Al to 1 mm Cu. Dosimetric measurements were performed and data were collected under reference conditions as stipulated within each separate protocol under investigation. The Monte Carlo method was used to derive backscatter factors for the actual x-ray machine. In general, the agreement of the dosimetric data at the surface of a full-scatter water phantom obtained using the guidelines of the various protocols was fairly good, i.e. within 1-2%. However, the in-air calibration method using the IPEMB and AAPM TG-61 protocols yielded an absorbed dose about 7% lower than the IAEA TRS-398 protocol in the 120 kV beam. By replacing the backscatter factors given in the protocols with Monte Carlo calculated backscatter factors, the convergence between the protocols improved (within 4%). The internal consistency obtained for protocols supporting more than one geometry for dosimetry under reference conditions was better than 0.2% for the DIN protocol (120 kV beam), 2-3% for the AAPM TG-61 (120 and 200 kV beams) and about 2% for the IPEMB protocol (200 kV beam). The present study shows that the current-supported dosimetry protocols in the kilovoltage range were in fairly good agreement, and there were only a few exceptions of clinical significance.     

Quality assurance of U.S.-guided external beam radiotherapy for prostate cancer: report of AAPM Task Group 154

Task Group 154 (TG154) of the American Association of Physicists in Medicine (AAPM) was created to produce a guidance document for clinical medical physicists describing recommended quality assurance (QA) procedures for ultrasound (U.S.)-guided external beam radiotherapy localization. This report describes the relevant literature, state of the art, and briefly summarizes U.S. imaging physics. Simulation, treatment planning and treatment delivery considerations are presented in order to improve consistency and accuracy. User training is emphasized in the report and recommendations regarding peer review are included. A set of thorough, yet practical, QA procedures, frequencies, and tolerances are recommended. These encompass recommendations to ensure both spatial accuracy and image quality.     

A comparison of image characteristics and convenience in panoramic radiography using charge-coupled device,storage phosphor,and film receptors

This study compared the image layer characteristics, dose requirements, and convenience in use of panoramic radiography using each of four different image receptors including traditional indirect exposure x-ray film, a storage phosphor system, and two solid-state sensors. The OP 100 D (Instrumentarium Imaging, Tuusula, Finland) charge-coupled device (CCD) sensor provided an instant image with a wide focal trough, making patient positioning error unlikely, but at the same time required a patient dose higher than that used with film. While the DigiPan (Trex/Trophy, Marne-la-Vallée, France) CCD significantly reduced the patient dose to radiation and also provided an instant image, the focal trough was narrower making patient positioning error more likely. The storage phosphor system provided high resolution and a reasonable focal trough width, but the procedure took longer than traditional film radiography and did not provide a dose saving.     

An evaluation of semiconductor and ionization chamber detectors for diagnostic x-ray dosimetry measurements

Dosemeters for performance testing of x-ray equipment may utilize semiconductor technology or ionization chambers (ICs). Semiconductor dosemeters incorporate several elements into the detectors from which compensation for variations in response with photon energy is derived. The design of the detectors influences their response with angle and this is different from that of ICs. The responses of semiconductor detectors (SDs) and ICs to x-ray beams with a variety of radiation qualities have been measured in order to assess differences in response. Measurements have been made with experimental arrangements simulating use of the detectors in performance testing of digital radiography and fluoroscopy equipment. Results show that differences in photon energy responses between the detectors are small, but because ICs are sensitive to radiation incident from all angles, they record more scattered radiation than SDs. Implications of differences in detector responses are discussed and recommendations made about their use. SDs are more appropriate for measurements of image receptor doses and are recommended for setting up automatic exposure control devices for digital radiography. ICs are suitable for assessment of patient entrance surface dose rate measurements. Correction factors that could be applied to allow comparisons between measurements with different dosemeters are proposed.     

Anniversary paper. Development of x-ray computed tomography: the role of medical physics and AAPM from the 1970s to present

The AAPM, through its members, meetings, and its flagship journal Medical Physics, has played an important role in the development and growth of x-ray tomography in the last 50 years. From a spate of early articles in the 1970s characterizing the first commercial computed tomography (CT) scanners through the "slice wars" of the 1990s and 2000s, the history of CT and related techniques such as tomosynthesis can readily be traced through the pages of Medical Physics and the annals of the AAPM and RSNA/AAPM Annual Meetings. In this article, the authors intend to give a brief review of the role of Medical Physics and the AAPM in CT and tomosynthesis imaging over the last few decades.