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At least 1,063 records · Page 59Linked to original sources
PubMed2006

AAPM Task Group 108: PET and PET/CT shielding requirements.

The shielding of positron emission tomography (PET) and PET/CT (computed tomography) facilities presents special challenges. The 0.511 MeV annihilation photons associated with positron decay are much higher energy than other diagnostic radiations. As a result, barrier shielding may be required in floors and ceilings as well as adjacent walls. Since the patient becomes the radioactive source after the radiopharmaceutical has been administered, one has to consider the entire time that the subject remains in the clinic. In this report we present methods for estimating the shielding requirements for PET and PET/CT facilities. Information about the physical properties of the most commonly used clinical PET radionuclides is summarized, although the report primarily refers to fluorine-18. Typical PET imaging protocols are reviewed and exposure rates from patients are estimated including self-attenuation by body tissues and physical decay of the radionuclide. Examples of barrier calculations are presented for controlled and noncontrolled areas. Shielding for adjacent rooms with scintillation cameras is also discussed. Tables and graphs of estimated transmission factors for lead, steel, and concrete at 0.511 MeV are also included. Meeting the regulatory limits for uncontrolled areas can be an expensive proposition. Careful planning with the equipment vendor, facility architect, and a qualified medical physicist is necessary to produce a cost effective design while maintaining radiation safety standards.

Positron-Emission Tomography↗
PubMed2006

Monte Carlo characterization of an ytterbium-169 high dose rate brachytherapy source with analysis of statistical uncertainty.

An ytterbium-169 high dose rate brachytherapy source, distinguished by an intensity-weighted average photon energy of 92.7 keV and a 32.015 +/- 0.009 day half-life, is characterized in terms of the updated AAPM Task Group Report No. 43 specifications using the MCNP5 Monte Carlo computer code. In accordance with these specifications, the investigation included Monte Carlo simulations both in water and air with the in-air photon spectrum filtered to remove low-energy photons below 10 keV. TG-43 dosimetric data including S(K), D(r, lamda), lambda, gL(r), F(r, lamda), phi an(r), and phi(an) were calculated and statistical uncertainties in these parameters were derived and calculated in the appendix.

Brachytherapy↗
PubMed2006

MCNPX simulation of a multileaf collimator.

Our purpose in this work was to validate a very detailed Monte Carlo model, developed in MCNPX, of a Millennium 120 multileaf collimator integrated into a Varian Clinac 21EX treatment head. The Monte Carlo results were compared with measurements for both the 6-MV and 18-MV photon modes. The following comparisons were performed: depth-dose curves, lateral profiles, multileaf collimator leakage, the tongue-and-grove test, and the round leaf-end test. The good agreement between the Monte Carlo simulations and measurements showed that our model is accurate. Consequently, the benchmarks provided by our study can be used in future Monte Carlo studies.

Computer Simulation↗
PubMed2006

Comparison of radiation shielding requirements for HDR brachytherapy using 169Yb and 192Ir sources.

169Yb has received a renewed focus lately as an alternative to 192Ir sources for high dose rate (HDR) brachytherapy. Following the results of a recent work by our group which proved 169Yb to be a good candidate for HDR prostate brachytherapy, this work seeks to quantify the radiation shielding requirements for 169Yb HDR brachytherapy applications in comparison to the corresponding requirements for the current 192Ir HDR brachytherapy standard. Monte Carlo simulation (MC) is used to obtain 169Yb and 192Ir broad beam transmission data through lead and concrete. Results are fitted to an analytical equation which can be used to readily calculate the barrier thickness required to achieve a given dose rate reduction. Shielding requirements for a HDR brachytherapy treatment room facility are presented as a function of distance, occupancy, dose limit, and facility workload, using analytical calculations for both 169Yb and 192Ir HDR sources. The barrier thickness required for 169Yb is lower than that for 192Ir by a factor of 4-5 for lead and 1.5-2 for concrete. Regarding 169Yb HDR brachytherapy applications, the lead shielding requirements do not exceed 15 mm, even in highly conservative case scenarios. This allows for the construction of a lead door in most cases, thus avoiding the construction of a space consuming, specially designed maze. The effects of source structure, attenuation by the patient, and scatter conditions within an actual treatment room on the above-noted findings are also discussed using corresponding MC simulation results.

Brachytherapy↗
PubMedDate not supplied

Saturation curve in gases of high atomic number at pressures up to 8 atm. I. Krypton and xenon.

The saturation curve has been studied in xenon and in krypton up to a pressure of 8 atm. An empirical formula has been found that describes the fraction of current collected over a wide range of voltages, pressures, ionization intensities, and electrode spacings. This is of practical value in the design of ionography chambers. For krypton the collection fraction fKr = (1 + 0.25eta-1.74)-1, and for xenon fXe = (1 + 0.16eta-1.88)-1, where eta = Fp-0.7Vd-2q-1/2 with F = 3.61 X 10(-7) and 2.50 X 10(-7) for krypton and xenon, respectively. The ranges of the variables covered in the experiments were p = 1-8 atm, V = 5-25000 V, d = 0.3-1.3 cm, and q = 4 X 10(-9)-6 X 10(-8) A/cm3.

Atmospheric Pressure↗
PubMedDate not supplied

Use of a pencil-shaped ionization chamber for measurement of exposure resulting from a computed tomography scan.

A simple method of comparing and/or monitoring the radiation output level of a Computed Tomography (CT) system, using a specially developed pencil-shaped ionization chamber, is presented. When the measurement is made with a phantom for the period of one tomographic scan, the result can be interpreted as the average exposure along the chamber axis resulting from the entire series of scans to be performed by the CT system. An interesting feature of this method is that no detailed knowledge of the beam distribution or slice thickness is necessary in order to measure the radiation output level, to compare the performance of CT systems or the estimate patient's exposure for a particular combination of CT operation parameters.

Mathematics↗
PubMedDate not supplied

Radiation protection aspects of a new high-energy linear accelerator.

The Therac-25 is a new 25-MeV linear accelerator manufactured by Atomic Energy of Canada, Ltd. The first two units have recently been installed in Toronto, Ontario and Halifax, Nova Scotia. Calculations and measurements of primary and secondary radiation levels were made. Neutron dose-equivalent rates were measured inside and outside the room. The maximum leakage rate at 1 m from the accelerator target was 0.4% Sv per peak photon Gy. The tenth value layer for neutrons from the Therac-25, at the entrance to a one-legged maze was found to be 5.5 cm of polyethylene. Measurements were done to estimate daily technologist exposure due to induced activity in the treatment room.

Electrons↗
PubMedDate not supplied

The neutron dose and energy spectrum outside a 20-MV accelerator treatment room.

A maze design is discussed for a Therac 20 linear accelerator (manufactured by Atomic Energy of Canada, Ltd.) which reduces the flux of neutrons at the door to permissible levels in controlled areas. The L-shaped design allows for a relatively light door at the end of the maze, consisting of 5.08-cm (2-in.) borated polyethylene and 2-mm lead. A comparison is made between the neutron dose equivalent (DE) calculated by various methods and the DE measured with a variety of portable neutron survey meters. In addition, the neutron energy spectrum outside the maze at 1 m from the door, measured with a polyethylene multisphere LiI system, is reported.

Humans↗
PubMedDate not supplied

Measurement and calculation of neutron leakage from a medical electron accelerator.

The leakage neutron spectra and dose equivalent were systematically measured in the irradiation field, treatment room, maze, and outside the shielding door at the microtron medical electron accelerator facility of the National Cancer Center, Tokyo. For these measurements, we used two types of multimoderator neutron spectrometers (Bonner spheres containing indium activation detectors and 3He detector), an aluminum activation detector, and a commercially available neutron rem counter. The measured results were compared with the combined calculation of the one-dimensional ANISN and two-dimensional DOT3.5 discrete ordinates transport codes. The calculation was performed by using a measured source spectrum in the irradiation field and by computer modeling of the maze entrance. The calculation indicated good agreement in spectral shape and agreement with experiment within a factor of 2 in absolute dose-equivalent values. This transport calculation was systematically repeated for different geometrical and material parameters, and simple analytical formulas and their parameters applicable for shielding design of a medical electron accelerator facility were obtained in general form.

Fast Neutrons↗
PubMedDate not supplied

A chamber and electrometer calibration factor as determined by each of the five AAPM accredited dosimetry calibration laboratories.

The same calibration grade ionization chamber and electrometer were sequentially sent to all five currently AAPM Accredited Dosimetry Calibration Laboratories (ADCL). A Cesium-137 based check device was utilized to ensure chamber and electrometer factor constancy and showed a maximum deviation of 0.32% (typically less than 0.1%) over the 227 days needed to complete the intercomparison. The chamber and electrometer calibration factor provided by each of the five ADCLs were analyzed for consistency. The maximum percent difference in reported chamber factor between all five ADCLs was 1.40%. The reported electrometer factor had a maximum discrepancy of only 0.50%. System (chamber plus electrometer) factors as provided by three of the five ADCLs had a maximum discrepancy of 1.51%.

Humans↗
PubMed1999

Room scattered neutrons.

A method of evaluating the photoneutron fluence in the maze of accelerator facilities due to room scattered neutrons is presented. Measurements were made to demonstrate that the room scattered fluence is reduced by a factor of 2pi in going from the treatment room to the inner maze entrance.

Gamma Rays↗
PubMed2001

Shielding assessment of a mobile electron accelerator for intraoperative radiotherapy.

A new approach to intraoperative radiotherapy makes use of a mobile electron linear accelerator delivering therapeutic radiation doses in an operating room suite. This unconventional technology has raised questions concerning protection for personnel and the necessity of shielding the adjoining areas. In this study, the leakage and scatter radiation from the mobile electron accelerator is measured and characterized in a series of spherical projections. An analysis is performed to determine the need for shielding or, alternatively, patient-based load restrictions in the operating room. This investigation provides a resource to assess shielding and/or patient load restrictions for any facility performing intraoperative radiotherapy with a similar unit. The data presented indicates that the mobile electron accelerator may be operated in an area with little or no shielding under nominal patient load expectations.

Electrons↗
PubMed2003

Shielding evaluation for IMRT implementation in an existing accelerator vault.

A formalism is developed for evaluating the shielding in an existing vault to be used for IMRT. Existing exposure rate measurements are utilized as well as a newly developed effective modulation scaling factor. Examples are given for vaults housing 6, 10 and 18 MV linear accelerators. The use of an 18 MV Siemens linear accelerator is evaluated for IMRT delivery with respect to neutron production and the effects on individual patients. A modified modulation scaling factor is developed and the risk of the incurrence of fatal secondary malignancies is estimated. The difference in neutron production between 18 MV Varian and Siemens accelerators is estimated using Monte Carlo results. The neutron production from the Siemens accelerator is found to be approximately 4 times less than that of the Varian accelerator resulting in a risk of fatal secondary malignancy occurrence of approximately 1.6% when using the SMLC delivery technique and our measured modulation scaling factors. This compares with a previously published value of 1.6% for routine 3D CRT delivery on the Varian accelerator.

Air Pollutants, Radioactive↗
PubMed2004

Evaluation of an automatic needle-loading system.

The purpose of this paper is to evaluate the dosimetric capabilities and the radiation protection (RP) performance of a new automatic needle-loading system for permanent prostate implants, the Isoloader (Mentor Corp.). The unit has been used in more than 100 clinical cases at our institution. The Isoloader is a computerized workstation that allows automated seed testing by a solid-state CdZnTe radiation detector and loading in surgical needles. The seeds are received in a shielded and ready-to-use cartridge. Radiation protection measurements were done on a cartridge filled with 67 (125)I seeds and during dosimetric seed verification and needle loading. The reproducibility of the detector was tested and its accuracy was determined by comparison to specified activities of six calibration seeds and to their measurements in a calibrated well-chamber (WC). Finally, the times required to complete dosimetric verification and needle loading were evaluated. The cartridge was found to be adequately shielded, since no significant amount of radiation was detected around it. Radiation during seed assay was found to be worst at the cartridge's bottom, where it has a value of 15.2 microSv/h (1.4 microSv/h at 10 cm). For the needle-loading task, measurements were performed with a typical needle (three seeds) at the shielded needle holder surface yielding 307.2 microSv/h (8.3 microSv/h at 20 cm). Seed dosimetric verification takes an average of 15 s/seed, while it takes a mean time of 50 s/needle to complete the loading task. Measurements of the six seed activities were within 0.65% of the ordered activities and 1.9% higher on average than those from the WC (min = 0.7%; max = 3.5%). The reproducibility of the measurements of the CdZnTe detector was excellent, with an average of 0.01% of deviation from a reference measurement (N = 120; = 1.9%). We therefore conclude that the Isoloader is a safe, fast, and effective needle-loading system.

Brachytherapy↗
PubMed1975

Chromosome damage in liver cells from low dose rate alpha, beta, and gamma irradiation: derivation of RBE.

Relative biological effectiveness (RBE) for chromosome damage in liver cells was determined after low dose rate exposures to alpha, beta, or gamma irradiation. Protracted exposures to beta and gamma irradiation were equally effective, whereas low dose rate exposures to alpha emitters were 15 to 20 times more damaging than exposures to beta or gamma irradiation. These data support the use of the quality factor of 10 recommended by the International Commission on Radiological Protection and the National Council on Radiation Protection for estimating the biological hazard from internally deposited alpha emitters. When the dose rates were low, all types of chromosome damage observed were produced by single-hit processes.

Alpha Particles↗