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Biomedical subjects

A L Boyer

Publications and source records attributed to A L Boyer.

At least 73 records · Page 4Linked to original sources
PubMed1987

Backscatter radiation at bone-titanium interface from high-energy X and gamma rays.

Occasionally head and neck cancer patients treated with high-energy X rays and gamma rays have titanium metal dental implants in their maxillae or mandibles. The resulting effect of the bone-metal interface on the radiation dose is of interest. Ionization measurements for 60Co gamma rays and 6 MV and 25 MV X rays were made. A thin-window parallel-plate chamber was used to determine the magnitude of the dose enhancement that was due to the backscattered electrons from titanium. The results showed that for 60Co there is a 15% increase in dose to solid bone at the entrance side of the titanium. For higher energy X rays, the increase in dose was about the same or slightly lower than for 60Co. Monte Carlo calculations substantiated the measurements. This increase in dose fell off rapidly and became negligible at 1-2 mm from the interface. This backscattered dose should be taken into account when planning radiation therapy treatment for patients with dental implants.

Bone and Bones↗
PubMed1984

Computer assisted instruction for radiotherapy residents.

In order for radiotherapy residents to become proficient with physics concepts, a certain amount of reinforcement and repetition is necessary. An assembly of computer programs has been developed for the RSX operating system with the aim of shifting these repetitive tasks from the physics staff to a computer. Three types of interactions are employed: (1) textual summaries of key concepts, (2) numerical problems, and (3) typical board exam questions. A file is maintained which records the resident's performances as they work through the programs.

Computer-Assisted Instruction↗
PubMed1984

Correction factors for survey meter calibrations.

A factor is derived to correct the response of a large cylindrical air ionization chamber in the radiation field of a point-like calibration gamma-ray source. The calculated factor is compared with the observed response of commercially available instruments used for radiation safety surveys. It is concluded that proper use of the factor can improve the accuracy of survey meter calibrations when point-like standard sources are employed. Furthermore, it is shown how the design of a chamber can reduce the response error when using point-like sources for calibration.

Calibration↗
PubMed1982

An electron beam pseudoarc technique for irradiation of large areas of chest wall and other curved surfaces.

An electron beam technique has been developed for the treatment of large areas of the chest wall and other curved surfaces employing the stationary field electron beam mode ordinarily available on a linear accelerator. The technique simulates an arc through the use of multiple fixed fields. The electron collimator is not used. Field shaping is achieved through the combined effects produced by the photon jaw settings, the arc limits, and secondary lead shielding on the patient's skin. Thus, there is no limitation on the circumferential extent of the field size that can be used. Electron beam energies of 6 MeV, 9 MeV, 12 MeV, 15 MeV and 18 MeV have been studied. Our findings indicate that, for these energies, this technique produces isodose curves parallel to the cylindrical surface at all depths beyond maximum build-up. This paper discusses the physical characteristics of the single beams, the degree of dose homogeneity achieved with the multiple fields, and the dosimetry technique developed to implement the therapy.

Electrons↗
PubMedDate not supplied

Laser "cross-hair" sidelight.

A technique is described for forming an optical laser cross-hair image for use as a radiotherapy sidelight. The technique is developed from a concept introduced earlier by which a line image is formed from a laser beam using a cylindrical lens. By using two cylindrical lenses and appropriate reflectors, two line images can be made to form a cross.

Humans↗
PubMedDate not supplied

A fundamental accuracy limitation on measurements of brachytherapy sources.

The dose rate gradients in the first few centimeters away from brachytherapy sources are extremely high. Attempts at direct measurements of the dose rates in the near vicinity of such sources are made with the smallest detectors available. Nevertheless, it is not obvious that the agreement between reported measurements and theoretical calculations can be justified when the dose rate gradients across the detector volumes employed are so great. A figure of merit is derived here which indicates how well the dose averaged over the volume of the detector corresponds to the dose at the center of the detector. This figure of merit provides a means to assess the maximum accuracy one can expect to achieve in a measurement made at a given distance from a linear source based on the dimensions of the source and the dimensions and shape of the detector.

Brachytherapy↗
PubMedDate not supplied

Simulator mounted Moiré topography camera for constructing compensating filters.

A technique has been developed and implemented by which individualized Ellis compensating filters can be rapidly and accurately produced from Moiré topographic photographs. A novel Moiré camera has been designed such that it has been possible to mount it on a radiotherapy simulator. The camera does not interfere with normal use of the simulator but can be deployed as needed. Depth of missing tissue contours are produced by the camera over the treatment field, accurately registered to the patient's simulated treatment position. An optical enlargement technique is used to transfer the depth of missing tissue contours in the photograph to a lead sheet compensating filter.

Humans↗
PubMedDate not supplied

Postal intercomparison of absorbed dose for high energy x rays with thermoluminescence dosimeters.

This study concerns the accuracy and precision of the IAEA/WHO LiF TLD system used in intercomparison by mail of absorbed doses from 60Co gamma-radiation and 4-25 MV x rays. The system employs 160 mg LiF powder in polystyrene capsules, which are placed at 5 or 7 cm depth in water and irradiated to doses close to 200 rad (2.00 Gy). The dosimeters are mailed to the IAEA Dosimetry Laboratory and read out under conditions to minimize variatons in instrument sensitivity. The precision of the readout technique, using 3 capsules per irradiation and the readout of 5 aliquots per capsule, is characterized by 0.2% standard deviation of the resulting mean. Since random errors during the irradiation are added, the detectable systematic descrepancy in dose delivery, at the 95% confidence level, is +/- 2% for 60Co gamma and +/- 3% for high-energy x rays.

Cobalt Radioisotopes↗
PubMedDate not supplied

Compensating filters for high energy x rays.

In order to produce individualized compensating filters for use with high energy x rays, a study was undertaken to study lead as a filter material using 10-MV x rays. As was expected, it was found that lead filters could be constructed for 10-MV x rays by extending concepts developed for cobalt-60 gamma rays. A quantity called the "effective attenuation coefficient" (mueff) was determined for a range of field sizes, initial depths in water, and lead thicknesses and for 100-cm SSD and 150-cm SSD treatment techniques. Slight variations of mueff with field area and filter thickness were discernable. However, a single value of this parameter could reproduce the original depth of missing tissue data with an rms error of 2 mm. Tests were then performed on filters whose shapes were calculated using the effective attenuation coefficient. Compensating filters restored the flatness of 10-MV x-ray beams to better than 5%, which was better than comparable filters using cobalt-60 gamma rays. The technique is presented as generally reliable for constructing compensating filters for use with high energy linear accelerators.

Radiotherapy, High-Energy↗
PubMedDate not supplied

Encoding patient contours using Fourier descriptors for computer treatment planning.

Frequently it is desirable to digitize patient's external and internal contours from computer-assisted tomography (CAT) scan images and to use them for computer treatment planning. After the contours are digitized, each contour could contain over 1000 points. It is a common practice to reduce the number of contour points by interpolation methods in order to use them in a treatment planning program, and in order to save storage space. This paper describes an alternative method for encoding contours. The x-y coordinates of each contour point are represented as a complex number, x + jy. The discrete Fourier transform (DFT) of the array of complex numbers is then computed. Only the 50 lowest frequency components of the DFT are retained. Each contour is then represented by these 50 complex numbers, known as Fourier descriptors. The original contour is restored by performing the inverse Fourier transform. All the frequency components higher than 50 are assumed to be zero during the inverse Fourier transform. The algorithm is described in detail.

Computers↗
PubMedDate not supplied

Calculation of photon dose distributions in an inhomogeneous medium using convolutions.

A formulation of the single energy photon transport problem is cast in the form of a series of convolutions over the primary fluence distribution with invariant kernels. A method is developed for compensating for the variant intervoxel attenuation in the scattered dose distribution by means of an invariant correction kernel. Results of applying the formulation to 60Co distributions in the presence of slab and block inhomogeneities are presented.

Cobalt Radioisotopes↗