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Biomedical subjects

P D LaRiviere

Publications and source records attributed to P D LaRiviere.

13 recordsLinked to original sources
PubMed1989

The quality of high-energy X-ray beams.

Supplement 17 of the British Journal of Radiology is a survey of central-axis depth doses for radiotherapy machines, patterned largely on BJR Supplement 11 published some 11 years earlier. Inspection of the high-energy X-ray depth doses for a 10 x 10 cm field at an SSD of 100 cm disclosed large differences between the two sets of data, especially for qualities above 8 MV. For example, a depth dose of 80% at 10 cm is rated at about 19 MV according to BJR Supplement 11, and 23 MV according to BJR Supplement 17. It was found that the Supplement 17 depth-dose data above 8 MV were erratic, but that the Supplement 11 data could be represented by an analytical expression, providing a unique means of assigning MV quality. It was also found that the dose-weighted average energy of the filtered beam plotted smoothly against depth dose. For dosimetric purposes, it is suggested that this parameter be used as a true measure of beam quality, removing the discrepancies introduced by the use of nominal MV for this purpose.

Humans↗
PubMed1985

Radiotherapy technologist dose from high-energy electron medical accelerators.

Whole-body doses incurred by radiation therapy technologists working with 18- and 24-MV medical linear accelerators have been measured at two clinics under normal working conditions with a sensitive solid-state personal dosimeter. The results based on 84 treatments indicate that average monthly doses in the range 12-19 mrem can be expected for 30 daily treatments similar to those monitored. It was also found that 76% of the in-room dose was due to activated components in the treatment head, the balance of the in-room dose being due to activated concrete. Other statistics were obtained on patient in-room times and technologist times and doses outside the treatment room. Although the monthly film dosimetry records show no detectable doses for technologists working with these particular machines, in the spirit of ALARA, steps have been taken to reduce residual activities even further by choice of material less susceptible to activation.

Electrons↗
PubMed1984

Primary and leakage radiation calculations at 6, 10 and 25 MeV.

Primary and leakage x-ray spectra for typical clinical accelerators have been calculated using the EGS Monte Carlo code for 3 energies (6, 10 and 25 MeV) and 4 angles (0 degree, 45 degrees, 90 degrees and 135 degrees). Broad-beam transmission curves have been calculated for ordinary concrete using the MORSE program with the EGS spectra as input. A simple analytical model, which is shown to agree rather well with both experimental data and with the MORSE results, is presented and initial and subsequent tenth-value layers are extracted. Finally, the photon spectrum after the concrete shield is obtained with MORSE.

Monte Carlo Method↗
PubMed1984

Transmission in concrete of primary and leakage x rays from a 24-MV medical linear accelerator.

The transmission of leakage radiation from a 24-MV/6-MV linear accelerator was measured through 52 in. of concrete forming the roof of a test cell, at angles of 30, 45, 90, 135 and 180 degrees relative to the direction of the electron beam at the target. Transmission of the primary beam was also included as a partial check on the suitability of the method. The latter results were in good agreement with recommended values. The leakage radiation tenth-value layers agreed well with theory at 90 degrees but, at greater and lesser angles, interpretation was less straightforward, owing it is believed to the multiple leakage sources and complex shielding geometries inherent in a bent-beam machine. The implications of the findings concerning shielding barrier design are discussed, including consideration of the new case presented by dual photon energy machines.

Construction Materials↗
PubMed1983

Electron depth-dose dependence on energy spectral quality.

Electron depth-dose curves and spectra have been measured from an experimental accelerator on which both energy and spectral shape could be varied independently. The measurements were made at nominal energies of 6 and 12 MeV, with the energy spread (FWHM) varied from 1.7% to over 10% of average energy. Changes in depth-dose fall-off characteristics correlated only weakly with the width of the accelerator's spectrum. The use of a single accelerator and experiment set-up eliminated the differences that have complicated previous comparisons of beams of different spectral quality, which have been based upon data from different accelerator designs.

Particle Accelerators↗
PubMedDate not supplied

Neutron energies in medical electron accelerator rooms.

Two experimental methods are described for obtaining average energies of neutron spectra produced by an electron accelerator in a medical treatment room. In the first method, the dose equivalent is measured by means of indium activation foils in an Andersson-Braun moderator. Fluences are determined by the usual activation foil techniques. These data lead to the conversion factor, neutrons/(cm2s) per mrem/h, which uniquely defines the effective energy of the unknown spectrum. In the second method, the fast neutron broad-beam attenuation in polyethylene is obtained from measurement of the thermalized fluence versus depth curve, whose slope in the exponential region parallels that of the fast neutron spectrum. It was found for a 24-MV linear accelerator that the effective energy varied from 0.28 MeV at the isocenter under closed collimators to 0.016-0.024 MeV in the entrance maze. These results compare favorably with other measurements and calculations. It is concluded that the dose equivalent can be measured to within +/- 50% with an activation foil in an Andersson-Braun moderator and that the characteristic energy obtained by attenuation at the isocenter closely approximates the average energy of the neutron spectrum escaping the treatment head.

Electrons↗
PubMedDate not supplied

Neutron sources in a 24-MV medical linear accelerator.

During the assembly of the prototype Clinac 2500, a unique opportunity arose to measure the cumulative total neutron yield of the machine as the major subassemblies were added, section by section. Differentiation of the results led to an inventory of the relative strength of the several neutron sources identified. The method utilized the fact that the scattered neutron fluence in a cavity in concrete is nearly uniform throughout the space and proportional to the fast neutron source strength. When normalized to the neutron output of the guide with bend magnet vacuum chamber, the relative neutron source strengths found were target, 1.9; primary collimator, 2.4; steel filter, 0.6; tungsten filter, 1.5; and jaw system, 0.8 open and approximately 2.0 closed. There is reasonable agreement between these measurements and published estimates of the yields from the major components of the treatment head.

Humans↗