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Biomedical subjects

P C Shragge

Publications and source records attributed to P C Shragge.

8 recordsLinked to original sources
PubMed1994

Calibration of film for accurate megavoltage photon dosimetry.

An accurate method of converting film density to dose is presented. For films oriented parallel to the beam's central axis, calibration curves were produced at several depths using Kodak XV-2 film for cobalt-60, 4 MV, and 18 MV beams. Then the appropriate curve was employed to convert the film density to dose at a specific depth. It is hypothesized that the change in film response with depth is due to changes in the photon spectra at depth in a phantom.

Calibration↗
PubMed1981

Ontario accelerator dose intercomparison study.

An ionometric dose intercomparison has been carried out on eight accelertors, with maximum photon energies from 6 to 32 meV, at five radiotherapy centres in Ontario. The ratio of the dose based on the clinically-employed rad/monitor unit to the dose measured by the committee representative had a mean value of 0.994, with a range of 7.3% and a coefficient of variation of 2.5%. The ratio of the dose measured by the institution's physicist to the dose measured by the committee representative had a mean value of 1.000 with a range of 6.9% and a coefficient of variation of 2.3%. Eight recommendations regarding dose calibration procedures are presented.

Humans↗
PubMedDate not supplied

Characteristics of an 18 MV photon beam from a Therac 20 Medical Linear Accelerator.

The 18 MV photon beam characteristics of a Therac 20 Medical Linear Accelerator manufactured by Atomic Energy of Canada Ltd, are presented. Tissue phantom ratios (TRP's) and percent depth dose data are given; for a 10 x 10 cm field, the percent depth dose at a depth of 10 cm is 78.5 (SSD 100 cm). The relative dose factors (RDF'S) are given and are analyzed to elucidate the relative contributions from phantom scatter, collimator scatter, and backscatter from the top of the collimators into the monitor chambers. The effect of field size and depth on the penumbra is described. Crossplots of the beam at a depth of 5 cm indicate that the flattening filter could be improved; there are hot spots of 108% near the corners of 40 x 40 fields.

Humans↗
PubMedDate not supplied

Improved method for the design of tissue compensators.

A program, WBEAM, is described which calculates dose distributions in planes perpendicular to the beam axis, taking into account both field shape and patient contour. WBEAM can be used to design compensators which when placed in the beam will produce uniform dose distributions in the plane of calculation. The program was tested in three different situations: a 30 x 30 cm field with a flat contour, a "mantle" field with a flat contour, a "mantle" field with a human-like contour. The program performs as designed: the dose distributions are accurate, and the compensators flatten the radiation beams to the specified limits.

Computers↗
PubMedDate not supplied

A semianalytical method for the design of a linac x-ray beam flattening filter.

The purpose of this study was to design an improved flattening filter for a Therac 20 medical linear accelerator. Profiles of the 18-MV x-ray beam produced by this accelerator measured along the diagonal of a 40 X 40 cm field at a depth of 5 cm were measured, and it was found that there were regions near the corners of the field where the dose was 109% of the central axis dose. An iterative algorithm for designing flattening filters was developed which required, as input, precise measurements of the following data: the unflattened primary beam profile, the fraction of the beam due to contamination radiation arising from interactions of primary photons with the flattening filter and the collimator assemblies, and the attenuation of the primary photons in water and lead as a function of angle from the central axis of the beam. A new flattening filter was designed and profiles of the beam were measured at a number of depths. These measurements showed that the beam was flattened to within +/- 1% out to 24 cm along the diagonal of a 40 X 40 cm field at a depth of 5 cm.

Equipment Design↗