Robert J. Barish

Thermal characteristics of a common polystyrene phantom

We have examined the thermal characteristics of a polystyrene phantom of the SCRAD type. In the case of a calibration phantom which is moved from one location to another, many hours may be required to achieve thermal equilibrium with the new environment. By using a correction term based on the temperature of the phantom, not of the room, it is possible to calibrate a therapy unit without waiting for thermal equilibrium.

Patient immobilization with a low-temperature splint/brace material.

The authors describe a low-temperature plastic which is useful for the construction of immobilizing devices where beam orientation is critical.

Long‐term use of an isotope check source for verification of ion chamber calibration

A radioactive check source is recommended for operational and constancy checks of dosimetry systems used for the calibration of therapeutic x-ray generators, including linear accelerators. Eight years of data have been analyzed for two ion chambers (and their associated electrometers) irradiated at fixed geometry in such a device. These dosimetry systems have also been calibrated every 2 years at a single Accredited Dosimetry Calibration Laboratory. Our analysis suggests that when a check source is used, and the results are consistent, the interval between formal calibrations can be lengthened.

Development of optical process for accessing three-dimensional patient topology.

A commercial optical system developed by Solid Photography, Inc., has been used to generate precise paraffin--polyethylene tissue compensators. A program with the objective of constructing a medical facility has been undertaken to access patient topology for treatment planning, tissue-compensator fabrication, and patient positioning verification on the treatment table. Currently, patients are taken to a specially equipped studio where a series of microprocessor-controlled reticles are projected on the surface to be analyzed and photographs are taken automatically for computer processing. The topology data then drives a machine tool to sculpt the tissue compensators and may be transmitted via acoustic-coupled terminals to a treatment-planning computer at the Medical Center. The tissue compensators have been found to conform accurately to patient contours. At present, data bases can be generated to accommodate volumes of approximately 70,000 cm3.

An improvement in the use of plaster bandage for patient contouring.

The setting time of plaster bandage used for patient contouring can be decreased by adding potassium sulfate to the dip water. A small amount of this inexpensive chemical can have a significant effect. This improvement gives the physicist sufficient control of the process so that contours may be made from plaster as quickly as they are made with stiff wire, but with improved accuracy.

Shielding block mounting with double-stick tape

We describe a system of mounting radiotherapy shielding blocks using a double-stick adhesive tape as an alternative to the screw-and-nut system generally employed. The method is easy to use and eliminates the dose perturbations that are caused by holes or slots in a blocking tray. More than 2 years of experience with this method has demonstrated its reliability in a clinical setting.

Radiation Protection and Dosimetry: An Introduction to Health Physics

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Source–surface distance and source–axis distance beam data acquisition

Radiation beam characteristics are usually obtained by scanning in a water phantom. Central axis depth dose data, beam profiles, and isodose curves are derived from these measurements. Certain treatment planning systems require that beam profiles be scanned isocentrically rather than at a constant source-surface distance. We have looked at profiles taken both ways, with appropriate field-size correction, and find no significant difference between them. This should simplify data acquisition in many cases.

Extended expression for tissue-maximum ratio fitted to the Varian Clinac-4 and Clinac-6 accelerators.

An expression which generates tissue maximum ratio (TMR) data for the Varian Clinac-4 and Clinac-6 accelerators is presented. This semiempirical expression can be programmed easily for radiotherapy computer systems or the new generation of scientific calculators. Our technique leads to good agreement with the published data, as demonstrated by a total average percent deviation of the measured TMR values of -0.115% for the Clinac-4 and -0.002% for the Clinac-6.