Robert J. Shalek

Fetal dose from radiotherapy with photon beams: Report of AAPM Radiation Therapy Committee Task Group No. 36

Approximately 4000 women per year in the United States require radiotherapy during pregnancy. This report presents data and techniques that allow the medical physicist to estimate the radiation dose the fetus will receive and to reduce this dose with appropriate shielding. Out-of-beam data are presented for a variety of photon beams, including cobalt-60 gamma rays and x rays from 4 to 18 MV. Designs for simple and inexpensive to more complex and expensive types of shielding equipment are described. Clinical examples show that proper shielding can reduce the radiation dose to the fetus by 50%. In addition, a review of the biological aspects of irradiation enables estimates of the risks of lethality, growth retardation, mental retardation, malformation, sterility, cancer induction, and genetic defects to the fetus.

Electron beam central axis depth dose measurements

Central axis depth dose measurements were made by the Radiological Physics Center on over 70 electron-producing machines used in radiation therapy. These data were consistent for each machine model and nominal energy. However, the data show that depth dose relations can vary significantly among different machine models for electron beams having the same nominal energy. Analysis shows that both the method used to achieve beam flatness and the mean incident electron energy determine the central axis depth dose curve past the depth of maximum dose. A linear relation of depth dose versus mean incident electron energy is used to predict depth dose to within 2 mm for most electron beams used clinically at depths greater than d95.

A review of computer techniques for dosimetry of interstitial and intracavitary radiotherapy

Abstract The use of computer for dosimetry calculations in interstitial and intracavitary radiotherapy is reviewed, with a summary of features of twenty computer programs which have been reported in the literature. A discussion of the history of manual calculations and their relationship to computer calculations is included, with comments on the impact of computers in this area of medical physics.

A review of the reliability of chamber factors used clinically in the United States (1968--1976).

One of the principal concerns of a physicist responsible for calibrating megavoltage radiotherapy equipment is the validity and stability of the 60Co exposure correction factor assigned to his ionization-chamber and electrometer system. It is the practice of the AAPM Radiological Physics Center (RPC) to perform an intercomparison between the RPC chamber and electrometer system and the chamber and electrometer in use at each of the various institutions visited by the RPC. The results of 202 such intercomparisons are reviewed to determine (1) the consistency in the assignment of exposure correction factors by a calibrating agency with itself and with other calibrating agencies, and (2) the dependence of the reliability of the exposure correction factors upon the type of field instrument and the time since calibration.

The RMP Medical Physics Program in Texas

The TRMP (Texas Regional Medical Physicists) Program formally began in 1968 as an organization of Texas medical physicists with the support of the Regional Medical Program in Texas. TRMP was formed to help its member physicists extend and improve their services to the medical profession. This is accomplished by offering services to the members that they do not have available as individuals and by serving as a means of communication and cooperation between the members. Services provided by the TRMP central office include dosimetry calculations by computer, mailed thermoluminescent dosemeters to check machine output, mailed films to check light localizers, loan of equipment and information services. In addition, a physicist is available for on-site review of dosimetry procedures at a members institution. The TRMP services are available only to physicists and TRMP employees are not permitted to do private consulting; therefore, there is no interference with established relationships between members and the radiotherapy community.

Zero field size TAR for high energy X-ray

A theoretical calculation of zero field size tissue-air ratios (TAR) for MV X-rays produced by a linear accelerator was presented. The method involves the generation of a thin-target bremsstrahlung spectrum attenuation in the beam flattening filter and the energy absorption of the resulting X-ray spectrum in tissue. The spectrum is considered in intervals of 50 keV from 0 to 4.0 MeV, and the TARS are calculated from 1.0 to 20.0 cm in one centimetre increments. All calculations were performed by a Nova computer. The results were compared with empirical data reported by Golden and Peterson. The agreement was within +or-1.0% from a depth of 1.0 to 15.0 cm. The method is being extended to 6 MV and 8 MV X-rays produced by linear accelerators.