John E. Aldrich

Dental enamel as an in vivo radiation dosimeter

The determination of the radiation exposure history of the population has become increasingly important in the study of the effects of low-level radiation. The present work was started to try to obtain an in vivo dosimeter that could give an indication of radiation exposure. Dental enamel is the only living tissue which retains indefinitely its radiation history, and electron spin resonance measurements have shown that the radiation signal can be resolved down to about 10 cGy. Measurements on samples from the general population give radiation exposure estimates that are reasonable, and one measurement on a patient who had radiotherapy to the mouth area showed a good correlation with tumor dose. We believe that this is an important new indicator of radiation dose and taken together with exposure histories should provide important data for epidemiological studies as well as accidental exposures.

An analysis of paramagnetic centers in irradiated dentin using electron spin resonance.

SummaryThe ESR spectra produced in irradiated dentin have been studied over a range of incident radiation energies from 50 kVp to 25 MVp. The behavior of the dentin ESR signal strength is similar to that of enamel as a function of the energy of the incident radiation. The magnitude of the dentin ESR signals are, however, up to 10 times smaller than the signals of dental enamel for a given radiation energy. The possible contributions of radiation interaction coefficients, chemical structure, and crystallite size to the differences in ESR spectra are discussed.

Radiation protection aspects of a new high‐energy linear accelerator

The Therac-25 is a new 25-MeV linear accelerator manufactured by Atomic Energy of Canada, Ltd. The first two units have recently been installed in Toronto, Ontario and Halifax, Nova Scotia. Calculations and measurements of primary and secondary radiation levels were made. Neutron dose-equivalent rates were measured inside and outside the room. The maximum leakage rate at 1 m from the accelerator target was 0.4% Sv per peak photon Gy. The tenth value layer for neutrons from the Therac-25, at the entrance to a one-legged maze was found to be 5.5 cm of polyethylene. Measurements were done to estimate daily technologist exposure due to induced activity in the treatment room.

A microcomputer-based system for radiotherapy beam compensator design and patient contour plotting.

A microcomputer-based system for radiotherapy beam compensator design has been constructed. A patient surface measuring device which projects a grid pattern on the patient is attached to the collimator of a therapy treatment simulator. A linear motion potentiometer, pivoted at one end on a ball joint attached to the collimator, is extended to touch each grid point in turn. The extension of the potentiometer is monitored by a microcomputer which calculates the coordinates of the patients surface as each grid point is entered. A template for compensator construction is subsequently printed on the microcomputer printer. The patient surface profile along any grid line can also be plotted and used in conjunction with the compensator data to produce an isodose distribution for the compensated field.

Characteristics of the photon beam from a new 25-MV linear accelerator

The Therac 25 is a relatively compact therapy machine, the heart of which is a double-pass electron linear accelerator. The electron beam is injected into the accelerator at the treatment head end of the machine and is accelerated back down the arm to an energy of 13 MeV. At this end of the machine a magnet system reflects the beam back into the structure where it gains up to an additional 12 MeV of energy. After leaving the linear accelerator the beam is bent by an achromatic head magnet through 270 degrees to the treatment head. The machine produces eight electron beams and a 25-MV photon beam. In this work only the parameters of the photon beam are addressed based on measurements at the first two clinical sites. Percentage depth doses, tissue phantom ratios, and beam symmetry and stability are presented and discussed.

Radioactive gold ring dermatitis

A superficial squamous cell carcinoma developed in a woman who wore a radioactive gold ring for more than 30 years. Only part of the ring was radioactive. Radiation dose measurements indicated that the dose to basal skin layer was 2.4 Gy (240 rad) per week. If it is assumed that the woman continually wore her wedding ring for 37 years since purchase, she would have received a maximum dose of approximately 4600 Gy.

Characteristics of electron beams from a new 25-MeV linear accelerator

Clinically useful electron fields are produced on the Atomic Energy of Canada, Limited Therac 25 linear accelerator by computer-controlled scanning of the electron beam. Measurements were made to determine the properties of these electron fields. Central axis percentage depth dose and bremsstrahlung background were compared for these fields and for the fields from selected machines that use scattering foils. Dose calibrations were made in both water and polystyrene using the American Association of Physicists in Medicine Task Group 21 protocol. Measurements were made to determine the relative output factors, virtual source position, and the attenuation of the electron fields by lead.

Self-adhesive lead for compensator production and radiation shielding

Sheet lead is a commonly used material for the manufacture of compensators and for radiation shielding. However, it suffers from such problems as nonuniformity of thickness, difficulties in accurate cutting and alignment, and toxicity. A double-sided adhesive lead sheet has been developed which overcomes these problems.

Comments on “ESR dosimetry for atomic bomb survivors and radiologic technologists”

Abstract After renormalization it has been shown that the data of Tatsumi-Miyajima agrees very closely with our data. It is suggested that further information on the origin of the radiation exposure could be obtained by using the differential attenuation properties of the tooth itself.

A review of the oncology curriculum at Dalhousie medical school

The teaching of oncology at Dalhousie medical school is currently left to the discretion of the 30 university departments. There is no central coordination. This organization leaves no monitoring of curriculum content to see that what should be covered is indeed covered, nor does it provide teaching from the perspective of oncology rather than the perspective of the other individual disciplines. Following an inventory of the oncology curriculum and a survey of interns graduating from Dalhousie in 1991, we recognize deficiencies in the clinical portion of the curriculum that could be rectified with a small number of hours designed to teach the various aspects of the management of cancer patients in case-scenario, student participation sessions.