An improved optical backpointer for isocentric radiotherapy machines.

A simple optical backpointer has been designed as a retrofit for isocentric radiotherapy machines. Two lasers are used to project two fan beams of light which intersect in line with the central ray of the radiation beam. The system is mechanically rugged, and the projected cross is visible even in room light.

Access control systems for radiotherapy treatment rooms.

The role of the time-delay or start-up switch as a safety feature in radiotherapy treatment rooms is discussed. A simple optical system which can be retrofitted to any treatment room is described.

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.

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 patient's 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.

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.

A video-based patient contour acquisition system for the design of radiotherapy compensators.

Recent advances in computer and video technology have enabled significant advances in the field of surface topography measurement. This paper will describe a digital video patient contour acquisition system for the design of radiotherapy beam compensators. The system is mounted on a tray which slides into the collimator of a radiotherapy simulator. The tray holds a source which projects a 1-mm wide fan beam of light along the simulator beam axis onto the patient. The tray also holds a small solid state video camera which views the projected line of light on the patient surface. Video images are digitized and processed on an 80386 based IBM PC-compatible computer. Transverse contours are obtained from one digitized video image or, in case of opposed fields, from two or more image frames taken at different gantry angles. Entire surfaces are measured by scanning the bed longitudinally and acquiring a number of transverse contours. Posterior surfaces are obtained by scanning a mold. A printer produces a template for making the compensator from sheet lead.

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.

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.

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.

Changes in the paramagnetic centres in irradiated and heated dental enamel studied using electron paramagnetic resonance.

The EPR signals in dental enamel produced by radiation and by heat were studied. The inherent background signal at g = 2.005, and a radiation-produced signal at g = 2.002 have different saturation behaviour with microwave power, and this affords a method of signal optimization. Heating enamel at temperatures from 100 degrees C to 450 degrees C produces a range of radical species from g = 2.002 to g = 2.005, which have been characterized by their g-values, line widths and saturation behaviour. Standard dental drilling produces a range of radicals which appear to be similar to those produced by heat.

A comparison of manual and remote afterloading in the treatment of carcinoma of the cervix: increasing dose rate with a flexible applicator.

A historical cohort design was used to assess the effect of introducing a remote afterloading system with a flexible applicator for the intracavitary treatment of cervical cancer. One hundred and sixty-eight patients treated for the 5 years prior to its introduction were compared to 84 patients treated for the first 3 years of its use. Patients were comparable with respect to age and stage. Treatment policies and techniques remained the same during the study period. Average dose rate to point A increased from 0.46 Gy/hr to 0.67 Gy/hr. The 3-year actuarial survival was equivalent between the two treatment modalities: 67.7% (manual), and 60.6% (remote). Patterns of local recurrence and distant failure were also equivalent between the two groups. Severe complication rates were comparable (8.3% manual, 6.3% remote) despite the increase in dose rate, confirming that the remote afterloading system can be safely adapted to a flexible applicator. The introduction of the remote afterloading system resulted in a 72% reduction in radiation exposure to staff.

Surface topography for patient repositioning.

For many years, this group has been interested in the measurement of patient surface contours in order to prepare radiotherapy compensators. Recently, these surface measurements have been made using optical ranging techniques (Medical Physics 16:425; 1989). A laser line is projected onto the patient surface and the video image of the line is digitized and used to calculate the coordinates of each point on the line. This surface information can be used not only for the calculation of compensators but can also be used to verify patient position from simulation to treatment and from one treatment to another. Because the calculation of surface coordinates takes approximately 30 seconds, the technique has been modified to allow the comparison of setup position in real time. In the new system, a series of narrow fan beams is projected to intersect the patient surface. At simulation, the reference video image of the lines is digitized and stored. At treatment setup, repeat video images are digitized and compared with the reference in real time. Operator adjustments to patient position are immediately shown on the composite image, and a congruency value is calculated to help in this process of alignment. Uses and accuracy of the technique will be discussed.

The surface doses from orthovoltage x-ray treatments.

Measurements have been made of the surface dose with treatment cones and lead shielding used with orthovoltage x-ray systems. Both devices significantly increase the surface dose, probably by the generation of photoelectrons. The surface dose depends upon the shape and material of the cone and the x-ray energy. The surface dose can be 500% higher than the dose measured with a Baldwin-Farmer chamber. The significance of these doses is discussed in the light of recent information on the thickness of the epithelium. It is suggested that every effort be made to reduce the surface dose by using low atomic number material between the lead and the patient surface.

The use of digitized port films in assessing new treatment modalities.

An inexpensive computer-based imaging system has been used to compare simulator and portal radiographs. The films were digitized, normalized, and then overlaid for measurement. In a test group of patients, a new treatment protocol for pelvis irradiation was evaluated using this technique. For 90% of the patients, the average error was found to be smaller than 5 mm, and there were no field placement errors greater than 1 cm.

Collective biodosimetry as a dosimetric "gold standard": a study of three radiation accidents.

Quantification of the biologically relevant dose is required for the establishment of cause-and-effect between radiation detriment or burden and important biological outcomes. Most epidemiological studies of unanticipated radiation exposure fail to establish cause and effect because of an inability to construct a valid quantification of dose for the exposed population. No one biodosimetric technique (biophysical or biological) meets all the requirements of an ideal dosimeter and thus qualify as a "gold standard." This report combines new results with previously published data in order to establish a collective biodosimetry as a dosimetric "gold standard" for the victims of three radiation accidents. Combining new and previously published data is necessary as execution and planning of a comprehensive dosimetry is rarely done at the initial stages of a radiation accident. The first subject was a fireman during the initial moments of the Chernobyl nuclear accident; the second was the victim of an unspecified occupational accident; and the third was exposed to a 60Co sterilization source. There was generally good agreement among the various biodosimetric techniques used for the three accident victims.