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.

Chronic infiltrative lung disease: comparison of diagnostic accuracies of radiography and low- and conventional-dose thin-section CT.

PURPOSE: To compare the accuracy of low-dose thin-section computed tomography (CT), chest radiography, and conventional-dose thin-section CT in diagnosis of chronic infiltrative lung disease (CILD). MATERIALS AND METHODS: Chest radiographs and low-dose (80-mAs) and conventional-dose (340-mAs) thin-section (1.5-mm) CT scans from 50 patients with CILD and 10 healthy control subjects were randomized and analyzed retrospectively. For CT, three images were obtained: at the aortic arch, at the tracheal carina, and 1 cm above the right hemidiaphragm. Two independent observers listed three likely diagnoses and recorded the degree of confidence in their first choice. RESULTS: A correct first-choice diagnosis was made more often with either CT technique than with radiography (P < .02). A high confidence level in the diagnosis was reached in 42% of radiographic, 61% of low-dose CT, and 63% of conventional-dose CT interpretations, which were correct in 92%, 90%, and 96% of the studies, respectively. CONCLUSION: The superior diagnostic accuracy of low-dose thin-section CT was achieved without an increase in effective radiation dose.

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.

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.

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.

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.

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.

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.

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

The 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.

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.

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.

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.

Development of a hospital radiation emergency protocol.

Many hospitals are faced with the development of radiation emergency plans. Details are given of the relevant literature and audio-visual materials to assist in the development of such a plan.

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.