As part of the responsibilities of the Medical Internal Radiation Dose Committee of the Society of Nuclear Medicine, new phantom models for heart walls and heart chambers have been developed. Estimates and methods of calculations for specific absorbed fractions to 21 target organs are included for photon sources in the heart. (DLS)

The problem of validating estimates of absorbed dose due to photon energy deposition is examined. The computational approaches used for the estimation of the photon energy deposition is examined. The limited data for validation of these approaches is discussed and suggestions made as to how better validation information might be obtained. (ACR)

TWENTY-FIFTH ANNUAL MEETING OF THE HEALTH PHYSICS SOCIETY: Abstracts of papers presented at the meeting: PDF Only

TWENTY-FIFTH ANNUAL MEETING OF THE HEALTH PHYSICS SOCIETY: Abstracts of papers presented at the meeting: PDF Only

Monte-Carlo simulation studies have been undertaken to study the transport of uranium L X-rays and the 241Am 59.5-keV gamma photon in a heterogeneous mathematical phantom. The phantom transmission function is found to be expressible in the form f =alfa1 ealfa2(micros xs+microL xL) for transport through soft tissue and lung, respectively. For Pu counting in the front position, alfa1 = 0.21 ± 0.03 and alfa2 = 1.42 ± 0.09 while for Am counting, alfa1 = 0.48 ± 0.06 and alfa2 = 2.36 ± 0.16. For rear counting of Pu, a, = 0.24 ± 0.03 and alfa2 = 1.55 ± 0.06, while for rear counting of Am, alfa1= 0.40 ± 0.05 and alfa2 = 2.10 ± 0.16. The Monte-Carlo method predicts an effective soft tissue thickness (ESTT) of 5.2±0.9cm for the mathematical phantom, in excellent agreement with an average ESTT of 5.0 ± 1.0 cm calculated from previous 51Cr-103Pd experiments using human volunteers. These results are also utilized to predict phoswich calibration factors and to predict the L X-ray line shape for the phoswich detector.

The purpose of this study was to determine, by theoretical calculation and experimental measurement, the absorbed dose distributions in two heterogeneous phantoms representing one-year- and five-year-old children from typical radiographic examinations for those ages. Theoretical work included the modification of an existing internal dose code which uses Monte Carlo methods to determine doses within the Snyder-Fisher mathematical phantom. A Ge(Li) detector and a pinhole collimator were used to measure x-ray spectra which served as input to the modified Monte Carlo codes which were used to calculate organ doses in children. The calculated and measured tissue-air values were compared for a number of organs. For most organs, the results of the calculated absorbed doses agreed with the measured absorbed doses within twice the coefficient of variation of the calculated value. The absorbed dose to specific organs for several selected radiological examinations are given for one-year-old, five-year-old, and adult phantoms.

The radiation dose to the bladder wall following the administration of radionuclides to patients can be reduced by a factor between 25 percent and 75 percent when the effective half-life for the radioactivity entering the urine is two hours or less. A significant but smaller reduction in dose to the gonads may also be achieved in situations where the major fraction of the administered activity is rapidly excreted in the urine. This reduction in dose is achieved by ensuring that the patient has between 50 and 150 ml of urine in his bladder when the radioactivity is injected, and is encouraged to void between one and two hours after the activity has been administered. The interrelationship of voiding schedule, effective half-life, initial urine volume, and demand urination has been analyzed in these studies. In addition, the significance of the rate of urine production and volume of urine in the bladder on the radiation dose to the bladder is demonstrated. (auth)

Chest cavity monitoring by gamma spectrometry generally utilizes detectors of 12.7, 20.3, or 22.9 cm in diameter, which are positioned according to autopsy data, chest X-rays, or the operator's estimate of lung position. In this study, Monte Carlo techniques and a computer representation of a reference human figure were used to determine optimum detector size and positioning. Escape efficiency for uncollided photons of 16–185 keV was determined for chest, back and sides of the thorax. The effect of nonuniform source positioning was also studied. Given uniform particle distribution, two detectors of 12.7-cm diameter were found to register essentially the same number of photons as one detector of 22.9-cm diameter, even though the latter has nearly 60% more surface area. The optimum detector appears to be a rectangle of approx 16 × 16 cm. If a detector is to be placed over each lung, the center of each detector is most effectively positioned approx 20 cm below the top of the shoulder and 10 cm from the midline of the sternum.

A dosimetric system was developed which provides estimates of mean radiation dose to organs from photon sources distributed uniformly in one or more organs. Although the sources of photons are assumed to be distributed uniformly, it is not true that dose from these photons is uniformly distributed. In particular, when a source of photons is located in a particular organ, nearby tissues will be irradiated at doses which decrease markedly with distance from the source. The mean dose may give a poor approximation to the actual dose if the tissues over which dose is averaged are extensive, for example, the remainder of the body. A set of enveloping organs was devised for liver, lungs, etc., which give mean dose at distances from zero to one centimeter from the source organ, from one to two centimeters, etc. These can be used to yield estimates of the extent of inhomogeneity of the dose distribution from a source of photons located in the source organ.