Calculating the absorbed dose from radioactive patients: the line-source versus point-source model.

UNLABELLED: In calculations of absorbed doses from radioactive patients, the activity distribution in such patients is generally assumed to be an unattenuated point source and the dose to exposed individuals at a given distance is therefore calculated using the inverse square law. In many nuclear medicine patients, the activity distribution is widely dispersed and does not simulate a point source. In these cases, a line-source model is proposed to more accurately reflect this extended activity distribution. METHODS: Calculations of dose rate per unit activity were performed for a point source and for line sources of lengths of 20, 50, 70, 100, and 174 cm, and the ratios of line-source values to point-source values were calculated. In addition, radionuclide-independent conversion factors, to convert exposure rate constants to dose rates per unit activity, for these line-source lengths at various distances were determined. RESULTS: The calculated values, substantiated by published data, indicate that the inverse square law approximation is not valid for a line source until a certain distance is reached, dependent on the length of the line source. For the 20-, 50-, 70-, 100-, and 174-cm line sources, the dose rate values estimated by the inverse square law approximation are within approximately 10% of the values estimated using the line-source approach at distances of 20, 45, 60, 85, and 145 cm, respectively. At closer distances, use of the point-source model for a patient with an extended activity distribution will overestimate the radiation absorbed dose to exposed individuals, sometimes by a very significant amount. CONCLUSION: The line-source model is a more realistic and practical approach than the traditional point-source model for determining the dose to individuals exposed to radioactive patients with widespread activity distributions.