title: Calculating the absorbed dose from radioactive patients: the line-source versus point-source model.
abstract: 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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Nuclear Physics Enterprises, Cherry Hill, New Jersey 08003, USA. siegelja@aol.com
authors.forename: Jeffry A
authors.initials: JA
authors.lastname: Siegel
authors.forename: Carol S
authors.initials: CS
authors.lastname: Marcus
authors.forename: Richard B
authors.initials: RB
authors.lastname: Sparks
mesh_terms: D004781:Environmental Exposure; D006801:Humans; D011829:Radiation Dosage; D011835:Radiation Protection; D011874:Radiometry / Q000379:methods; D011878:Radiotherapy