Comparing Dose Rates Near a Radioactive Patient Evaluated Using Various Source Models

Abstract

This study investigated radiation exposures from nuclear medicine patients by systematically comparing the dose rates calculated using various source models, ranging from simplified point, line, and cylinder sources to high-quality anthropomorphic phantoms. Three widely used radionuclides, (99m)Tc, (18)F, and I(131), were considered in these source models with uniform or organ-dependent distributions. Conducting Monte Carlo simulations with anthropomorphic phantoms is a realistic but time-consuming approach. The point source model is simple but too conservative, overestimating dose rates by approximately a factor of 2 at a distance of 30 cm and by 30-40% at 1 m. Both the line and cylinder source models provided improved estimates, reducing the overestimation of dose rates to 10-20% at distances of interest. The line source model was comparable to the cylinder source model because of the offset of two competing effects (i.e., attenuation and buildup) caused by the source volume. The influence of various photon energies and cylinder sizes on the result of compensating errors was examined to evaluate the effective range of the line source model. The line source model, which is relatively easy to implement and predicts slightly conservative dose rates, is considered the most practical method for calculating dose rates near radioactive patients. An application of the line source model to 51 post-thyroidectomy patients in Taiwan was demonstrated. The consistency between calculations and measurements was satisfactory after considering the room-scattering effect.

The Nuclear Medicine Patient as a Line Source: The Source Length Is Certainly Not the Patient Height, But It Is a Reasonable Approximation

“…The biokinetic parameters describing the whole-body retention (Liu et al 2014) are also of importance and should be chosen with care. In Liu et al (2015b), mathematical phantoms and a static activity distribution have been used to calculate dose rate at 30 and 100 cm by Monte Carlo simulations. It was concluded that the line source model is a good approximation for 99m Tc, 18 F, and 131 I used for remnant ablations.…”

“…It should be acknowledged that despite this, the approach is usually accepted (Matheoud et al 2004; De Carvalho et al 2011; D'Alessio et al 2015; Liu et al 2015b). The experimental evidences for such an approach are limited to a few studies (Willegaignon et al 2007; Yi et al 2013; D'Alessio et al 2015) and limited to a small number of short distances or a single distance (Liu 2015b).…”

“…To compute the exposure rate correctly, particularly for close contact distances, it has been suggested to consider the patient as a line source (Castronovo et al 1982; Siegel et al 2002). It should be acknowledged that despite this, the approach is usually accepted (Matheoud et al 2004; De Carvalho et al 2011; D'Alessio et al 2015; Liu et al 2015b). The experimental evidences for such an approach are limited to a few studies (Willegaignon et al 2007; Yi et al 2013; D'Alessio et al 2015) and limited to a small number of short distances or a single distance (Liu 2015b).…”

“…Scaling factor : 100 to 10 cm Scaling factor : 100 to 30 cm Scaling factor : 100 to 200 cm Default with fitted parameters Default/Fitted Default with fitted parameters Default/Fitted Default with fitted parameters Default/whole-body retention(Liu et al 2014) are also of importance and should be chosen with care. InLiu et al (2015b), mathematical phantoms and a static activity distribution have been used to calculate dose rate at 30 and 100 cm by Monte Carlo simulations. It was concluded that the line source model is a good approximation for 99m Tc, 18 F, and 131 I used for remnant ablations.…”