Monte Carlo modeling of radiation dose distributions in intravascular radiation therapy.

Radiation dose distributions are developed for balloon and wire sources of radioactivity within coronary arteries. The Monte Carlo codes MCNP 4B and EGS4 were used to calculate dose distributions for photons and electrons at discrete energies around such sources, with and without the presence of a high-density atherosclerotic plaque. An interactive computer program was developed which then calculates dose distributions for many radionuclides by applying the emission spectra to the discrete energy grids calculated by the Monte Carlo codes, weighting appropriately for electron energy and abundance. Results for Re-186 and Re-188 balloon sources are shown in comparison to an Ir-192 wire source. The program provides dose distributions as well as estimates of activity levels needed to deliver prescribed doses to the vessel wall at selected distances from the lumen in a selected time interval. In addition, dose calculations are presented in this paper for other organs in the body, from photon radiation as well as from possible loss of liquid activity into the bloodstream in the case of a balloon rupture. These results, especially the interactive computer program permitting easy comparison of various radionuclides and their physical characteristics, will greatly facilitate the comparison process and aid in the selection of the best candidate(s) for clinical use.

Re-evaluation of absorbed fractions for photons and electrons in spheres of various sizes.

UNLABELLED: Absorbed fractions for unit density spheres in an infinite unit density medium, previously calculated for photon emitters and electron emitters, were reevaluated with the Monte Carlo codes EGS4 and MCNP4B. METHODS: Activity was assumed to be distributed uniformly throughout the spheres, and absorbed fractions for self-irradiation were calculated at discrete photon and electron energies. RESULTS: For electrons, the codes were in very good agreement with each other (+/-5%) and with published values, except at higher energies in the very smallest spheres, where some differences exceeded 10%. For photons, the codes were again in good agreement with each other but produced results that varied considerably from published MIRD values. For energies <1 MeV and sphere sizes <50 g, the absorbed fractions determined using the Monte Carlo codes were typically 20%-40% higher than values in MIRD 3 and 8. For energies >1 MeV, the Monte Carlo values were sometimes lower than those in the MIRD documents. Recommended values, generally the average results from the 2 Monte Carlo codes, are given for all sphere sizes and energies for both electrons and photons. CONCLUSION: The absorbed fractions calculated using the Monte Carlo codes should replace the older values and are helpful in evaluating tumor doses, doses to small organs, and other situations in which a uniform distribution of activity throughout a spherical structure of unit density can be assumed.