Dosimetric characteristics of a linear array of γ or β‐emitting seeds in intravascular irradiation: Monte Carlo studies for the AAPM TG‐43/60 formalism

In principle, the AAPM TG‐43/60 formalism for intravascular brachytherapy (IVBT) dosimetry of catheter‐based sources is fully valid with a single seed of cylindrical symmetry and in the region comparable to or larger than the mean‐free path of emitting radiation. However, for the geometry of a linear array of seeds within the few millimeter range of interest in IVBT, the suitability of the AAPM TG‐43/60 formalism has not been fully addressed yet. We have meticulously investigated the dosimetric characteristics of catheter‐basedγ   ( 192 Ir ) andβ   ( 90Sr / Y ) sources using Monte Carlo methods before applying the AAPM TG‐43/60 formalism. The dosimetric perturbation due to radiation interactions with neighboring seeds is at most 2% over the entire region of interest for the192 Ir source, while it increases to about 5% for the90Sr / Y source. As the transaxial distance ( y ) increases beyond 3 mm, the sum of the dose contributions from neighboring seeds exceeds the dose contribution from the center seed for both sources. However, it continues to increase with the increasing y for192 Ir but is saturated beyond y = 5   mm for90Sr / Y . Even within a few millimeters from the seeds, the dose from the low‐energy betas of192 Ir is still less than 1% of the total dose. The radial dose and anisotropy functions are reformulated in reduced cylindrical coordinate with the reference point at y = 2   mm . The dose rate constant of192 Ir and the dose rate of90Sr / Y at the reference point showed a fairly good agreement ( within   ± 2 % ) with earlier studies and the NIST‐traceable value, respectively. We conclude that the dosimetric perturbation caused by close proximity of neighboring seeds is nearly negligible so that the AAPM TG‐43/60 formalism can be applied to a linear array of seeds.