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Treatment planning dosimetric parameters for 192 Ir seed at short distances: Effects of air channels and neighboring seeds based on Monte Carlo study
Author(s) -
Patel Neil S.,
ChiuTsao SouTung,
Shih J. Allen,
Ho Yunsil,
Tsao HungSheng,
Harrison Louis B.
Publication year - 2004
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.1753432
Subject(s) - monte carlo method , dosimetry , brachytherapy , computational physics , physics , anisotropy , optics , nuclear medicine , mathematics , statistics , medicine , radiation therapy
The dose distributions around two different arrangements of a single radioactive192 Ir seed in water, (1) with air channels at the ends, and (2) surrounded by two nonactive (“dummy”) seeds on both longitudinal ends, were calculated using MCNP4C Monte Carlo simulations at distances up to 1 cm. The contributions from beta particles and electrons emitted by192 Ir were included in the calculations. The effects of (a) the air channels at the seed ends and (b) the interference effect of the dummy seeds on the dose distribution were quantified and compared. It was found that the dummy seeds do not cause significant dose reduction for radial distances beyond 0.05 cm from the seed center. It is decided to report the dose rate values and the dosimetric parameters in TG43 format for a single seed with air channels for use in treatment planning computer systems. The dose rate constant (at 1 cm) of192 Ir seed, Λ, is 1.108 cGy U − 1 h − 1 . The values of radial dose function, g ( r ) , are within 1% from the TG43 recommended polynomial fit, except for distances within 0.08 cm. The anisotropy function, F ( r , θ ) , attains large values near the seed ends and shallow angles (up to 8), as well as many values greater than 2 at the 20 ° polar angle. Treatment planning systems involving intravascular brachytherapy do not compromise the accuracy for dosimetry of multiple seed trains by summing single seed values in water.