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Dosimetric characteristics and a standard for the 198 Au seed used in interstitial brachytherapy
Author(s) -
Dauffy Lucile S.
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 - 10.1118/1.1644511
Subject(s) - brachytherapy , kerma , dosimetry , thermoluminescent dosimeter , ionization chamber , imaging phantom , dosimeter , nuclear medicine , thermoluminescent dosimetry , monte carlo method , materials science , physics , medical physics , medicine , ionization , radiation therapy , mathematics , radiology , ion , statistics , quantum mechanics
The American Association of Physicists in Medicine (AAPM) Task Group 43 report (TG‐43) provides a dosimetry protocol for brachytherapy dose calculations, and results for some sources. Gold 198 seeds( T1 / 2= 2.70 days, E γ = 412 keV ) have been used for brachytherapy treatments including brain, eye, and prostate tumors. However, the National Institute of Standards and Technology (NIST) does not have a standard for198 Au seeds as for103 Pd and125 I seeds, and the TG‐43 report gives no data for198 Au . For these reasons and with the conversion of treatment planning systems to TG‐43 based methods, both Monte Carlo calculations ( MCNP 4C) and thermoluminescent dosimeters (TLD's) are used in this work to determine TG‐43 data. MCNP is also used to model a standard well chamber's response for a198 Au seed relative to both an192 Ir seed and a137 Cs seed. After activation, seed activity is determined using both a well chamber and a HPGe detector. The geometric variation in dose is measured using TLD's in a solid water phantom. The results for air kerma strength, S k , per unit apparent activity, are 2.06 ( MCNP ) and 2.09 (measured) U mCi − 1. The former is identical to the AAPM TG‐32 report results. The dose rate constant results, Λ, are 1.12 ( MCNP ) and 1.10 (measured) cGy h − 1 U − 1. The radial dose function, g ( r ) , anisotropy function, F ( r , θ ) , and anisotropy factor, φ an( r ) , are given. The anisotropy constant values are 0.973 ( MCNP ) and 0.994 (measured) and are consistent with both source geometry and photon energy. The calculated response of a Sun Nuclear ionization chamber, model 1008, demonstrates that to convert known192 Ir and137 Cs seed activities to “ 198 Au equivalent” activities, they should be multiplied by 1.95 and 1.49, respectively.