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Calibration of 1 9 2 Ir high‐dose‐rate afterloading systems
Author(s) -
Goetsch Steven J.,
Attix F. H.,
Pearson D. W.,
Thomadsen B. R.
Publication year - 1991
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 - 10.1118/1.596649
Subject(s) - calibration , dosimetry , ionization chamber , brachytherapy , physics , ionization , inverse square law , kerma , radioactive source , dose rate , nuclear medicine , optics , medical physics , ion , radiation therapy , detector , medicine , classical mechanics , quantum mechanics , gravitation
A method is described for calibration of 1 9 2 Ir high‐dose‐rate (HDR) brachytherapy afterloading systems. Since NIST does not offer calibration of ionization chambers with the gamma‐ray spectrum of iridium‐192, an interpolation procedure is employed, using calibrations above ( 1 3 7 Cs, 662 keV) and below (250 kVcp, 146‐keV x rays) the exposure‐weighted average 1 9 2 Ir energy of 397 keV. The same total wall+cap thickness must be used for both calibrations, and for the 1 9 2 Ir measurements. A wall+cap thickness of 0.3 g/cm 2 is recommended to assure charged particle equilibrium and to exclude secondary electrons emitted from the source encapsulation. Procedures are described for determining the corrections for source–chamber distance and room scatter during the source calibration in inverse‐square‐law geometry. A new well‐type ionization chamber has been designed specifically for convenient routine use with the HDR afterloading system. It can be calibrated by means of a previously calibrated 1 9 2 Ir source, and offers a simple means for verifying the decay rate and for calibrating 1 9 2 Ir replacement sources.