An experimental and calculated dose distribution in water around CDC K-type caesium-137 sources

Abstract

The radiation distribution in water around caesium-137 K-type sources has been measured and the experimental results used to provide data for an expression for dose calculations which may be conveniently applied in computer programs. The calculated absorbed dose rate obtained in this manner is estimated to be within 3% of the actual dose rate for any point in water up to 8 cm from the source. It is also suggested that the strength of a brachytherapy source be expressed in terms of an experimental exposure rate at some well-defined distance since this quantity may be determined more precisely and with less ambiguity than source activity.

Code of practice for brachytherapy physics: Report of the AAPM Radiation Therapy Committee Task Group No. 56

“…For 137 Cs, the Sievert model has been shown to model accurately ͑within 5%͒ single source dose distributions when tested against dose measurements and Monte Carlo calculations. 26,27 Pd. TG-43 provides a set of data for some designs of these sources ͑except gold͒.…”

“…After validating the algorithm's accuracy and implementing it for the desired source type, the dose distribution should be checked against manual calculations at several points. Such checks can be performed using the general Sievert integral table 35 or by means of the unfiltered line source formula 26 in the case of lightly filtered sources.…”

“…For 137 Cs, the Sievert model has been shown to model accurately ͑within 5%͒ single source dose distributions when tested against dose measurements and Monte Carlo calculations. 26,27 This report recommends that readers proceed cautiously in applying the Sievert model to lower-energy sources, including 192 Ir wires and seeds. Although the Sievert model accurately models the dose rate distribution near the transverse axis of 192 Ir, errors in reconstructing the dose distribution near the longitudinal axis ͑where oblique filtration effects are important͒ as large as 20-40% have been reported.…”

Monte Carlo calculation of dose rate distributions around wires

“…TG43 recommends treating the attenuation coefficients as free parameters of best fit chosen to minimize the deviations from Sievert model against the results obtained by the most adequate method. Diffey et al 23 and Williamson 4 have successfully used this approach to ''fit'' the Sievert model to measured and Monte Carlo generated dose distributions for 137 Cs intracavity sources. As indicated by TG43, the Sievert model probably cannot accurately describe sources for which the scatter-to-total-dose ratio varies significantly with distance, because the effect of filtration is assumed to be independent of distance.…”

Determination of 137Cs dosimetry parameters according to the AAPM TG-43 formalism

“…The 137 Cs intracavitary sources are cylindrical sources with a typical physical length of 2 cm. Dose distributions around various 137 Cs sources have been calculated and measured by various investigators [1][2][3][4][5][6][7] using techniques ranging from numerical integration of point source dose distributions ͑i.e., Sievert-type integrals͒, and Monte Carlo simulations, to dose measurements using ion chamber, diode, and film. Published dosimetry data for these sources are in the form of dose rate tables on a Cartesian grid along the source axis and the transverse axis away from the source center.…”