2 Brachytherapy Physics-Sources and Dosimetry

Brachytherapy Physics: Sources and Dosimetry

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THE GEC ESTRO HANDBOOK OF BRACHYTHERAPY | Part I: The basics of Brachytherapy Version 1 - 01/12/2014

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Fig. 2.10 Radial dose function values, g(r) , for seven cesium-137 low-dose rate sources plotted against radial distance, r , (a) for the full range of r up to 20 cm, and (b) zoomed in for the clinically relevant distances near to the sources, for r ≤ 2.5 cm (3).

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Fig. 2.11 Anisotropy function values, F(r,θ) , for four iodine-125 sources. F(r,θ) polar angle profiles are shown for 2 radial distances, (a) for r = 0.5 cm and (b) for r = 1.0 cm (3).

5.4 Examples of one- and two-dimensional dose distributions One dimensional dose profiles, radial dose functions A radial dose profile is a one-dimensional representation of the dose as a function of distance along a given trajectory. Dose pro- files can be calculated with a treatment planning system to show high and low values of dose near a source or within an implant. This section is confined to a few examples for a point source and single line source geometry. The influence of the absorption and scatter correction, expressed in the radial dose function (TG-43; to be compared with the Meisberger correction in the conventional formalism), is shown for a cesium-137 point source in Fig. 2.12. This graph demon- strates that the dose profiles near this source have gradients that are predominantly determined by the inverse square law, while the influence from scatter and absorption effects is only approx. 5% at a distance of 5 cm.

basic dosimetric practice standards. It is then the responsi- bility of the user to perform an analysis to ensure that seed calibrations are traceable with an acceptable level of accuracy and that available published and unpublished dosimetry data exhibits an acceptable level of rigor and redundancy to safely treat patients. Early adopters of a new seed product should consider working closely with the vendor, the vendor’s do- simetry consultant, and the treatment planning software vendor to ensure that the conditions outlined in the relevant AAPM recommendations are met. If source data are availa- ble, these datasets can be found on the Websites of ESTRO or Carleton University. See: (7, 18). Data provided by the vendor for the given source must always be validated by the user before clinical use!