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Brachytherapy

Introduction

Dosimetry in brachytherapy involves firstly, knowledge of the strength of the source (usually expressed as air-kerma rate at a standard distance from the source) and secondly, calculation of dose to tissue in proximity to the source when it is placed in the treatment position.

Important Principles

For hospitals, it is recommended that the source strength of all brachytherapy sources should be measured on receipt with a local dosimeter and compared with the manufacturer’s certificate of source strength. This is typically performed with either a Farmer-type chamber or a well-type chamber. ESTRO Booklet no. 8 provides a formalism for determination of air-kerma rate with both types of ionization chamber. Calibration of the hospital’s ionization chamber in terms of air-kerma is performed by an SSDL or PSDL at relevant energies.

Dose calculation formalisms have been developed for brachytherapy such as the AAPM TG 43 report. In that report, the starting point is the air-kerma strength of the source and various factors are introduced to calculate the dose to water surrounding the source. Those factors include the geometry, radial dose and anisotropy functions, as well as the dose rate constant in water. These factors are tabulated and disseminated for various brachy­­therapy sources. It is noted that dosimetry in brachytherapy is complicated by high dose gradients and the influence of source geometry on the dose distribution.

Introduction to References 

ESTRO Booklet no. 8 provides comprehensive advice on brachytherapy dosimetry. AAPM TG 43 and its updates provide a formalism for dose calculation, whilst the ESTRO database provides a link to TG-43 brachytherapy dosimetry parameters for relevant brachytherapy seeds/sources. The IAEA TECDOC 1274 gives guidelines on calibration of photon and beta ray sources used in brachytherapy for both SSDLs and hospitals.