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

age from the vendor value by 0.3% (10). Measurements with the well chambers were 0.5% ± 0.1% higher than the vendor-pro- vided source strength. Measurements with the Farmer chamber were 0.7% lower than the average well chamber results and 0.2% lower than the vendor-provided source strength. The compar- isons were made using equipment with traceability to different standards from the laboratories. 4.3 Quality control of sources Quality management is a responsibility of the medical physicist and/or the RSO in the radiation therapy department. A detailed description of radiation safety issues and a general overview of quality assurance and quality control procedures regard- ing sources is given in chapter 3 on Radiation Protection in Brachytherapy. A very comprehensive overview of quality man- agement in brachytherapy is given in the book ‘Achieving Quali- ty in Brachytherapy’ by Thomadsen (53). With regard to brachytherapy sources, in this paragraph only the following important topics that must be addressed are listed: • The necessity to have an accurate logbook of all available sourc- es, their identification numbers, their strengths, and storage position. • The necessity to perform leakage testing using a wipe test pro- cedure, according to the requirements of the licence of the de- partment with regard to the minimum frequency. • The necessity to perform inspections on a regular basis of safe- ty measures, the loggings, the vaults, and the (written) proce- dures. • The necessity to perform a source strength verification prior to clinical use of a brachytherapy source, as described in the previous section. • The necessity to perform a check of the uniformity of the ac- tivity distribution, e.g. the uniformity of linear sources, source needles, ribbons, or the uniformity of source strength in a batch of sources. • The necessity to –if possible- inspect the source integrity before clinical use. • The necessity to validate the dosimetric parameters of the sources that are used in the dose calculations. It is noted that high-dose rate sources constitute a high risk of ra- diation exposure. Therefore direct wipe testing and visual inspec- tion are not recommended for these sources. Often checks are per- formed on the applicators or appliances such as transfer tubes to inspect if any radioactive contamination from a leaking source is present on these materials.

Fig. 2.6 A typical in-air calibration jig suitable for calibrating an HDR or PDR 192 Ir source to be positioned 10 cm left and right from a centrally placed Farmer type ionisation chamber with a build-up cap. A small plastic tube is used in this jig to keep the catheters exactly 20 cm apart. Readings with the source in the left and right catheter are averaged to correct for positional inaccuracies of the tubes. (Courtesy: J. Venselaar)

Fig. 2.7 Example of a well-type chamber with electrometer, a system nowadays generally recom- mended in societal reports for use in the clinic. (Courtesy: J.Venselaar)

A source calibration accuracy of ≈3% relative to existing air ker- ma standards is recommended. If the institution’s verification of source strength disagrees with the manufacturer’s data by more than 3%, the cause of the disagreement should be investigated. It should be noted that the recommended 3% tolerance between manufacturer and institution calibrations discussed above ap- plies to the mean of a batch of sources . Since individual sources may differ from the mean by a greater amount, a maximum de- viation from the mean of 5% is acceptable for individual sources in such a batch (6). Although traceability to primary standards for HDR iridium-192 sources is still not readily available for all users world-wide, those standards that have been developed seem to be at an acceptable level. In a paper on a comparison of HDR source strength meas- urements for these sources using equipment with traceability to different standards, Carlsson Tedgren et al . showed that the measured source strengths varied by 0.8% and differed on aver-

5. DOSE CALCULATION IN BRACHYTHERAPY

A full description of what is called here the conventional meth- odology for the calculation of the absorbed dose around a brachytherapy source can be found in many other text books. In present-day practice, however, this conventional method has been abandoned in modern treatment planning systems as it has been replaced by another formalism. This new formalism was published in a report of AAPM Task Group 43 , generally indicat- ed as the TG-43 formalism (35). This report was updated in 2004 (44), supplemented in 2007 (45), and has become a standard practice in computerised dose planning systems. Nevertheless,