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

3.4 Clinical application in brachytherapy Although the radionuclide radium-226 is very well-known, it was abandoned for clinical use many years ago because of its long half life and the associated environmental risks. Another reason is that the daughter product radon-222 in its gaseous form can escape from damaged sources, leading to severe risks of radioactive contamination. In the first instance, the radionu- clide cesium-137 was used as a replacement for radium-226, and cesium-137-containing sources were constructed in such a way that a very similar dose distribution was obtained around the sources. These sources were commonly available in the form of needles and tubes. Cesium-137 material is readily available from nuclear power reactor waste products. Both radium-226 and cesium-137 decay with relatively high energy photons, as does cobalt-60, which is apparent from the higher values of the half value layer ( HVL , the thickness of a radiation protection shield or barrier that reduces the exposure with a factor 2; values for lead are shown in Table 2.1) of these radionuclides. Therefore, protective walls and -devices need to be thicker for these sources and are more expensive than with most other radionuclides. Iridium-192 is produced in nuclear reactors and has been the most popular radionuclide in brachytherapy since the nine- teen-sixties. It combines a relatively high mean photon energy with an HVL which is considerably smaller than, e.g., with cesi- um-137 or cobalt-60. Therefore, less shielding thickness in lead barriers or concrete walls is required, thus reducing facility costs. The main advantage of the iridium-192 in radioactive sources, however, is the very high specific activity . The specific activity is defined as the maximum activity of a radionuclide that can be contained in 1 g (or 1 mg) of the material. These values are shown for the sources listed in Table 2.1. A very small amount of pure iridium material can still have a very high yield of photons, which means that the source can be small. This property allows the construction of thin iridium wires of only 0.3 mm diameter for manual brachytherapy applications, or miniaturized highly activated sources for use in HDR or PDR afterloaders. Source guiding tubes or needles can have an outer dimension of 1.9 mm or less. The half-life of iridium-192 is close to 74 days, requiring sources to be exchanged at intervals of 3-4 months.

speak the same language, three different lengths of a source are defined: the physical length, PL , the active length, AL , and the equivalent active length, EL , as shown in the Fig. 2.3. These sources can usually be delivered in a wide variety of source strengths. In addition, designs of four seed type sources developed specif- ically for use in prostate permanent implant brachytherapy are shown here with their construction details in Fig. 2.4. These four examples form a small selection of the many seeds, each with their own details that, have been proposed to the market by a multitude of vendors. See for example in the ESTRO Web pages where dosimetry data useful for brachytherapy treatment plan- ning have been collected: http://www.estro.org/about/govern- ance-organisation/committees-activities/tg43 (18).

Fig. 2.3 Different types of sources used in brachytherapy. AL : active length, PL : physical length, EL : equivalent active length, s : spacing between the centres of seeds.

Fig. 2.4 In this graph iodine-125 seed sources for use in prostate implants from 4 different vendors are shown: the Oncura/GE-Healthcare (previously Amersham Health) model 6711, the Best model 2301, the Bebig/Theragenics model I25.S06, Elekta (previously Nucletron) I-125 SelectSeed. The seeds have a typical outer diameter of 0.8 mm and a length of 4.5 - 5.0 mm. Figures taken from (18).