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

decrease of strength during treatment time. A simple approxi- mation to the calculation of total dose of an implant is to use the dose rate value half-way through the planned application. Note: the activity of all sources at the end of 7 half-lives is less than 1%. Ten half-lives will reduce the source activity by about 10 -3 . This is a simple rule of thumb which is often used in calculations of remaining activity, e.g. for dealing with radioactive waste disposal. Sometimes sources are permanently implanted into patients, such as iodine-125 seeds in prostate treatment. It is then nec- essary to know the total number of disintegrations that occur in an infinite period of time. A radioactive source that decays expo- nentially over an infinite time with a half-life T 1/2 is equivalent to a virtual source that radiates at a constant rate (thus, equal to its initial rate) for a given time period, T a . T a is called the “average life” of the radionuclide and equals. The factor 1.44 (or: 1 / 0.693) derives from a simple mathematical operation ( integration over time ), and is universally applicable. As a result, the average life T a of iodine-125, commonly used for permanent implants of the prostate, is about 86 days (59.49 / 0.693 = 85.8 d). The total dose of an implant is equal to the prod- uct of the initial dose rate and T a . Many radionuclides undergo successive transformations in which the original parent nuclide gives rise to a daughter radio- nuclide (3, 16). If the half-life of the parent is longer than that of the daughter, then after a certain period of time a condition of equilibrium will be achieved. This means that the ratio of daugh- ter activity to parent activity will become constant. The apparent decay rate of the daughter nuclide is then governed by the half- life of the parent. Two examples were given in Table 2.2, parent strontium-90 together with daughter yttrium-90 and parent ru- thenium-106 together with daughter rhodium-106. 3.2 Decay schemes of radionuclides The choice of a radionuclide to be applied in brachytherapy sys- tems depends on a number of considerations and on the physical properties. The radiation emitted by the radionuclide determines how a source can or should be constructed and is therefore im- portant for the possible source geometry and structure. The half- life, T ½ , determines if a source can be used in permanent or tem- porary implants or in both types. The specific activity defines the possible source size and dose rate. The energy of emitted radia- tion influences the dose distribution within tissue, and also im- poses the measures to be taken for radiation protection. Density and atomic number of the radionuclide are important for radio- graphic visibility and source localisation, and influence the (an) isotropy of the resulting dose distribution around the source. The radiation emitted by a radionuclide can be clarified with a graphical representation of the energy levels of the moth- er-daughter nuclides. Fig. 2.1 and Fig. 2.2 present two such de- cay schemes of commonly used radionuclides, iodine-125 and cesium-137. The schemes of these two nuclides are relatively simple, compared to a very complex decay scheme of the irid- T a = T 1/2 0.6 93 = 1.44 T 1/2 (2.3)

Fig. 2.1 Schematic representation of electron capture decay of iodine-125 to the first excited state of tellurium-125. The disintegration energy for the decay is Q EC = 0.1858 MeV. There is a single γ-ray of 0.035 MeV with an intensity of 6.68% emitted, whereas there are several characteristic x-rays emitted in the range 0.027 to 0.032 MeV as a result of internal conversion processes. The average number of photons emitted per disintegration with energy above 10 keV is 1.4. The half- life for iodine-125 decay is 59.49 days (3). (Courtesy: D. Baltas)

Fig. 2.2 Schematic representation of β- decay of cesium-137 which decays mainly (94.4%) to the second excited state of barium-137. The disintegration energy for the decay is Q β - = 1.1756 MeV. In practice a single γ-ray of 0.6617 MeV is emitted with an absolute intensity of 85.1%. The aver- age number of photons emitted per disintegration with energy above 10 keV is 0.9. The half-life for cesium-137 decay is 30.07 years (3). (Courtesy: D. Baltas)

ium-192 nuclide which has a multitude of photon energies in its spectrum. The iridium-192 scheme is not presented in this publication. Decay schemes, emission spectra, production methods and in- formation on source construction of other widely used radio- nuclides and sources can be found in other textbooks, such as in chapter 5 of Baltas et al. (3) or Ballester et al. (2). 3.3 Types of sources In brachytherapy different types of sealed or encapsulated sourc- es are used. They all contain a certain amount of a radionuclide that is encapsulated in layers of a metal such as platinum, titani- um, or stainless steel, or in some kind of thin foil in the case of ß-emitters. Various types of sources such as tubes, needles, wires, pellets, seeds, and a single stepping source connected to a cable, are available (see Fig. 2.3 and Fig. 2.4). The definition of the length of a source should not lead to confu- sion among the members of the brachytherapy team. In order to