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

part of the work of the radiation safety officer (RSO), a job often assigned to the medical physics team. Therefore the content of this chapter is confined to a descrip- tion of sources, source types, and their characterization. The ba- sic theory of dose calculations is explained. It does not address the general issues of radiotherapy physics which are already covered in many other textbooks. However, the reader will find separate chapters in this Handbook on Radiation Protection in Brachytherapy (chapter 3), Brachytherapy Equipment and Qual- ity Assurance (chapter 4), Radiobiology (chapter 5), Modern Im- aging in Brachytherapy (chapter 6), Principles of Brachytherapy Planning Systems (chapter 7), Treatment Planning and Evalu- ation (chapter 8), and Reporting in Brachytherapy (chapter 9). Today, gamma-ray sources are the most commonly used sources in the treatment of malignancies. Beta radiation is used only in very specific types of treatments, and neutron emitting sources are rarely used. The most widely used radionuclide in brachyther- apy is iridium-192, in the form of the miniaturised pulsed-dose rate (PDR) and high-dose rate (HDR) stepping sources used in dedicated afterloading equipment. Iridium-192 has also been applied in many manual afterloading techniques in the form of thin wires which are cut to the desired lengths, and in ribbons containing small iridium seeds. Other radionuclides common- ly used in brachytherapy are iodine-125, palladium-103, and cesium-137. There are other gamma-ray emitting radionuclides which are discussed in this chapter for their specific properties, such as cobalt-60, ytterbium-169 and thulium-170. Eye lesions are sometimes treated with plaques that are covered with a layer of the β-emitter ruthenium-106. Specific sources containing the β-emitting radionuclides strontium-90 and yttrium-90 have been developed for use in endovascular brachytherapy, but in practice this technique was more or less abandoned several years ago (11, 37). The focus of this chapter will be on photon emitting sources. For further information on the production methods and phys- ical properties of sources that are currently in use or were used more extensively in the past, such as radium-226, gold-198, or on developments of other nuclides like americium-241 and samar- ium-145, the reader is referred to other publications, e.g. (2, 3). Note: in this physics and dosimetry chapter the radionuclides will be referred to by their name and atomic weight. In the clinical chapters of this Handbook, the atomic weight is usually omitted for reasons of brevity. A clarification will be given on the methods of calculation of dose deposition to a point in tissue at a given distance from a source. This chapter shows the mathematical background of the algorithms used in treatment planning systems. In addition, some aspects of 2-D and 3-D dose calculation other than dis- tance, such as oblique filtration and source anisotropy, are briefly discussed. Dose characteristics of a single point-source and of line-sources are shown. Brachytherapy is team work. Radiation oncologists, medical phys- icists and radiation technologists work jointly to obtain the best results for their patients. Education and training are crucial parts of each successful team, a statement that cannot be repeated too often as shown in several reports on incidents and accidents (22, 25). The clinical parts of this Handbook will probably be read and used most frequently by the radiation oncologists. As medical specialists, they are responsible for the treatment of their patients

and specifically for the correct geometric localisation of the appli- cator in order to treat the defined target volume adequately. Con- sistent applicator placement is crucially dependent on the skills of the radiation oncologist. The radiation oncologist is responsible overall for the whole procedure, in which it is primarily the phys- icist’s task to ensure that the treatment is delivered accurately and safely during all steps and in accordance with the radiation oncol- ogist’s prescription. The physicist must ensure that sources of cor- rect strength and type are accurately positioned in the applicators, and that the source positions are accurately reconstructed either from the 2D or –preferably- 3D reconstruction images together with the organ and tumour structures. An accurate and consistent treatment planning procedure is key. High-quality performance of the afterloading systemmust be guaranteed. The radiation tech- nologist is a co-ordinating specialist in the brachytherapy team, supporting the work of oncologists and physicists, with dedicated tasks that may differ from institute to institute. 3.1 Radioactivity Radioactivity is a phenomenon, discovered more than 100 years ago, in which ionising radiation is emitted by the nuclei of ra- dionuclides. This radiation can be in the form of particles, or electromagnetic radiation, or both. The process of radioactive decay or disintegration is a statistical phenomenon. The number of atoms that disintegrate per unit of time is proportional to the number of atoms in a given amount of source material. This rate of decay of a radioactive source is referred to as its activity. The SI (Système International) unit for activity is the bequerel (Bq), which equals to 1 disintegration per second. Another unit, the curie (Ci) has been in use for many decades and was defined by the activity contained in 1 g of the radionuclide radium-226. It is equal to 3.7 10 10 Bq. Consequently, 1 Bq equals 2.70 10 -11 Ci. Note: in practice, units as those discussed in these chapters can be altered by various factors of 10 through the use of appropriate prefixes. For instance, 10 -12 = pico (p), 10 9 = giga (G). So one can write: 1 Ci = 37 GBq or 1 MBq = 27 µCi. Radioactive decay occurs spontaneously, and there is no way to predict when each individual atom will disintegrate. However, on the average, one can state that in a given time frame, called the half-life , T 1/2 , half of the atoms will disintegrate. In the next half-life, one-half of the remaining atoms will decay, etc. Conse- quently, the number of atoms and the activity, A , of any radionu- clide vary exponentially over time as given in the formula: 3. RADIOACTIVE SOURCES

period) (time -e n)

A

A

(actual)

=

calibratio (at

•

(2.1)

1/2 0.693 T

=

In this equation the decay constant of the radionuclide λ is used (in units s -1 ). The decay formula can also be written as follows: