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

2 Brachytherapy Physics: Sources and Dosimetry Jack Venselaar, Dimos Baltas

1. Summary 2. Introduction

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5. Dose calculation in brachytherapy

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6. Key messages 7. References

3. Radioactive sources 4. Source dosimetry

1. SUMMARY

should be available with traceability to a primary standard.

to know the strength of the sources. Dose calculation algorithms were developed for clinical use from very simple calculation meth- ods, via parameter based approaches, to complex, so-called mod- el-based dose calculation algorithms (MBDCA’s, including Monte Carlo techniques). To be able to perform such calculations for real clinical cases, the target definition needs to be determined. So, im- aging techniques are employed to reconstruct the source positions, target and treatment volumes, organs at risk, and prescription points and volumes using different types of imaging modalities. There must be a clear understanding of the volume definitions so that the strength of brachytherapy (a high dose fall-off due to the inverse square law, and therefore high dose conformity to the tar- get volume) can be used to optimize the individual treatment plan in terms of highly conformal coverage of the target and limitation of surrounding tissues. Treatment delivery systems are commer- cially available to treat our patients, but these systems have to be commissioned, sources need to be replaced on a regular basis, and servicing must be organized. This part falls within the scope of a departmental quality management system for which the medical physicist is usually responsible. Radioprotection is an important Calculation of the absorbed dose to the target is a key ele- ment in the process of clinical treatment preparation. In this chapter, two methods for the calculation of dose to a point in water at a given distance from a source are explained: the conventional approach and the methodology described in the AAPM Task Group 43. The latter has become a standard of practice in all present-day brachytherapy treatment plan- ning systems. References to the open literature and practical webbased databases are provided so that the reader can find reliable data for the validation of the TG-43 calculated dose of a brachytherapy treatment planning system. Some examples are shown of one- and two-dimensional dose distributions to demonstrate typical dependence of the dose deposition in relation to the energy of the emitted photons and absorption phenomena in the source walls, such as anisotropy. However, these two methodologies of dose calculation have their limitations such as their inability to account for shielding and inhomogeneities with, for example, lack of scattered radi- ation. New solutions are being developed to avoid such draw- backs. The last section of the chapter therefore discusses the present status of so-called Model Based Dose Calculation Al- gorithms (MBDCAs) and their possible impact on dosimetry.

2. INTRODUCTION The physics of radiotherapy, and specifically the physics of brachytherapy covers a large number of topics that are all strongly interrelated. It deals with the understanding of the phenomenon of radioactivity and radioactive decay. Interaction of the emitted ionizing radiation with matter -with the different processes of ab- sorption and scatter effects: the photoelectric effect, compton ef- fect, and pair production at high energies- is essentially the same in brachytherapy as in external beam therapy and will therefore only be touched on in this chapter. Factors that influence the tissue re- sponse to the absorption of radiation, with variations in dose dep- osition over time, fraction size and interfraction intervals, is the realm of radiobiology. What remain as important topics to discuss here start with the characterization of the radioactive material and the physical sources used in brachytherapy: the different source types and the influence of the encapsulation of the sealed sources. Then, for accurate dosimetry, i.e. predictive dose planning using dedicated brachytherapy treatment planning computers, we need This chapter deals with the first steps in understanding of brachytherapy physics in this rapidly evolving field of treat- ment with ionizing radiation. The chapter starts with an ex- planation of the physical phenomena of radioactive decay and the properties of radionuclides that are (or have been) widely used for the superficial, interstitial, intracavitary, and endolu- minal treatment of tumours. The properties of these nuclides are discussed in relation to their relevance for certain types of clinical application. Drawings of real sources are shown to demonstrate the wide variation of commercial source designs that have been developed as solutions for clinical demands. Treatment duration is a critical factor for the comfort of the patient and the potential workload of an afterloader. It is re- lated to the prescription dose to the target. It needs to be de- termined with high accuracy as the accuracy of dose delivery is directly dependent on knowledge of the time of treatment. The quantity source strength therefore needs to be clearly defined according to national or international recommenda- tions. Methods for determining this strength and performing in-house calibration or validation of newly acquired sources is discussed. Awell-type chamber is usually the preferred type of equipment for this purpose, for which a calibration factor