Transcription of Chapter 6 EXTERNAL PHOTON BEAMS: PHYSICAL ASPECTS
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Chapter 6 EXTERNAL PHOTON BEAMS: PHYSICAL ASPECTS
161 Chapter 6 EXTERNAL PHOTON BEAMS: PHYSICAL PODGORSAKD epartment of Medical Physics,McGill University Health Centre,Montreal, Quebec, INTRODUCTIONR adiotherapy procedures fall into two main categories: EXTERNAL beam radiotherapy and brachytherapy. In EXTERNAL beam radiotherapy the radiation source is at a certain distance from the patient and the target within the patient is irradiated with an EXTERNAL radiation beam. In brachytherapy (see Chapter 13) radiation sources are placed directly into the target volume (intracavitary or interstitial brachytherapy) or on to a target (surface mould or intraoperative radiotherapy). Most EXTERNAL beam radiotherapy is carried out with PHOTON beams, some with electron beams and a very small fraction with more exotic particles such as protons, heavier ions or Chapter deals with EXTERNAL PHOTON beam radiotherapy. PHOTON EXTERNAL beams are all characterized by the same PHYSICAL parameters, but fall into various categories depending on their origin, means of production and energy.
CHAPTER 6 166 medium, k(r med) ª 0.985 for 60Co photons and approximately 1 for lower photon energies. Step 5: ‘Dose to small mass of medium in free space’ D′med is obtained from the following relationship: (6.19) where b is a proportionality constant equal to 1.003, 1.001 and 1.0 for
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