Transcription of Chapter 6 EXTERNAL PHOTON BEAMS: PHYSICAL ASPECTS
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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).
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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