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Dosimetry of Small Fields
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Measurement of small photon beams is complicated by
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loss of lateral electronic equilibrium,
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volume averaging,
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detector-interface artifacts,
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collimator effects,
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and detector position-orientation effects
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Recommendation: use an appropriate dosimeter with a spatial resolution of
approximately 1 mm or better (stereotactic detectors)
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Collimator with a diameter of 5 mm => dose falloff over a radius of 2.5 mm
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Thickness of 1 euro coin is 2.3 mm!