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