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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!