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• The signal from the monitor chamber can be affected from particles which have
backscattered from the upper part of the jaws into the chamber (dependent on linac head
and monitor chamber design, )
Backscatter into the monitor chamber
• At narrower collimations there is more backscatter than in larger fields
⇒
monitor
chamber reaches faster its pre-set value
⇒
linac relative output decreases with
decreasing field size
Verhaegen & Seuntjens, Phys. Med. Biol, 48 (2003) R107-R164