Physics Athens 2016

IMRT – the small field challenge

Small fields - Inhomogeneities - electronic disequilibrium effects Fraction of scatter becomes significant for small fields Small no. of MU’s (step-and shoot !) STATUS: OK, proves the stable relationship between the dose monitor setting and the linac CAX output in static circumstances at 0° gantry angle. The exact curve fitting methodology should be included in the acceptance procedure. UZ Brussel 17/12/2009 v4.0

deviation from linear fit Dose-MU linearity

10 15 20

-5 0 5

VN Hansen et al,Phys. “ Quality assurance of the dose delivered by small radiation Segments” Med. Biol. 43 (1998) 2665–2675

1 500MU/min 400MU/min 200MU/min 100MU/min Figure 3: Deviation from linearity of the output in function of the amount of delivered MUʼs. 10 100 1000 deviation of absorbed dose [%] MU

Minimum number of MU at RMH is 3MU