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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.
-5
0
5
10
15
20
1
10
100
1000
deviation from linear fit
deviation of absorbed dose [%]
MU
500MU/min
400MU/min
200MU/min
100MU/min
Figure 3: Deviation from linearity of the output in function of the amount of delivered MUʼs.
UZ Brussel 17/12/2009 v4.0
Dose-MU linearity
Minimum number of MU at RMH is 3MU
VN Hansen et al,Phys. “
Quality assurance of the dose
delivered by small radiation Segments”
Med. Biol.
43 (1998) 2665–2675