Dosimetric leaf gap (DLG):
Dosimetric consequences
Lars H. Præstegaard, Mai-Britt K. Jørgensen
and Lone Hoffmann
Aarhus University Hospital
January 2012
Dosimetric leaf gap (DLG)
Dosimetric leaf gap:
Increase of field size due to transmission trough leaf end
Field size (mm) = MLC field size (mm) + DLG (mm)
Aarhus University Hospital, Århus Sygehus
Direct measurement of DLG
MLC transmission
(function of W)
T. LoSasso et. al., Med. Phys. 28, 2209–2219 2001.
Aarhus University Hospital, Århus Sygehus
Aarhus:
Adjustment of DLS in Eclipse using chair plan
Constancy check of DLG in Aarhus
Sweeping gap test:
10  dose of sw eepingfield (5 mm)
Sw eepinggap ratio (SGR) 
dose of open field
Aarhus University Hospital, Århus Sygehus
DLG for Varian Clinacs in Aarhus
• 10 Varian Clinac accelerators in Aarhus with MLC version 7.2
• Adjustment of DLG using MLC HyperTerminal:
– Command: diagAdjustSysOffsets
– Parameter: System Leaf Gap Offset
• Sweeping gap ratios (SGRs) for System Leaf Gap Offset = 0:
Accelerator
SGR
1
2
3
4
5
6
7
8
9
10
0.782
0.772
0.772
0.795
0.806
0.790
0.791
0.797
0.803
0.772
Aarhus University Hospital, Århus Sygehus
Max. range of the sweeping
gap ratios in Aarhus:
0.772-0.806
Dosimetric effect of different DLG
Mean dose relative to
SGR=0.773
Accelerator 3
Sweeping gap ratio (SGR)
0.826
0.808
0.773
System Leaf Gap Offset [mm]
0.6
0.5
0
Dose #Fractions
Modulation
IMRT
68/66/60/50-34
892 MU
prost + LN
RA
74/55-37
500MU
prost + LN
RA
74/55-37
prost + LN
RA
prost + LN
Plan
Site
1
HH
2
#Arc
s
Comments
1.9
1.5
0
Typical IMRT plan
2
1
0.8
0
Standard modulation
1000MU
2
2
1.5
0
High modulation
74/55-37
500MU
1
0.9
0.8
0
Standard modulation
RA
74/55-37
1000MU
1
2
1.6
0
High modulation
tonsil
RA
68/60/50-34
500MU
1
1.5
1.1
0
Standard modulation
tonsil
RA
68/60/50-34
1000MU
1
2.8
2.2
0
High modulation
tonsil
RA
68/60/50-34
500MU
2
1.7
1.3
0
Standard modulation
tonsil
RA
68/60/50-34
1000MU
2
3.1
2.6
0
High modulation
Mean prostate, 500 MU
1.0
0.8
0.0
Mean prostate, 1000 MU
2.0
1.6
0.0
Mean tonsil, 500 MU
1.6
1.2
0.0
Mean tonsil, 1000 MU
3.0
2.4
0.0
3
4
5
Analysis:
Aarhus University Hospital, Århus Sygehus
Dosimetric effect of changes of DLG
• Change of DLG: Largest effect on dose for highly
modulated test plans (opposing MLC leafs are closer during
treatment)
• Dose change is twice as large for 1000 MU in comparison to
500 MU (half the distance of opposing MLC leafs)
• Largest effect on dose (plan 4): 1.3% (500 MU) and 2.6 %
(1000 MU) for a change of the sweeping gap ratio from 0.773
to 0.808
• System Leaf Gap Offset=0: For a highly modulated plan the
dose will change up to 2.6 % if patient is moved from
accelerator 3 to 5 (systematic effect)
Aarhus University Hospital, Århus Sygehus
Adjustment of DLG in Aarhus
Clinical demand:
Free movement of patients between all accelerators for
effective handling of:
– Scheduled accelerator maintenance
– Accelerator errors

All Clinacs are adjusted for the same value of SGR (DLG):
SGR = 0.825 (System Leaf Gap Offset = -0.7 to -0.3)
MLC Version 7.2:
Rough scale for leaf gap offset (step of 0.1 mm)
 Spread of SGR (DLG) around 0.825
 Need for increased tolerance for SGR (0.010.015)
Aarhus University Hospital, Århus Sygehus
Stability of SGR (DLG)
System Leaf Gap Offset ≠ 0:
• Change of SGR during the day or following MLC initialization
• Rough scale for leaf gap offset
 SGR variation for specific accelerator: Up to 0.015
 Random dose error of up to 1.2 % (for 1000 MU)
System Leaf Gap Offset = 0:
Max. variation of sweeping gap ratio = 0.002
Result:
Better SGR (DLG) stability for System Leaf Gap Offset = 0
Need for same SGR (DLG) for all accelerator for System
Leaf Gap Offset = 0. How to do that?
Aarhus University Hospital, Århus Sygehus
Discussion
How do you measure DLG?
What is the range of DLGs for System Leaf Gap
Offset = 0 in your clinic?
Can the DLG for System Leaf Gap Offset = 0 be
adjusted?
How much modulation do you use for RapidArc
plans?
Do you think the dosimetric consequence of
different DLGs is significant?
Other comments?
Aarhus University Hospital, Århus Sygehus