1. Dosimetric leaf gap (DLG): Dosimetric consequences
Lars H. Præstegaard, Mai-Britt K. Jørgensen
and Lone Hoffmann
Aarhus University Hospital
January 2012

2. Dosimetric leaf gap (DLG)
Increase of field size due to transmission trough leaf end
Field size (mm) = MLC field size (mm) + DLG (mm)

3. Direct measurement of DLG
MLC transmission (function of W)
Aarhus:
Adjustment of DLS in Eclipse using chair plan
T. LoSasso et. al., Med. Phys. 28, 2209–

4. Constancy check of DLG in Aarhus
Sweeping gap test:

5. 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
0.782 2
0.772 3 4
0.795 5
0.806 6
0.790 7
0.791 8
0.797 9
0.803 10
Max. range of the sweeping gap ratios in Aarhus:

6. Dosimetric effect of different DLG
Accelerator 3
Mean dose relative to SGR=0.773
 Sweeping gap ratio (SGR)
0.826
0.808
0.773
System Leaf Gap Offset [mm]
0.6
0.5
Plan
Site
Dose -#Fractions
Modulation
#Arcs
Comments 1 HH
IMRT
68/66/60/50-34
892 MU 
1.9
1.5
Typical IMRT plan 2
prost + LN RA
74/55-37
500MU
0.8
Standard modulation
1000MU
High modulation 3
0.9
1.6 4
tonsil
68/60/50-34
1.1
2.8
2.2 5
1.7
1.3
3.1
2.6
Analysis:
Mean prostate, 500 MU 
1.0
0.0
Mean prostate, 1000 MU 
2.0
Mean tonsil, 500 MU 
1.2
Mean tonsil, 1000 MU 
3.0
2.4

7. 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 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)

8. 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 = (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)

9. 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?

10. 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?