Dosimetric leaf gap (DLG): Dosimetric consequences

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

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, 2209–2219 2001.

Constancy check of DLG in Aarhus Sweeping gap test:

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: 0.772-0.806

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

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)

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)

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?

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?