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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

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Aarhus University Hospital, Århus Sygehus 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)

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

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Aarhus University Hospital, Århus Sygehus Constancy check of DLG in Aarhus Sweeping gap test:

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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: AcceleratorSGR Max. range of the sweeping gap ratios in Aarhus:

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Aarhus University Hospital, Århus Sygehus Dosimetric effect of different DLG Accelerator 3 Mean dose relative to SGR=0.773 Sweeping gap ratio (SGR) System Leaf Gap Offset [mm] PlanSite Dose - #Fractions Modulation #Arc s Comments 1HHIMRT68/66/60/ MU Typical IMRT plan 2prost + LNRA74/ MU210.80Standard modulation prost + LNRA74/ MU221.50High modulation 3prost + LNRA74/ MU Standard modulation prost + LNRA74/ MU121.60High modulation 4tonsilRA68/60/ MU Standard modulation tonsilRA68/60/ MU High modulation 5tonsilRA68/60/ MU Standard modulation tonsilRA68/60/ MU High modulation Analysis: Mean prostate, 500 MU Mean prostate, 1000 MU Mean tonsil, 500 MU Mean tonsil, 1000 MU

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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 to •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)

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Aarhus University Hospital, Århus Sygehus Adjustment of DLG in Aarhus Clinical demand: Free movement of patients between all accelerators for effective handling of: All Clinacs are adjusted for the same value of SGR (DLG): SGR = (System Leaf Gap Offset = -0.7 to -0.3) –Scheduled accelerator maintenance –Accelerator errors MLC Version 7.2: Rough scale for leaf gap offset (step of 0.1 mm) Spread of SGR (DLG) around Need for increased tolerance for SGR (0.01 0.015)

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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 Random dose error of up to 1.2 % (for 1000 MU) System Leaf Gap Offset = 0: Max. variation of sweeping gap ratio = 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?

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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?

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