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Dosimetric leaf gap (DLG): Dosimetric consequences

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.

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Dosimetric leaf gap (DLG): Dosimetric consequences

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  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) Dosimetric leaf gap: 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, 2209–2219 2001.

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

  5. 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: DLG for Varian Clinacs in Aarhus Max. range of the sweeping gap ratios in Aarhus: 0.772-0.806

  6. Dosimetric effect of different DLG

  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 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)

  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 = 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)

  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?

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