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Session: Therapy Dose Calculation and Verification - I [Return to Session]

Ideal Dosimetric Leaf Gap as a Function of Equivalent Square Field Size

J Seekamp, D Pearson*, University of Toledo, Toledo, OH


WE-C1000-IePD-F4-4 (Wednesday, 7/13/2022) 10:00 AM - 10:30 AM [Eastern Time (GMT-4)]

Exhibit Hall | Forum 4

Purpose: To evaluate the relationship between the ideal dosimetric leaf gap (DLG) and changing field size in Eclipse beam modelling for the AcurosXB15.6.05 algorithm.

Methods: A Varian TrueBeam accelerator was modelled in Eclipse for Acuros15.6.05. A series of 6FFF VMAT plans of varying average equivalent square fields (1.71-9.58cm) were delivered to an ArcCHECK. These measured plans were then compared against exported dose files from the treatment planning system using a gamma analysis. All measurements were evaluated using 1%/1mm, 2%/2mm, 3%/3mm, and 4%/4mm pass criteria. All measurements were compared against 11 different exported dose files from Eclipse, each calculated with a varying DLG value. In each case only the DLG parameter was changed in the beam model. A curve of best fit of the gamma pass rate was determined for each measurement. The curve was then solved for the optimal DLG value to maximize the gamma pass rate for each criterion. All optimal DLG values were plotted against the equivalent square field size of the plan to find the relationship between DLG and field size.

Results: A strong relationship between ideal DLG and field size was found. R squared values of 0.952, 0.974, 0.995, and 0.961 were found for linear fits of the 1%/1mm, 2%/2mm, 3%/3mm, and 4%/4mm gamma pass criteria, respectively. The ideal DLG of a 1.71cm equivalent square field plan at 3%/3mm was found to be 1.239mm and for a 9.58cm equivalent square field plan at the same criteria was found to be 2.522mm.

Conclusion: The DLG parameter is highly dependent upon the equivalent square field of the plan delivered. A change from a 1.71cm square field to a 9.58cm square field doubles the ideal DLG length.


Not Applicable / None Entered.


TH- External Beam- Photons: Small field computational dosimetry

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