The Effect of Collimator Setting On Small Photon Field Output in a Commercial Treatment Planning System
Presentations
PO-GePV-T-356 (Sunday, 7/25/2021) [Eastern Time (GMT-4)]
Purpose: More than one third of SBRT/SRS fields treated in our clinic were less than 5 mm in size. Due to limitation of linear accelerator collimator jaw positioning accuracy (Y: +/- 2 mm, X: +/- 1 mm, manufacture specification) the effect of the collimator setting on small photon field output needs to be studied.
Methods: A commercially available treatment planning system (Eclipse TPS, Version 15.5.11, Varian Medical System) was used in this study. The outputs of small photon fields of collimator setting in 10 x 10 mm and 20 x 20 mm were first calculated to validate the accuracy of small field dose calculation accuracy of our treatment planning system. A number of treatment plans were then created with a small photon MLC field of 5 x 5 mm in size with various X and Y jaw collimator settings. The calculated outputs of the small photon fields were compared, as well as to that of 100 x 100 mm reference field.
Results: The outputs for 10 x10 mm and 20 x 20 mm photon field shown comparable results with publishing results in the literature, validating the dose calculation accuracy of our dose calculation algorithm. The outputs of the same field size of 5 x 5 mm with various collimator setting from 5 x 5 mm to 20 x 20 mm shown a difference up to 24.9%. For the same collimator setting 5 x 20 mm, by interchanging X, Y jaw settings the output also shown 4% difference.
Conclusion: There was significant difference among plans with the same MLC field size but different collimator jaw settings, which would be verified by measurement with plastic scintillation detector. Recently we have adopted new planning guideline to have fixed jaws of 20 x 20 mm for all small field conformal arc plans,
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