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Brain SRS Dose Variations Due to TrueBeam STx Isocenter Uncertainty

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V Stakhursky

V Stakhursky1*, C Xie2 , J Chinault2 , (1) Caritas Christi Health Care, Methuen, MA, (2) Mary Washington Hospital Regional Cancer Center, Fredericksburg, VA


SU-E-T-57 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: Stereotactic Radiosurgery (SRS) is a valuable option for treatment of cranial melignancies. Well defined isocenter position (uncertainty <1 mm) of Varian TrueBeam STx, HD MLC, linac allows for tighter PTV margins(1mm). We report the effect of daily isocenter fluctuations established by Winston-Lutz test on target and cranial organ at risk (OAR) dosimetric parameters.

Methods: 10 patient treatments were prepared using BrainLab iPlan softare. GTV and critical structures were contoured by neurosurgeon. A uniform margin of 1mm was used to expand GTV to PTV. Dose of 18 to 20Gy was prescribed to cover 99% of PTV volume using 3 or 4 dynamic arcs. Winston-Lutz test was performed daily to monitor linac isocenter stability, with 8 images (4 for various gantry angles, 4 for couch angles) analyzed by Standard Imaging PIPSpro software. Singular Value Decomposition method is used to determine the values for isocenter x, y and z coordinate offset, subsequently fed into treatment planning software to evaluate changes in dose coverage.

Results: The average displacement (distance from lightfield center) of linac isocenter during 3 months of daily observations was 0.42±0.12mm (max observed displacement 0.656mm), with the average displacement of 0.27mm pt-superior, 0.22mm pt-right, 0.04mm pt-posterior direction. With all plans normalized (PTV V100=99%) the daily isocenter walk resulted in average loss of V100PTV volume coverage of 4.2%. PTV Dmin dropped 8.9%, while GTV Dmin was 3.1% lower (108.1% planned) on average. Changes for PTV and GTV Dmean and Dmax parameters were less than 1%. Most of the cases showed insignificant changes in OAR dose coverage, with exception of vermis lesion where brainstem Dmax changed 2.4%.

Conclusion: Daily monitoring of TrueBeam STx linac isocenter showed that GTV V100 remained at 100% of target volume in all cases. The effect of isocenter uncertainty could be further reduced by Brainlab ExacTrac X-ray imaging.

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