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Distribution and Prediction of Stereotactic Body Radiation Therapy Delivery Errors Originating From Linac Radiation Isocenter Instabilities

E Izaguirre

E IZAGUIRRE1,2*, A WILES1 , T KNEWTSON 1 , (1) Methodist LeBonheur University Hospital - West Cancer Center, (2)University of Tennessee Health Science Center, Memphis, TN


SU-I-GPD-T-241 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: We present a study designed to correlate linac radiation isocenter shifts detected using the morning Winston Lutz (W-L) test with dosimetric errors in the dose prescribed to the planned target volume (PTV).

Methods: Our theoretical analysis of dosimetric errors originating from mechanical instabilities, specifically shifts of the radiation isocenter, proves the existence of universal hyperbolic curves (HC) that correlate the differences between intended delivered dose and actual delivered dose to the PTV as a function of the daily radiation isocenter shift. The HC are parametrized according to daily mechanical shifts to create a family of curves suitable to predict maximum daily dose errors to the treated malignancy. We validated the error estimating accuracy of a family of HC statistically matched to historical data of daily radiation isocenter shifts from a VERSA-HD and a Synergy Elekta linac. Predicted dosimetric errors were compared against PTV dose computed in isocenter shifted plans using the Pinnacle treatment planning system (TPS).

Results: Analysis of the dosimetric errors predicted using the universal HC as a function of PTV volume shows close trend agreement to the TPS simulations. This study is based on 18 randomly selected stereotactic body radiation therapy cases including lung, brain, liver, and spine treatment sites. The parametric family of HC predicted a fast increase of the PTV dose errors for volumes smaller than 30 cc and an asymptotic constant error for PTV’s larger than 30 cc. These results are in agreement with all the cases included in this study for isocenter shifts up to 1.4 mm.

Conclusion: The assessment of the dosimetric daily error in SBRT treatments using HC parametrized according to the daily radiation isocenter shifts is a promising tool to estimate the upper limit of the daily dose error to the treated malignancy.

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