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Modeling Biological Effects of Residual Uncertainties For Stereotactic Radiosurgery

L Ma

L Ma1*, A Sahgal2 , D Larson1 , M McDermott1 , P Sneed1 , (1) UCSF Comprehensive Cancer Center, San Francisco, CA, (2) University of Toronto, Toronto, Ontario,


SU-D-16A-6 Sunday 2:05PM - 3:00PM Room: 16A

Purpose: Residual uncertainties on the order of 1-2 mm are frequently observed when delivering stereotactic radiosurgery via on-line imaging guidance with a relocatable frame. In this study, a predictive model was developed to evalute potentiral late radiation effects associated with such uncertainties.

Methods: A mathematical model was first developed to correlate the peripherial isodose volume with the internal and/or setup margins for a radiosurgical target. Such a model was then integrated with a previoulsy published logistic regression normal tissue complication model for determining the symptomatic radiation necrosis rate at various target sizes and prescription dose levels. The model was tested on a cohort of 15 brain tumor and tumor resection cavity patient cases and model predicted results were compared with the clinical results reported in the literature.

Results: A normalized target diameter (D₀) in term of D₀ = 6V/S, where V is the volume of a radiosurgical target and S is the surface of the target, was found to correlate excellently with the peripheral isodose volume for a radiosurgical delivery (logarithmic regression R² > 0.99). The peripheral isodose volumes were found increase rapidly with increasing uncertainties levels. In general, a 1-mm residual uncertainties as calculated to result in approximately 0.5%, 1%, and 3% increases in the symptomatic radiation necrosis rate for D₀ = 1 cm, 2 cm, and 3 cm based on the prescription guideline of RTOG 9005, i.e., 21 Gy to a lesion of 1 cm in diameter, 18 Gy to a lesion 2 cm in diameter, and 15 Gy to a lesion 3 cm in diameter respectively.

Conclusion: The results of study suggest more stringent criteria on residual uncertainties are needed when treating a large target such as D₀≥ 3 cm with stereotactic radiosurgery.

Funding Support, Disclosures, and Conflict of Interest: Dr. Ma and Dr. Sahgal are currently serving on the board of international society of stereotactic radiosurgery (ISRS)

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