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Personalized Treatment Gating Thresholds in Frameless Stereotactic Radiosurgery Using Predictions of Dosimetric Fidelity and Treatment Interruption

R MacDonald1*, Y Lee1, J Schasfoort2, H Soliman1, A Sahgal1, M Ruschin1(1) Sunnybrook Health Sciences Centre, Odette Cancer Centre, Toronto, ON, CA,(2) Gamma Knife Center Tilburg


(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: Gamma Knife Icon (GKI) enables user-defined gating threshold for intra-fraction motion tracking during stereotactic radiosurgery (SRS). An optimal threshold ensures dosimetric fidelity of the planned distribution and minimizes prolonged treatment time as caused by gating events. A prediction of motion performance can be made based on a database of traces and a short acquisition of motion to help define an optimal threshold selection for a given patient.

Methods: database of 2,490 motion traces (717 individuals) was analyzed using previously published methods to characterize patient intra-fraction motion performance on the GKI. A 600 second sample of motion was used to classify a patient’s motion and identify cases in the database with similar metrics. These motion traces were used to perform a predictive reconstruction of that patient’s actual delivered dose distribution for a range of motion thresholds. The remaining patient’s fractions were then reconstructed and compared to prediction. Four patient cases (16 fractions) were used to predict number of interruptions (n=16), change in target coverage (n=16), and change in brainstem maximum dose (n=10). The difference between mean predicted value and reconstructed value were compared for accuracy.

Results: difference between mean prediction and reconstructed values were 0.83 ±1.39 interruptions, 0.42 ± 0.44 % in target coverage change, and 32.3 ± 41.9 cGy for brainstem maximum dose. 35 of the 42 predictions were within one standard deviation of the predicted mean.

Conclusion: Large databases of motion traces can be used to characterize patient performance and predict motion performance. Dosimetric deterioration due to motion and extension to treatment duration can be accurately predicted in some cases using only a short acquisition of motion and the treatment plan. This reconstruction may provide benefit in generating a patient-specific motion threshold which balances dosimetric accuracy and treatment duration.

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Funding Support, Disclosures, and Conflict of Interest: Dr. Ruschin - co-inventor & intellectual-property on the GammaKnife-Icon. Dr. Soliman - honoraria from Elekta Dr. Sahgal - served role with Varian & Elekta; board member to ISRS; honorarium from Elekta, Accuray, Varian, Brainlab, Medtroni; travel accommodations/expenses by Elekta, Varian, and Brainlab; member of Elekta-MRL, Spine, Oligometastases and Linac-SRS Consortia.


Gamma Knife, Radiosurgery, Patient Movement


TH- External Beam- Photons: Motion management - intrafraction

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