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Dosimetric Validation of Mask-Based Stereotactic Gamma Knife Radiosurgery Using Pseudo in Vivo 3D Dosimetry: End-To-End SRS Quality Assurance

K Osterman

K Osterman1*, P Storey2 , E Pappas3 , D Kondziolka1 , J Silverman1 , I Das1 , A Xu1 , J Xue1 , K Han1 , S Lymberis1 , (1) NYU Medical Center, New York, NY, (2) New York University School of Medicine, New York, NY, (3) Technological Educational Institute, Athens, Greece


SU-I-GPD-T-539 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To assess end-to-end treatment delivery accuracy of an Icon Gamma Knife system using patient-specific 3D-printed phantoms.

Methods: A vestibular schwannoma case with CT and MRI scans was de-identified and anonymized to adhere to IRB and institutional requirements. Two 3-D printed phantoms based on CT bone anatomy were created, one polymer gel-filled and a second with A16 and W1 inserts within the acoustic GTV. A CBCT with mask immobilization defined the stereotactic space with a TMR-algorithm used in planning. After irradiation, 1.5T MR with 20-channel head/neck coil and 4-contrast HASTE sequence (TE=43/553/1060/1570ms) was run onsite for T2 mapping of the gel’s radiation response, generating a high spatial resolution 3-D dose distribution. 1, 2 and 3-D qualitative and quantitative comparisons of dose distribution and localization. A gamma index (criterion: 2mm distance-to-agreement, 5% dose difference) was evaluated along dose profiles and in 2-D dose maps. DVHs were calculated for targets and OARs. The plan was re-run twice in the matched phantom for measurement with W1 and A16 detectors.

Results: For 6Gy dose prescribed to the 50% isodose line, mean doses for plan and gel measurements were: GTV 7.9 and 7.5Gy, vestibular nerve 2.35 and 2.1Gy. Measurements at the GTV with the W1 and A16 detectors showed dose differences of -6.8% and -7.2% relative to plan. At the level of the GTV, both 1-D dose-profile checks and 2-D planar dose analysis resulted in passing gamma evaluation scores. With 1mm error bars applied, most points were observed to fall within a tighter 1mm DTA tolerance. Dose overlays of the plan and irradiated gel were qualitatively indistinguishable.

Conclusion: Results of this End-to-End QA provide confirmation of the accuracy of delivery with the mask-based system. Measured dose differences on the order of 6-7% may be attributed to use of the TMR algorithm, which assumes tissue homogeneity.

Funding Support, Disclosures, and Conflict of Interest: RTsafe provided the phantoms for use in this study.

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