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Retrospective Analysis of OSMS Time Series for Potential Intervention in SRS

J Belliveau1*, K Thind1,2 , (1) Tom Baker Cancer Centre, Calgary, AB, (2) Department of Oncology and Physics, University of Calgary


(Sunday, 7/29/2018) 3:00 PM - 6:00 PM

Room: Exhibit Hall

Purpose: Recently, our center began treating our brain SRS patients using a Varian Edge linear accelerator with optical-surface monitoring system (OSMS). Current practice is to use passive monitoring; however, we have undertaken a preliminary study to incorporate OSMS for interventions. We retrospectively analyzed four patients' time series OSMS position.

Methods: We analyzed time series residual drift for translations and rotations using RANDO. Additionally, we analyzed the patients' time series position data treated under our SRS protocol, which currently requires a CBCT prior to each VMAT arc. Patient immobilization is achieved using CDR's Freedom 6 Couch extension, Intuition Click, and an open face mask. Current SRS imaging tolerances are translations under 0.5 mm or rotations under 0.3° before applying final couch shifts. A reference OSMS surface image was acquired before each arc assuming CBCT matching as the gold standard.

Results: The largest residual OSMS motion measured with RANDO across all couch angles was 0.2 mm translationally and 0.1° rotationally. Our patient analysis was performed incorporating residual drift and imaging tolerances (total 0.7 mm or 0.4°). It was observed that this tolerance would be acceptable for couch angles ± 45° but would exceed our tolerance of 0.4° for larger table angles due to static offsets.

Conclusion: While the time series data remains stable for the course of treatment, the use of OSMS for large table angles may pose a problem for monitoring. The ability to use OSMS with larger table angles must be investigated as well as the possibility of using OSMS as a marker for repeat CBCT imaging should we fall out of tolerance.


Not Applicable / None Entered.


TH- External beam- photons: Motion management (intrafraction)

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