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Remote, Real-Time and High-Resolution Optical Imaging of Small Beamlets Relevant in Radiotherapy

M Ashraf1*, P Bruza1 , B Williams2 , D Gladstone2 , N Nelson2 , B Pogue1 ,(1) Thayer School Of Engineering, Dartmouth College, Hanover, NH,(2) Dartmouth-Hitchcock Medical Center, Lebanon, NH,


(Saturday, 3/30/2019) 10:30 AM - 12:30 PM

Room: Osceola Ballroom C

Purpose: We propose a remote, real-time optical imaging technique, for quality assurance and commissioning of small beamlets relevant in radiotherapy. A pulsed intensified CMOS camera was used to capture optical photons generated due to radioluminescence in a cylindrical water tank doped with 1g/L quinine sulfate.

Methods: A range of static beams and dynamic VMAT plans were simulated in a treatment planning system (TPS). For static beams, the percentage dose depth (PDD) and cross beam profiles (CBP) were obtained from the optical images and compared against TPS data. Gamma analysis was performed on all dynamic plans relative to the TPS image data. The technique was tested for sensitivity again common errors (MLC position, Gantry Angle) by inducing deliberate errors in VMAT plans per control point. The technique’s detection limits for spatial resolution, the smallest beam size and the smallest number of control points that can be imaged reliably were also tested.

Results: The optical PDD’s agree to within 2 % of the TPS data for small static square (5, 10 and 50mm²) beams. For CBP’s, we were able to achieve a gamma pass rate >98% for the 3%/1mm criteria. All dynamic plans passed the 3%/3mm criteria with a >90% passing rate. With the camera 2m away from the isocenter, a theoretical spatial resolution 0.21mm was achieved. Beams as small as 5mm and a total of 20-10 Monitor Units can be reliably measured. The technique is sensitive to multileaf collimators (MLC) errors down to 2mm. In future, the technique will be tested against other errors.

Conclusion: Optical imaging provides high resolution quality assurance compared to other QA devices used commercially. Ability to image down to 20-10 MU potentially allows us to capture data per single control point for dynamic plans. The technique is sensitive to small offsets errors in MLC leaf positions.

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