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Use of Small 3D Radiochromic Dosimeters and a Motion Phantom for Dose Verification of Dynamic MLC Tracking Radiotherapy

F Costa1*, M Menten1 , S Doran2 , J Adamovics3 , I Hanson1 , S Nill1 , U Oelfke1 , (1) The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, London, (2) CRUK Cancer Imaging Centre, The Institute of Cancer Research, London, (3) Rider University, Lawrenceville, NJ


(Sunday, 7/29/2018) 2:05 PM - 3:00 PM

Room: Room 209

Purpose: To investigate the use of small size PRESAGE® 3D dosimeters in combination with a lung motion phantom for dosimetric verification of real-time MLC-tracked radiotherapy treatments.

Methods: A cylindrical insert holding a PRESAGE® sample of 3.5cm diameter and 5cm length was placed in the QUASAR™ MRI4D motion phantom, simulating a tumour in the lungs. A CT scan of this set up was acquired and transferred to the research Monaco treatment planning system. A plan with four equidistant spaced beams and a field size of 1x2.5cm² was created and calculated with Monte Carlo algorithm with a maximum dose of 9.7Gy. Three samples of PRESAGE® were irradiated in three different scenarios: (1) static: static phantom, non-tracking; (2) motion: moving phantom, non-tracking (3) tracking: moving phantom, with tracking. The in-house MLC-tracking software DynaTrack was used to interface with our research Elekta Synergy Linac and dynamically move the MLC based on the target position reported by the motion phantom. Log-files containing the recorded MLC and phantom positions every 40ms allowed to reconstruct the dose delivered to the samples under motion and tracking. PRESAGE® samples were imaged with an in-house telecentric optical-CT scanner and reconstructed with 0.2mm resolution.

Results: Dosimetric changes between samples under motion and tracking are visible. Comparison between simulated and measured 3D dose showed good agreement for all three irradiation scenarios (static: 99.2%; motion: 99.7%; tracking: 99.3% with a 3%, 2mm and a 10% threshold local gamma criterion), failing only at the edges of the PRESAGE® samples (~6mm) for doses ≥30% of maximum dose.

Conclusion: We showed that small PRESAGE® samples can be used to validate the delivery of dynamic MLC-tracked radiotherapy. Despite the discrepancies found at the edges of the samples, due to sample non-uniform properties, the methodology used was reproducible, easy to set up and provides valuable 3D dosimetric information.

Funding Support, Disclosures, and Conflict of Interest: The authors acknowledge Elekta AB, Stockholm, Sweden; NHS funding to the NIHR Biomedical Research Centre at The Royal Marsden and the ICR. The ICR is supported by Cancer Research UK under Programme C33589/A19727.


Quality Assurance, Treatment Verification, Dosimetry


TH- External beam- photons: Quality Assurance - IMRT

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