Room: Room 209
Purpose: To investigate the use of small size PRESAGEÂ® 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 PRESAGEÂ® sample of 3.5cm diameter and 5cm length was placed in the QUASARâ„¢ 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 1x2.5cmÂ² was created and calculated with Monte Carlo algorithm with a maximum dose of 9.7Gy. Three samples of PRESAGEÂ® 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. PRESAGEÂ® 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 PRESAGEÂ® samples (~6mm) for doses â‰¥30% of maximum dose.
Conclusion: We showed that small PRESAGEÂ® 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.