Program Information
Influence of Margins and Prostate Rotations On Tracked Dynamic MLC Deliveries
M Fast*, C Kamerling, P Ziegenhein, J Bedford, S Nill, U Oelfke, The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, London, UK
Presentations
TH-AB-303-10 (Thursday, July 16, 2015) 7:30 AM - 9:30 AM Room: 303
Purpose: Tracked dynamic MLC treatment deliveries offer an opportunity for margin reduction and healthy tissue sparing. With our newly developed real-time online dose accumulation tool we investigate the impact of clinical target volume (CTV) to planning target volume (PTV) margins and of a prostate rotation on CTV coverage and organ-at-risk dose.
Methods: We have previously developed an in-house dose accumulation tool for experimental MLC tracking deliveries on our Elekta Synergy/Agility research platform. The dose accumulation is based on pre-calculated dose influence data and updated in ≤30 ms for every MLC aperture reported by the linac. For this study, we have created a set of prostate treatment plans according to RTOG 0938 (5 Fx, 7-beams) using different CTV-to-PTV margins: 1, 3 and 5 mm isotropically; and 3 mm posteriorly + 5 mm otherwise. For each plan, the delivery was performed statically (no tracking, no motion) and dynamically (tracking, motion) with four different real-patient prostate trajectories. An additional prostate rotation of 20° about the LR axis was simulated for the dynamic deliveries.
Results: For the dynamically tracked deliveries, target coverage was maintained compared to the static reference case for all CTV-to-PTV margins. Baseline drifts of the prostate posteriorly and inferiorly increased the rectum D2 dose by 0.18 Gy while decreasing the bladder D2 dose by 0.28 Gy averaged over all plans. Sudden transient motion anteriorly and superiorly increased bladder D2 by 0.14 Gy and decreased rectum D2 by 0.67 Gy. For all margins, CTV dose appeared robust against a prostate rotation when using MLC tracking.
Conclusion: Residual tracking errors caused by system latency appear to be sufficiently covered by a 1 mm CTV-to-PTV margin. Initial evidence has been given that MLC tracking with smaller margins allows for uncompromised CTV coverage without overdosing the rectum or bladder compared to a conventional treatment strategy.
Funding Support, Disclosures, and Conflict of Interest: We acknowledge support from Elekta AB under a research agreement. Research at The Institute of Cancer Research is also supported by Cancer Research UK under Programme C33589/A19727. We acknowledge NHS funding to the NIHR Biomedical Research Centre at The Royal Marsden and The Institute of Cancer Research.
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