ESTRO38 Congress Report

Physics

3. MLC tracking for lung cancer SABR is clinically feasible: results of first-in-human clinical trial (E38-0898)

Jeremy Booth, Vincent Caillet, AdamBriggs, Nick Hardcastle, Kathryn Szymura, Ricky O’Brien, Ben Harris, TomEade, Paul Keall

Northern Sydney Cancer Centre, Australia

Context of the study Real-time adaptive radiotherapy provides activemonitoring and correction for motion of lesions during treatment delivery. The technology has been available on specialist linacs but is not commercially available for standard c-arm linacs. The potential benefits of real-time adaptation are higher accuracy, lower target dose volumes, and lower organ at risk dose. In this study we investigated the feasibility of using real-time adaptive radiotherapy on a standard c-arm linac to treat lung cancer SABR patients. Overview of abstract This study demonstrated that standard radiotherapy linacs can be used tomonitor and adapt to respiration-induced lung cancer motion during high dose radiotherapy. Seventeen patients were treated with a real-time adaptation technique known as MLC tracking. Comparison planning against standard technique showed reduced target volumes, lower lung dose and higher accuracy determined as mean beam to tumour alignment. What were the three main findings of your research? Real-time adaptive radiotherapy can be delivered with high accuracy on standard linacs. The study shows that withMLC tracking the planning target volume (volume of lesion plus lungmargin receiving the prescription dose was reduced by an average of 25% (range 2% - 47%), the surrounding lung tissue dose was reduced by an average of 14% (range 9% - 42%) and the overall system accuracy is 1.2mm. The top row of the figure below shows planning dose is smaller for MLC tracking than standard planning which uses an Internal Target Volume (ITV). Across the bottom row, the figure shows that delivery is more accurate; with MLC tracking the dose cloud (indicating the prescribed dose) conforms to red line (targeted volume). With standard ITV-based treatment the colour cloud is missing part of the targeted volume indicating tumour under-dose.

Figure Dose clouds for MLC tracking planned (top left) and delivered (bottom left) show high fidelity in delivery under motion and smaller dose clouds representing less organ at risk exposure. Dose clouds for same patient planned with standard ITV-based method (top left) and simulated delivered dose (bottom right) shows dose cloud not extending to edge of the target red contour representing under-dose. What impact could your research have? This research concludes that standard linacs can effectively deliver real-time adaptive radiotherapy and that this technique offers high accuracy. Enabling real- time adaptive radiotherapy is complementary to online adaptive techniques and could be used reduce radiation to nearby healthy organs, to increase the viable tumour size for stereotactic radiotherapy, improve safety for patients and facilitate re-treatment or increased tumour doses. Is this research indicative of a bigger trend in oncology? With improvements incomputational power and radiotherapy technology, we are seeing increases in radiotherapy treatment plan adaptation to daily anatomical changes which may be due to patient shape, organ deformation/ filling, internal tumour size/shape/position and even patient condition. We push this further by showing it is feasible to adapt to the tumour position as it moves during treatment delivery.

Congress report | PHYSICS

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