Abstract Book

ESTRO 37

S507

conformity in all cases. The general trend for this patient cohort is that the breath-sampled techniques, with a preference for CBS over BS, performs better than volumetric and layered rescanning with respect to both target coverage (an example is given in Figure 1) and variability in dose to OARs. For the slow energy switching time, the total delivery time for volumetric rescanning is very long (up to 44 minutes excluding time to change beam angles) and this technique is only viable in a clinical setting when fast energy switching time is available. In that case, volumetric rescanning is faster than the breath-sampled techniques, but the breath- sampled techniques are still the preferred choice due to the more favourable dose distribution.

PO-0935 Breath-sampled rescanning reduces interplay effects in active proton treatments E. Engwall 1 , L. Glimelius 1 , E. Hynning 1 1 RaySearch Laboratories AB, Research, Stockholm, Sweden Purpose or Objective We investigate four different rescanning techniques in active proton beam therapy of non-small cell lung cancer (NSCLC), and assess their ability to reduce interplay effects. NSCLC has been identified to be well-suited for protons, but the lung region poses many problems related to large density heterogeneities and organ motion, including the interplay effect. Material and Methods The techniques investigated are (1) volumetric rescanning, (2) layered rescanning, (3) breath-sampled (BS) layered rescanning, and (4) continuous breath- sampled (CBS) layered rescanning. The breath-sampled methods will spread the layer rescans over a full breathing cycle (based on the average breathing cycle length as obtained at the time of CT acquisition), resulting in an improved averaging effect at the expense of a longer treatment time. In CBS, we fit as many layer rescans as possible within one breathing cycle to achieve the highest possible averaging effect. The interplay effect was evaluated for robust optimized treatment plans (with and without rescanning) for seven different NSCLC patients in the treatment planning system RayStation. The optimization and final dose calculation was based on the Monte Carlo dose engine to fully account for the density heterogeneities in the lung region. The interplay evaluation was performed for varying starting phases and breathing cycle lengths, using a realistic treatment delivery time structure given from a direct connection to the IBA ScanAlgo simulation tool. Both slow (2.0 s) and fast (0.1 s) energy switching times were simulated. Results Without rescanning the interplay effect is severe for all seven studied patients. Rescanning improves the dose

Conclusion Breath-sampled techniques are superior to simple layered rescanning and volumetric rescanning in our study and they mitigate the interplay effect well also when the breathing cycle length is moderately varied (up to 15%) between planning and delivery. This allows for a straightforward implementation at the clinic without additional motion monitoring equipment. PO-0936 Feasibility of markerless bronchus motion monitoring using kilovoltage images for central lung SBRT L. Van der Weide 1 , C. Hazelaar 1 , M. Dahele 1 , B. Slotman 1 , W. Verbakel 1 Purpose or Objective Stereotactic body radiotherapy (SBRT) for central lung tumors is associated with an increased risk of toxicity of the central airways. Airway position monitoring during radiotherapy delivery could help to manage these risks and increase user-confidence. The aims of this study were to investigate the feasibility of markerless template matching with triangulation using kV projection images for motion monitoring of the proximal bronchial tree (PBT) and to compare the detected motion to the motion on the planning 4DCT scan. Material and Methods CBCT projections of 4 patients (2 CBCT scans/patient) treated with lung SBRT were retrospectively analyzed. 2D reference templates of the right PBT (extending ~1.5 cm above and ~5 cm below the carina) were created in the form of filtered DRRs from a single phase of the planning 1 VU University Medical Center, Department of Radiotherapy, Amsterdam, The Netherlands