ESTRO 2022 - Abstract Book

S1499

Abstract book

ESTRO 2022

1 Amsterdam university medical center, Radiation oncology, Amsterdam, The Netherlands

Purpose or Objective Stereotactic body radiotherapy (SBRT) for lung tumors is often performed during free-breathing, irradiating an internal target volume (ITV) incorporating all motion. If respiratory tumor motion is significant (e.g. ≥ 15mm) this can result in large target volumes and irradiation of more lung, increasing the chance of toxicity. In addition the tumor boundaries can be less clear on a 3D CBCT-scan, especially when the target has a low density, which makes image registration for setup more difficult. Deep inspiration breath hold (DIBH) may reduce the irradiated lung volume, butDIBH is not feasible for all lung SBRT patients and the tumor position can vary between breath holds. Therefore we implemented free-breathing expiration gating in our department. The free breathing removes any burden associated with breath-hold and generally, the tumor moves considerably less in the 50% expiration phases than in the 50% inspiration phases, reducing the target volume. We evaluated the treatment time, ITV and residual motion of expiration gating in lung SBRT patients. Materials and Methods Expiration gating was performed in 10 lung SBRT patients treated on a TrueBeam® linac. Patients were treated with VMAT to a total dose of 30-60Gy in 1-8 fractions. Typically, the 30-70% phases were used for treatment but this was patient dependent. At the LINAC, amplitude gating was performed based on the motion of an external marker block (RPM) after translating the phases to RPM amplitude thresholds. Positional set-up was performed using a gated CBCT prior to the 1 st arc, and 1-2 more between arcs. During CBCT acquisition, online tumor position monitoring, using non-clinical software (RTR), consisting of template matching of each kV image followed by triangulation, was used to confirm the motion of the tumor in the gating window. ITV size was compared to that for a non-gated treatment. The times for imaging and delivery are reported. Results Longitudinal tumor motion on the 10-phase (0-90%) 4DCT was 16-31mm (mean 18±6mm). For the selected expiration gating phases the longitudinal tumor motion reduced to 1-7 mm (mean 4±4mm). The ITV on all phases was 1.2-65cm 3 , mean=15±20cm 3 . For the expiration gating phases the ITV was 0.7-50cm 3 (mean 10±15): a mean reduction of 38%. The total time from first set-up imaging to the end of the last arc was 6-66 minutes (mean 19.4±10.8). 80% of the treatments required 24 minutes or less. RTR tracking showed that motion during gated CBCT acquisition was in good agreement with motion during the gating phases on the planning CT, and RTR positional verification corresponded with the average CBCT shifts. Conclusion Expiration gating has been successfully applied for lung SBRT. It achieved a clinically relevant reduction in ITV size and longitudinal tumor motion compared with non-gated free breathing. Average treatment time was clinically acceptable and the expiration phase led to a reproducible tumor position. 1 Delft University of Technology, Department of Radiation Science and Technology, Delft, The Netherlands; 2 Erasmus MC Cancer Institute, Department of Radiotherapy, Rotterdam, The Netherlands; 3 HollandPTC, Department of Radiation Oncology, Delft, The Netherlands Purpose or Objective Breathing interplay effects arise from the interaction between the moving tumor and scanning beam. Various mitigation techniques and planning methodologies account for tumor motion during dose delivery, yet a comprehensive comparison of how combinations of these perform is missing. This study aims to investigate the effectiveness of a wide range of interplay mitigation approaches under various fractionation schemes. Materials and Methods We statistically assess interplay effects in 4D-CT anatomically robust (using 5 phases) and ITV plans (originally planned for 33 fractions of 2 Gy/fraction based on clinical practice at HollandPTC, Netherlands) of two lung cancer patients: P1 with a CTV volume of 489.7 cm3 and maximum breathing amplitude of 8.7 mm, and P2 with a CTV of 39.1 cm3 and 4.1 mm maximum amplitude. The interplay dose calculation is based on 8 phase 4D-CT scans and breathing signals indicating the phase at which each spot is delivered. As motion mitigation techniques we simulated fractionation (with 1, 3, 5, 15 and 33 fractions); layered and volumetric rescanning (with 1, 3, and 5 repaints); random extra time addition between spots; random energy layer ordering; and all possible combinations of these. Using 100 different breathing scenarios (100 different breathing signals) per mitigation setting, our analysis is based on the 90th percentile of the 100 corresponding D98 values, expressed as a fraction of the prescribed dose. Doses for hypofractionated treatments are adjusted to be biologically equivalent to the 33 fraction scenario using biologically effective dose with α / β = 10Gy. Results Figure 1 shows that adding more repaints has little added value for >15 fractions, and that treatments delivered in 5 fraction with 5 repaints per fraction are almost equivalent to 33 fraction delivery. With similar values for the same number of repaints, rescanning provides the same level of smoothing regardless of being volumetric, layered or including randomness. Our results also indicate the inherent fragility of ITV, shown by the 4% (P1) and 0.5% (P2) decrease in D98 with respect to 4D-CT robust plans across all mitigation settings. For the small tumor (P2), adding 2 more repaints has no large effect on the D98, whereas for the larger tumor (P1) 5 repaints results in a higher D98 in the hypofractionation regime. For larger tumor volume and movement, the spread in D98 increases between the pure interplay scenario and the interplay mitigated scenarios (P1: 88%-99% vs. P2: 94%-97% of the prescribed dose), indicating that interplay mitigation is more important for larger tumor and motion. PO-1700 Evaluating the effectiveness of interplay mitigation techniques in proton therapy H. van der Wind 1 , O. Pastor-Serrano 1 , S. Habraken 2,3 , D. Schaart 1,3 , D. Lathouwers 1 , M. Hoogeman 2,3 , Z. Perkó 1