ESTRO 37 Abstract book

ESTRO 37

S491

dose reduction by 20.2% and 30.8%, respectively. Vessel V30 decreased from 91.6% to 34.7% and 20.2% for NS10- VMAT and NS5-VMAT plans, respectively. Conclusion We showed that nerve-sparing SABR using VMAT-SIB strategy is dosimetrically feasible, allowing nerve-sparing and highly conformal plans, benefits of hypo- fractionation, dose escalation to DIL and fast treatment delivery. The use of narrower leaf-width MLC increased the quality of plans. PO-0915 Integration of Trade-Off Exploration and RapidPlan(TM) for gynaecological treatment planning. S. Morris 1 , S. Currie 1 , E. Miguel 1 1 Beatson West of Scotland Cancer Centre, Radiotherapy Physics, Glasgow, United Kingdom Purpose or Objective To integrate the benefit of Multi-Criteria Optimization (MCO)–based Trade-Off Exploration into a RapidPlan™ model for gynaecological radiotherapy treatment planning, and to assess if further optimization is achievable by applying MCO to plans created using this model. Material and Methods Ten patients, previously treated for cervical cancers, were retrospectively planned using the Eclipse Treatment Planning System [Varian Medical Systems, Palo Alto, CA, USA] with a combination of RapidPlan™ and MCO-based Trade-Off Exploration. A RapidPlan™ model was created from standard treatment plans and another from plans subject to MCO trade-off exploration. Each patient was planned using both models, then plans created using the MCO based model were run through the ‘Explore Trade- Offs’ section of Eclipse again. All plans were normalised such that 100% of the prescription of 45Gy was applied to the mean value of the PTV. The effect of the incorporation of MCO in these plans was assessed by the change in mean dose to the OARs, and the change in the D95% of the PTV. Results For all OARs a statistically significant reduction (p<0.01) in dose was observed by using the RapidPlan™ model based on trade-off exploration compared to the standard RapidPlan™ model, and a further significant reduction (p<0.01) by subsequent application of MCO to these plans. The bladder, rectum and femoral heads saw a reduction in the mean dose of 2.8Gy, and 2.1Gy and 1.6Gy respectively, with a further 1.2Gy, 0.6Gy and 1.9Gy decrease by application of MCO. The greatest benefit was observed for the bowel, where use of the MCO based model resulted in a mean reduction of 66cc (range 8-106cc) for V15Gy, 70cc (range 29cc-136cc) for V20Gy and 30cc (range 9cc-47cc) for V30Gy. Conversely the V45Gy showed a small but significant increase of 7cc. All bowel parameters showed a significant dose reduction by subsequent application of MCO. This translated into a reduction in mean bowel dose of 1.7Gy with MCO contributing a further 0.5Gy reduction. In terms of the PTV there was a mean reduction in coverage at the 95% level of 0.5%, which for all plans was still within the 95% tolerance. After MCO had been applied the D95% was reduced by a further 1.2%, which for 7 of the 10 plans caused the D95% to be breached. Whilst a reduction of 1.2% is relatively small, visual observation of the dose distribution showed marked degradation of the coverage in the vicinity of the OARs and a reduction in the homogeneity index of 1.3. Conclusion

Trade-Off Exploration has been successfully built into a RapidPlan™ model and will provide an efficient way to incorporate the benefits of both techniques into new plans. Further benefit to the OARs can be achieved by subsequent application of MCO, but at the expense of PTV coverage. It is predicted that trade-off exploration will provide greater returns where the dose prescription is escalated and is the subject of further investigation. PO-0916 Explore the future of of proton hypo- fractionation treatment - Spot-scanning Proton Arc therapy. X. Ding 1 , X. Li 1 , J. Zhou 1 , C. Stevens 1 , K. Sura 1 , P. Chinnaiyan 1 , I. Grills 1 , Y. Di 1 , P. Kabolizadeh 1 1 Beaumont Health, Radiation Oncology, Royal Oak, USA Purpose or Objective The application of using proton Pencil Beam Scanning (PBS) technique in hypo-fractionation treatment is limited compared to the photon treatment due to its inferior dose conformity cause by the large lateral penumbra and sensitivity of range, setup and motion uncertainties. We hypothesis that Spot-scanning Proton Arc(SPArc) therapy, which is designed to deliver a continuously proton beam through one or more arc trajectories, is able to effectively improve dose conformity and mitigate the dose impact from those uncertainties. Herein, we conduct a comprehensive study by a peripheral brain target, a spine target Stereotactic Radiosurgery(SRS), and a lung mobile tumor patients to explore the dosimetric benefits of SPArc. Material and Methods We retrospectively select one case from each disease site as a representative geometry. Both SPArc and Robust Optimized Intensity Modulated Proton Therapy(RO-IMPT) were re-planned. For brain and spine SRS cases, the dosimetric evaluations including Conformity Index(CI), Gradient Index(GI), and Heterogeneity Index(HI) will be performed on the static dose for all the critical structures among SPArc, RO-IMPT, and the clinical plans: Gamma Knife(Brain) and VMAT(Spine) plans. For lung mobile tumor case, motion interplay effect is analyzed to evaluate the plan robustness in treating in a single fraction comparing with RO-IMPT. Results Overall, SPArc is able to provide an advanced dosimetric outcome over the current RO-IMPT technique in terms of the robustness and target coverage for hypo-fractionation treatment in brain, spine and lung. Compared to GK Brain SRS, SPArc is able to achieve equivalent or better dosimetric outcome in terms of GI, CI and mean body dose. Overall, SPArc is able to provide an advanced dosimetric outcome over the current RO-IMPT technique in terms of the robustness and target coverage for hypo- fractionation treatment in brain, spine and lung. Compared to GK Brain SRS, SPArc is able to achieve equivalent or better dosimetric outcome in terms of GI, CI and mean body dose. For spine SRS, The mean dose to the partial cord in SParc plans is significantly less than those of IMPT plans, 547±72 cGy vs. 607±78 cGy (p<0.03), but not for Dmax, D0.35cc and D10%. Despite the range uncertainties associated with the proton beam therapy, the conformity index from SParc plans is comparable to the VMAT plans, 1.1±0.1vs. 1.1±0.0, significantly higher than the IMPT of 1.3±0.1(p<0.01). The R50 of SParc plans is significantly superior, 3.3 vs 3.7(p<0.01) for IMPT plans. Both are significantly better than VMAT plans of 4.1(p<0.05). For lung mobile tumor treatment, the SPArc is able to mitigate the interplay effect, maintaining the