ESTRO 36 Abstract Book

S434 ESTRO 36 2017 _______________________________________________________________________________________________

Gustav Carus- Technische Universität Dresden- Helmholtz-Zentrum Dresden – Rossendorf, Dresden, Germany 2 University Hospital Carl Gustav Carus- Technische Universität Dresden, Department of Radiation Oncology, Dresden, Germany 3 German Cancer Consortium DKTK, partner site Dresden, Dresden, Germany 4 Helmholtz-Zentrum Dresden – Rossendorf, Institute of Radiooncology, Dresden, Germany 5 National Center for Tumor Diseases, partner site Dresden, Dresden, Germany Purpose or Objective To compare 4 different proton pencil beam scanning (PBS) treatment approaches for unilateral head and neck cancer (HNC) targets in terms of robustness, including anatomical changes during the treatment course. Material and Methods Eight patients with unilateral HNC treated with double scattered proton therapy were selected. Each patient dataset consists in a planning CT and several control CTs acquired by an in-room CT scanner during the treatment course. Four different proton PBS plans with simultaneous integrated boost and dose prescriptions of 50.3 Gy(RBE) to the low-risk CTV and 68 Gy(RBE) to the high-risk CTV in 34 fractions were calculated: conventional PTV-based single- field (SFO) and multifield optimization (MFO), and robustly optimized SFO and MFO plans on CTV level, considering ±3 mm and ±3.5% of setup and range uncertainty,respectively. The treatment plans were recalculated on the registered control CTs and the cumulative doses calculated and compared with the nominal plan. For robustness evaluation, perturbed doses using a probabilistic scenario-wise approach obtaining random setup shifts through Gaussian sampling, and range uncertainties of 0, +3,5% and -3,5% were calculated, using planning and control CTs, considering both anatomic changes and uncertainties. Cumulative doses from 30 different perturbed treatment courses were generated for each plan. Results The target coverage for the four nominal plans was similar, fulfilling the clinical specification of D98≥95% of the prescribed dose (range 96.9-100.5% for low-risk CTV, 97.4-100.8% for high-risk CTV), being slightly lower on the robust optimized plans. The doses to the organs at risk were similar for all plans; however, for the ipsilateral parotid, higher median doses up to 5 Gy were found on the SFO approaches (Table 1), whereas the contralateral parotid is completely spared. The target coverage throughout the treatment course with slightly changing anatomy remains in general constant.

bladder and small bowel are merged to a structure that is used as the single OAR. Next, a density override of 0.5 g/cm 3 is performed on any air pockets in the PTV that are identified using a density threshold. A dual arc VMAT plan is set up and the dose distribution is optimized using the Pinnacle 3 Auto-Planner. After the generation of the Auto- Plan, which takes about 45 minutes, it is presented to the dosimetrist for approval. The Pinnacle 3 Auto-Planner creates plans based on a set of dose optimization goals and a number of advanced settings called the “treatment technique”, which allows (indirect) control over the resulting plan. The main challenge is to develop a single treatment technique that leads to optimal plans, which meet our precise and high clinical demands, for a large patient population. After having optimized the treatment technique using a test set of 30 patients, we evaluated the Auto-Plans by performing a blind test where 4 physicians and 4 planning dosimetrists were asked to compare manual clinical plans with Auto-Plans for 10 new patients. Results The optimized treatment technique is shown in Table 1. On average, the mean dose to the small bowel + bladder is 2.5 Gy lower for the Auto-Plans compared with manual plans, at the expense of having a slightly increased dose in the lateral direction. An example of a manual plan and an Auto-Plan is shown in Figure 1. The result of the blind test was a unanimous preference for the Auto-Plans (20- 0), based on a better PTV coverage and a lower OAR dose. The slightly higher lateral dose was considered acceptable.

Conclusion We have successfully developed automatic rectum VMAT treatment planning using our automation framework FAST in combination with the Pinnacle 3 Auto-Planner. The Auto- Plans systematically differ from the manual clinical plans (with an average OAR mean dose reduction of 2.5 Gy) and are unanimously preferred by physicians and dosimetrists. This clearly demonstrates how the implementation of an Auto-Planner system, combined with the accompanying reconsideration of plan style and clinical trade-offs, can lead to improved treatment plans. As a result, automatic rectum VMAT planning has been introduced in our clinic as of July 2016. PO-0819 Robustness evaluation of single- and multifield optimized proton plans for unilateral head and neck M. Cubillos Mesías 1 , E.G.C. Troost 1,2,3,4,5 , S. Appold 2 , M. Krause 1,2,3,4,5 , C. Richter 1,2,3,4 , K. Stützer 1,4 , M. Baumann 1 1 OncoRay – National Center for Radiation Research in Oncology- Medical Faculty and University Hospital Carl