ESTRO 36 Abstract Book

S438 ESTRO 36 _______________________________________________________________________________________________

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.

Results Anatomical robust optimization resulted in adequate CTV doses if at least three artificial CTs were included next to the planning CT. Online plan adaptation also resulted in adequate CTV irradiation, whereas this could not be achieved using the SFUD approach, even with a PTV margin of 5 mm (Figure 2). Anatomical robust optimization provided considerable OAR sparing compared with the SFUD approach (5 mm margin), with an average reduction in max-dose and mean-dose parameters of 6.0 Gy (17%) and 5.8 Gy (24%), respectively. The use of online plan adaptation resulted in further OAR sparing compared with anatomical robust optimization, reducing max-dose and mean-dose parameters on average by 3.8 Gy (13%) and 3.4 Gy (23%), respectively.

Conclusion We have developed an anatomical robust optimization method that effectively dealt with the variation in nasal cavity filling, providing substantially improved CTV coverage and OAR sparing compared with the conventional SFUD approach. Online plan adaptation allowed for further OAR dose reduction and we therefore recommend this planning strategy to be pursued for future application in these patients. PO-0818 Improving plan quality and efficiency by automated rectum VMAT treatment planning G. Wortel 1 , J. Trinks 1 , D. Eekhout 1 , P. De Ruiter 1 , R. De Graaf 1 , L. Dewit 1 , E. Damen 1 1 Netherlands Cancer Institute Antoni van Leeuwenhoek Hospital, Department of Radiation Oncology, Amsterdam, The Netherlands Purpose or Objective To develop, evaluate, and implement fully automated VMAT plan generation for rectum patients that receive either palliative 39 Gy (13×3 Gy), or curative 45 Gy (25×1.8 Gy, postoperative), 50 Gy (25×2 Gy, preoperative) treatment. Material and Methods The automatic rectum VMAT plan generation is performed by a combination of our in-house developed automation framework FAST and the Pinnacle 3 Auto-Planner. The automatic planning starts after the physician has delineated the rectum target volume(s). FAST starts our TPS Pinnacle 3 , creates a patient record, and imports the CT. The patient’s skin and bladder are auto-segmented by Pinnacle 3 ’s module SPICE. In addition, the small bowel is delineated using a custom-made FAST module. The

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