ESTRO 2020 Abstract Book

S782 ESTRO 2020

and several organs-at-risk (OARs) such as the parotid glands and oral cavity D mean . Results The fully automated robustly optimized MLO plans fulfilled the D98>94% criteria for primary CTV in all five patients, see figure 1. The D 98 of the MLO plans was on average (± SD) higher than in the clinical plans (6783 ± 58 cGy for the MLO plans and 6624±143 cGy for the clinical plans). On average, the vw-min D98 of the elective CTV of the MLO and clinical plans were 5139 ±139 cGy and 5160±75 cGy, respectively. The dose to spinal cord and external in the MLO plans were comparable to the clinical plans and fulfilled the clinical criteria. For the other OARs, the dose in the clinical plans was lower compared to the MLO plans, see figure 2.

treatment planning system which models the delivery system that fires at 100 fixed gantry positions with a 1-cm fan-beam field size. Dose modulation is achieved by fast gantry rotation, couch step size of 2.1 mm, and each firing position being visited multiple times in the same axial plane. Material and Methods Included in this study are 10 spine SBRT patients who were treated in our clinic using VMAT under an IRB protocol. The plan objectives are adjusted to achieve the same PTV dose coverage as the clinically approved plans. OAR doses are optimized to achieve the lowest possible maximum and mean doses while keeping the PTV coverage the same. We analyzed optimization time as a function of dose as well as benefit of further optimization. Dose values after optimization and the final dose calculation were also compared. Results OARs included in this study were different for each patient depending on the location of the target. Nevertheless, the mean and maximum OAR doses were consistently lower and ranged between 50% and 99% of the clinical plans. Seven optimizations and one fine-grid final dose computation were performed for each patient. The first optimization took, on average, 11 minutes and all following optimizations took around 65 seconds. The fine- grid final dose computation time was 9 minutes on average. After 7 optimizations no significant improvement was observed. The fine-grid dose computation values ranged from -0.4% to +7.5% of the optimization values. Figure 1 shows how the PTV coverage and mean doses received by OARs changed by each successive optimization for patient 1. All values are normalized to the clinical values. Hence the value less than 1 means the mean dose with prototype TPS is less than the clinically approved plan. The mean doses provided by the prototype TPS were all lower than the clinical plan. All plans are done by an inexperienced planner who received only two hours training before this study in addition to doing a test case which took around 2 hours. First patient plan took 20 minutes for optimization, 10 minutes for fine-grid dose computation, and additional 30 minutes to adjust objectives during planning. These time values were comparable for the other patients in this study.

Conclusion The MLO method fully automatically creates robust IMPT plans for oropharyngeal cancer patients with similar target robustness as compared to the clinical reference plans. In a next step, an OAR dose reduction algorithm will be added to further improve the MLO plans. PO-1463 Evaluation of Plan Quality of a New BgRT Delivery Platform for Spine SBRT S. Balyimez 1 , S. Pitroda 1 , J. Partouche 1 , J. George 1 , D. Angela 2 , H. Cal 2 , B. Aydogan 1 1 University of Chicago Medicine, Radiation Oncology, Chicago, USA ; 2 Reflexion Medical Systems, Research, San Francisco, USA Purpose or Objective To evaluate the planning process, quality, and time of a prototype BgRT treatment planning system for spine SBRT. The new BgRT system combines a compact 6 MV linac and binary multileaf collimator with PET detectors, fan-beam kVCT, and MV imaging systems on the same ring gantry. Plans were generated using the prototype

Conclusion This study concludes that the prototype TPS delivery system has the potential to improve both the planning process and time for spine SBRT. Prototype TPS is easy to learn and to generate quality plans with very minimal experience and training. Further studies will be directed to determine the delivery time and treatment delivery accuracy.