ESTRO 36 Abstract Book

S235 ESTRO 36 2017 _______________________________________________________________________________________________

Conclusion A fully automated planning system has been developed that allows configuration by expert treatment planners and oncologists. The evaluative study presented shows high quality plans can be produced with no user input, following the initial site-specific configuration process. This simple process allows high-quality automated plans to be produced for new treatment sites in an efficient manner. OC-0447 CyberArc: a 4π-arc optimization algorithm for CyberKnife V. Kearney 1 , J. Cheung 1 , T. Solberg 1 , C. McGuinness 1 1 University of California UCSF, department of radiation oncology, San Francisco CA, USA Purpose or Objective To demonstrate the feasibility of 4π-arc radiotherapy using CyberKnife for decreased treatment delivery times. Material and Methods A novel 4π-arc optimization algorithm (CyberArc) was developed and evaluated in 4 prostate and 2 brain cancer patients previously treated with CyberKnife using Iris collimation. CyberArc was designed for continuous radiation delivery between beam and node positions using 4π treatment geometry. During beam delivery, the isocenter and Iris collimator diameter are allowed to freely move within machine tolerances. For comparison purposes, new plans were generated using the same total number of beams and range of Iris collimation. Dose calculation was based on the MatRad pencil beam algorithm, modified using the machine commissioning data to fit the CyberKnife flattening filter free beam profiles and percent depth doses. An initial 4π library of beam coordinates is cast over the allowed delivery space. A constrained subplex-based optimization algorithm then selects from an initial library of 6 node positions for each beam coordinate using a 5mm x 5mm fluence map resolution to obtain the first set of beam/node/collimator configurations. A preliminary monitor unit calculation is performed, and beam/node/collimator positions that fall under a threshold are discarded. A 3D traveling salesman problem is solved using a genetic algorithm to obtain the paths between beams ( Figure 1) . From the second set of beam/node/collimator positions, intermediate beam/node/collimator coordinates are calculated along the path between neighboring coordinates using cubic interpolation. A third set of continuous intermediate beam/node/collimator doses are calculated every 2° along the arc path with a 2mm x 2mm fluence map resolution. MUs are calculated for each beam/node/collimagor position using an L-BFGS-B optimization engine. All plans were normalized to the 70% dose volume of the PTV for comparison.

patient. On average CyberArc decreased treatment times by 1.76x ± 0.23x for the prostates cases and 1.62x ± 0.13x for brain patients, not taking into consideration the gantry speed limitations. Staying within the tolerance of the machine speed specifications, the average time decrease was 1.56x ± 0.19x for prostate patients and 1.39x ± 0.11x for brain patients.

Figure 2. DVH comparison between the original CyberKnife plan (solid line) and the corresponding CyberArc plan (dashed line). Conclusion CyberArc is able to deliver plans that are dosimetrically comparable to their CyberKnife counterparts, while reducing treatment times considerably. OC-0448 Near real-time automated dose restoration in IMPT to compensate for daily tissue density variations T. Jagt 1 , S. Breedveld 1 , S. Van de Water 1 , B. Heijmen 1 , M. Hoogeman 1 1 Erasmus MC Cancer Institute, Radiation Oncology, Rotterdam, The Netherlands Purpose or Objective Intensity-modulated proton therapy (IMPT) allows for very localized dose deposition, but is also highly sensitive to daily variations in tissue density along the pencil beam paths, induced for example by variations in organ filling. This potentially results in severe deviations between the planned and delivered dose. To manage this, we developed a fast dose restoration method that adapts the treatment plan in near real-time. Material and Methods The dose restoration method consists of two steps: (1) restoration of the geometrical spot positions (Bragg peaks) by adapting the energy of each pencil beam to the new water equivalent path length (Figure 1), and (2) re- optimization of pencil beam weights by minimizing the dosimetric difference with the planned dose distribution, using a fast and exact quadratic solver. Figure 1 Restoring spot positions. Left: The intended spot positions. Middle: An air cavity causes a displacement and a change in spot shape (not depicted). Right: The energy of the pencil beam has been adapted to restore the spot position. The method was evaluated on 10 prostate cancer patients, using 8-10 repeat CT scans; 1 for planning and 7-9 for restoration. The scans were aligned based on intra- prostatic markers. Prostate, lymph nodes and seminal vesicles were delineated as target structures. Dose was prescribed according to a simultaneously integrated boost scheme assigning 74 Gy to the high-dose planning target volume (PTV) (prostate + 4 mm) and 55 Gy to the low-dose PTV (lymph nodes and seminal vesicles + 7 mm). Results While substantial dose deviations were observed in the repeat CT scans without restoration, clinically acceptable dose distributions were obtained after restoration (Figure

Figure 1. The set of final beam positions and their corresponding paths for prostate patient 3. Results Among the six patients analyzed, the average difference in PTV min dose, max dose, and V95 was 2.47% ± 2.13%, 4.11% ± 2.62%, and 1.63% ± 3.01% respectively. The average conformity index (CI) was 1.09 ± 0.07 for the brain patients and 1.12 ± 0.09 for the prostate patients. Figure 2 shows the plan comparison DVHs for a prostate and brain