ESTRO 36 Abstract Book

S238 ESTRO 36 _______________________________________________________________________________________________

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.

2). This resulted in PTV V 95% ≤ 2% for all scans. For the bladder, the differences between the restored and intended treatment plans were below 2 Gy and 2%-point. The rectum differences were below 2 Gy and 2%-point for 90% of the scans. In the remaining scans the rectum was filled with air and partly overlapped with the PTV, resulting in unavoidably higher rectum doses. ≥ 98% and V 107%

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 2 Boxplots showing differences in dosimetric parameters between the distorted and intended (left) and re-optimized and intended dose distributions (right) for all 80 scans. Left to right, rectum parameters: D mean , V 45Gy , V 60Gy , V 75Gy and bladder parameters: D mean , V 45Gy , V 65Gy. The mean time needed for energy adapta tion was 5.4 seconds (3.5-10.6). The re-optimization time was on average below 5 seconds (maximum 9.0). T he most time consuming and currently limiting operation was calculating the dose distribution matrix (average 4.3 minutes (2.4-9.6)), performed once betw een the two steps. Conclusion The impact of density variations on the penci l beam path in IMPT can be reduced by performing an automated dose restoration consisting of a water equivalent path length correction of the pencil beams, followed by a re- optimization of the pencil beam weights. OC-0449 A novel and objective plan evaluation for limb sarcomas IMRT in the IMRiS phase II trial R. Simões 1 , H. Yang 1 , R. Patel 1 , F. Le Grange 2 , S. Beare 3 , E. Miles 1 , B. Seddon 2 1 Mount Vernon Cancer Centre, National Radiotherapy Trials Quality Assurance RTTQA Group, London, United Kingdom 2 University College Hospital, Sarcoma Unit, London, United Kingdom 3 University College of London, Cancer Research UK & University College London Cancer Trials Centre, London, United Kingdom Purpose or Objective IMRiS (Clinicaltrials.gov id:NCT02520128) is a multicentre phase II trial of intensity modulated radiotherapy (IMRT) in soft tissue and bone sarcomas. IMRT was implemented in the UK for limb soft tissue sarcomas (STS) in the context of this trial, which opened to recruitment in March 2016. As limb STS volumes are very variable, there are several ways of optimising the plans. It is often difficult to assess plan quality without understanding fully if the presented plan has been well optimised. We describe novel metrics used to evaluate IMRT plan quality for limb STS. Material and Methods A case of liposarcoma of the left thigh was available to the 29 IMRiS participating centres. The prescription was 50Gy in 25 fractions. The clinical target volumes and the Proffered Papers: Planning and quality assurance