ESTRO 36 Abstract Book

S236 ESTRO 36 2017 _______________________________________________________________________________________________

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%

relevant organs at risk (OAR) were pre-contoured. The planning target volume (PTV) was derived at each centre (5-10 mm). All plans were checked, PTV conformality (PTV V 95% /PTV V total ) and PTV compromise (OAR V 95% /OAR&PTV V Total overlap ) indexes were also calculated. The relevant OAR for this case were the femoral head and neck (FHN) and the femur in treatment field (FTF). The IMRT dose fall-off gradient for FHN (FHN V95%/ FHN V80%) was also assessed. Normal tissues and the joint were not analysed, as their tolerances were easily met for this specific case. Results 19 centres completed 20 IMRT plans. The plan quality of 9/20(45%) submissions was suboptimal and had to be repeated. The results (see table) include the resubmitted cases (total 29 plans). The case was particularly challenging near FHN and FTF, due to an overlap of OARs with the PTV. Depending on PTV margins, overlapping FHN volumes varied from 13.8% (for PTV margins of 5mm) to 33.0% (for PTV margins of 10mm). FTF overlapping volume with PTV ranged from 24.7% to 51.1%.Plans were very conformal to PTV; however, the PTV conformality index was not useful for areas where PTV overlapped with OAR. We therefore calculated a compromise index for the PTV areas overlapping with FHN and FTF, which support the visual assessment of plans. The graph below highlights plans in which V80% was suboptimal in relation to the V95% (in total 5 plans had a suboptimal IMRT fall-off dose gradient).

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

Conclusion Limb STS tumours are a heterogeneous group of tumours with significant variation in PTV shape and size. Evaluating the plans for a newly implemented technique can be challenging, particularly when determining if a plan is optimal. We developed an objective assessment method that is applicable to all limb STS. The first planning results show that 45% of plans had either compromised PTV coverage in favour of meeting OAR dose constraints, or had not created a steep enough dose gradient near the OAR. We attribute this to a change in the planning technique paradigm, as many of the centres were using IMRT for limb STS for the first time. OC-0450 Geometric variation of the axillary lymph node region in locoregional breast/chest wall irradiation. K.N. Goudschaal 1 , N. Bijker 1 , A. Bel 1 , N. Van Wieringen 1 , M. Kamphuis 1