Abstract Book

ESTRO 37

S564

USA] with 4-Pi and HyperArc. HyperArc is a non-coplanar SRS planning and treatment solution, available in Eclipse v15.5. These plans were then further optimised with Trade-Off exploration. Trade-Offs is the multi-criteria optimisation planning method, available in Eclipse v15.5. The cohort of patients included one 5 lesion case, one 4 lesion case, four 3 lesions cases and four 2 lesions cases. All plans were optimized using standard objectives to meet constraints based on SRS guidelines. All plans were normalized to 99% of the highest prescription target volume (primary target volume) receiving 100% of the prescribed dose. Additional Trade-Off optimization was applied to the 4Pi and HyperArc plans to determine if any further sparing of healthy brain or OARs could be achieved. Plan quality was evaluated by; volume of the brain receiving 4Gy(V4Gy), volume of brain receiving 12Gy(V12Gy), mean dose to brain, OAR doses and deviation from PTV prescription. Results For non-primary volumes, the deviation from prescription was reduced with HyperArc in comparison to 4-Pi, 2.6% to 7.3% (p=0.006). Trade-Off exploration reduced the deviation with 4-Pi to 4.4% but was not statistically significant (p=0.06), Trade-Off exploration with HyperArc increased this deviation to 4% (p=0.002). V4Gy and V12Gy to brain was reduced in all 10 cases when planning with HyperArc in comparison to 4-Pi (p=0.008 and p=0.01, respectively). The average reduction in V4Gy was 145cc and for V12Gy 6.3cc. The largest improvement is found in the 5 and 4 lesion cases. The addition of Trade-Off exploration to 4-Pi significantly reduced V4Gy (p=0.002) but was borderline significant in reducing V12Gy (p=0.05). There was no significant change in brain dose with the addition of Trade-Offs to HyperArc. The only significant change in OAR dose was in D0.5cc to Brainstem and D0.2cc to optic pathway which was increased when planning with 4-Pi alone. There was no significant improvement with HyperArc Trade-Offs in comparison to HyperArc alone although dose to optic pathway was reduced in 7 of 10 cases. Conclusion HyperArc planning provides significant improvements in PTV coverage, dose to healthy brain and OAR doses in comparison to 4-Pi. The addition of Trade-Off exploration to 4-Pi provides significant improvements in plan quality although not comparable to HyperArc. HyperArc is particularly beneficial for multi lesion cases. Trade- Off exploration of HyperArc plans can confer planning advantages in individual site specific cases to at risk OARs but provides no general improvement in dose to normal brain. PO-1010 A crowd-knowledge-based analysis of DVHs in SBRT: first steps towards a national virtual audit A. Savini 1 , M. Fusella 1 , M. Esposito 1 , C. Carbonini 1 , S. Clemente 1 , C. Fiandra 1 , C. Garibaldi 1 , F. Giglioli 1 , C. Marino 1 , E. Moretti 1 , S. Russo 1 , L. Strigari 1 , S. Strolin 1 , C. Talamonti 1 , E. Villaggi 1 , M. Stasi 1 , P. Mancosu 1 1 SBRT Working Group, AIFM - Italian Association of Medical Physics, Italy, Italy Purpose or Objective Currently, most of the multicenter analyses on treatment planning rely on selected data extraction from the dose volume histogram (DVH) of each plan. Different treatment planning systems employ different algorithms to generate the DVH. This can bias a grouped analysis due

to possible poor data consistency. In order to make data comparable, a homogeneous method for data extraction is necessary. In this work we used a consistent method to present a preliminary analysis of multiple data coming from a national survey on stereotactic body radiotherapy (SBRT) planning. Material and Methods A single spine case was shared among 9 radiation oncology centers. The dose prescription was 30 Gy in 3 fractions with specific constraints on target coverage and dose to nearby organs at risk. The VMAT delivery technique was employed by each center. All data were collected in DICOM-RT format. A script was developed in R language using the RadOnc R-Package for DVHs recalculating using a homogeneous algorithm. Specific DVH points collected from the 9 centers were compared with those recalculated with RadOnc. Using the data recalculated with RadOnc, a consistent multiple-DVH analysis was performed and relevant dose parameters were compared. This preliminary analysis was focused only on dose parameters relative to the planning target volume (PTV). Results Differences between collected and recalculated DVHs were minimal, however in some cases the deviations were up to 1.5%. The multiple-DVH analysis showed a notable variability on target dose level (Fig.1). The D2% to the target deviated from its median value up to 34%. Deviations of comparable magnitude were found for the mean dose. These differences were caused mainly by different planning optimization strategies, rather than by the use of a specific treatment technology. The variability was reduced when considering the dose level corresponding to the prescribed target coverage ( i.e. D90% in Fig.2). This could be attributed to a clear indication on the coverage constraint given in the planning guidelines.