ESTRO 36 Abstract Book

S182 ESTRO 36 2017 _______________________________________________________________________________________________

6 Azienda Ospedaliera Città della Salute e della Scienza, Medical Physics, Torino, Italy 7 Humanitas Centro Catanese di Oncologia, Medical Physics, Catania, Italy 8 Ospedale Ordine Mauriziano di Torino- Umberto I, Medical Physics, Torino, Italy 9 Istituto Regina Elena - Istituti Fisioterapici Ospedalieri, Medical Physics, Roma, Italy 10 AUSL di Piacenza, Medical Physics, Piacenza, Italy 11 Istituto Clinico Humanitas, Medical Physics, Rozzano, Italy Purpose or Objective SBRT planning for spinal metastases is particularly challenging due to the high dose required for covering the PTV complex shape, and to the steep dose gradient mandatory for sparing the spinal cord. Many combinations of delivery systems and TPSs are clinically available in different institutions. Aim of this study was to investigate the dosimetric variability in planning spine SBRT among a large number of centers. Material and Methods Two spinal cases were planned by 38 centers (48 TPS) with different technologies (table 1): a single dorsal metastasis, and double cervical metastases. The required dose prescription (DP) was 30 Gy in 3 fractions. Ideal PTV coverage request was: V DP >90% (minimum request: V DP >80%). Constraints on the organs at risk (OAR) were: PRV spinal cord: V 18Gy <0.35cm 3 , V 21.9Gy <0.03 cm 3 ; oesophagus: V 17.7Gy <5cm 3 , V 25.2Gy <0.03 cm 3 . As a last option, planners were allowed to downgrade DP to 27 Gy to fulfil OAR constraints. 3D dose matrixes were analyzed. DVH were generated and analyzed with MIM 6.5 (MIM Software Inc. Cleveland US). Homogeneity index (HI) was computed for each PTV as HI= (D 2% -D 98% )/DP. Planners did not meet the protocol constraints or PTV dose coverage were asked to re-plan the wrong case. Multivariate statistical analysis was performed to assess correlations between dosimetric results and planning parameters.

analysis showed, for both cases, a significant correlation (p<0.05) between Homogeneity Index (HI) and both OAR dose sparing and PTV coverage. Irradiation techniques correlated with spinal cord sparing; however institutions using similar/same delivery/TPS techniques produced quite different dose distributions, highlighting the influence of the planner experience on the optimization process (figure 1).

Fig1 Box plot relative to the single metastasis case. 18 plans were computed using VMAT, 8 VMAT FFF (linac Free of Flattering Filter), 6 Ciberknife, 5 Tomotherapy, 7 IMRT, 1 3dCRT. Conclusion At our knowledge, this is the largest non-sponsored multicentre planning comparison. Differences in DVH binning among centres could explain minor violations. HI is a key factor for planning optimization: prescribing to lower isodose generally leads to better OAR sparing and higher PTV coverage. Results have a dependence on the irradiation technique, although the planner's experience plays a not negligible role. A multicentre analysis as proposed in this study can have an impact on the standardization of plan quality for spinal SBRT. OC-0348 Reducing the dosimetric impact of variable gas volume in the abdomen during RT of esophageal cancer P. Jin 1 , J. Visser 1 , K.F. Crama 1 , N. Van Wieringen 1 , A. Bel 1 , M.C.C.M. Hulshof 1 , T. Alderliesten 1 1 Academic Medical Center, Radiation Oncology, Amsterdam, The Netherlands Purpose or Objective For middle/distal esophageal tumors, a varying gas volume in the upper abdomen could induce changes in the dosimetry of RT. In this study, we investigated the dosimetric impact of abdominal gas pockets as well as a density override (DO) strategy to mitigate dosimetric effects. Material and Methods We retrospectively included 1 patient with middle and 8 patients with distal esophageal cancer. For these patients, it was unclear whether re-planning was needed due to the varying gas volume during treatment. For each patient, we measured gas volumes in the planning CT (pCT) and 8–28 (median: 14) CBCTs to assess possible time trends. Further, we made IMRT and VMAT plans with a prescription

Table1: Linac , TPS, delivery technique and kind of inverse optimization used in the intercomparison. Results 14/96 plans did not meet the protocol requests. After the re-planning, still 6/96 plans with different technologies did not respect at least one constraint with differences >0.5 Gy. For the dorsal case, 3 minimum (<0.5Gy) deviations (1 VMAT, 1 IMRT, 1 Tomo), and 2 reduced DP (1 VMAT and 1 Tomo) occurred. For the cervical case, 3 minimum deviation (1VMAT 1IMRT 1Tomo), and 2 reduced DP (1 VMAT and 1Tomo) were observed. Multivariate