ESTRO 36 Abstract Book

S608 ESTRO 36 2017 _______________________________________________________________________________________________

Conclusion The presence of oligodendroglial features and IDH 1 mutation in patients underwent complete surgical resection allowed to identify a subgroup with better outcome in which radiation therapy can be delayed at disease progression. EP-1122 TSPO PET imaging RT treatment planning in malignant glioma D.F. Fleischmann 1,3 , N.L. Albert 2 , M. Unterrainer 2 , P. Bartenstein 2 , C. Belka 1,3 , M. Niyazi 1,3 1 LMU Munich, Radiation Oncology, Munich, Germany 2 LMU Munich, Nuclear Medicine, Munich, Germany 3 German Cancer Consortium (DKTK) German Cancer Research Center (DKFZ) Heidelberg, Germany, Purpose or Objective TSPO PET imaging has been recently hypothesized to accurately display biologically active tumor in high-grade glioma patients and is of major interest for radiotherapy (RT) treatment planning. Biological tumor volumes (BTVs) with different thresholds were analyzed retrospectively for their concordance with MRI-based gross tumor volumes (GTVs). Material and Methods TSPO PET images of 12 patients were retrospectively analyzed. Eleven GBM patients and one patient with anaplastic astrocytoma (IDH wt) were included into the analyses. Five patients underwent primary definitive radiochemotherapy (RCx) with temozolomide, three patients hypofractionated RT and four patients re- irradiation at HGG recurrence. Median dose was 2 to 60 Gy for primary RCx, 2.67 to 40.05 Gy for hypofractionated RT and 2.4/2 Gy to 43.2/36 Gy (three patients) with SIB or 2 to 36 Gy (one patient due to a very large recurrence) at re-irradiation with concomitant bevacizumab. Results Different BTV thresholds were tested, BTV1.6, BTV1.8 or BTV2.0. As per definition, BTV1.6 was largest (86.5 cc > 72.3 cc > 59.3 cc). Median PTV volume was 341.8 cc (primary RCx) or 82.2 cc (PTV36) and 34.9 cc (PTV43.2). The Sorensen-DICE coefficient of BTVs vs. GTV (solely MRI- based) was 0.48 (BTV1.6), 0.54 (BTV1.8) or 0.58 (BTV2.0). Volumetric comparisons revealed significantly larger BTVs in comparison to the median GTV volume of 29.4 cc (respectively p=0.002/0.003/0.008, paired Wilcoxon tests). It was tested whether the BTV volume was included within the PTV (60 Gy) or PTV36/43.2. For this purpose, the amount of BTV included within the PTV was calculated as (BTV ∩ PTV)/BTV. For a threshold of 1.6, the amount was median 0.99 in primary GBM and median 0.70 for PTV36 (four patients) and 0.43 for PTV43.2 (three patients). At a threshold of 1.8, the corresponding values were 0.996, 0.80 and 0.60, and at a threshold of 2.0, the values were 0.997, 0.88 and 0.71. Conclusion TSPO PET imaging seems to be a very interesting approach for GBM delineation at primary RT and re-RT. GTV and BTV concordance was poor, but almost the whole BTV content was included within the primary PTV (60 Gy). TSPO might have a high relevance for re-irradiation as margins are far tighter than for primary GBM (8 mm added to the GTV for PTV36, 3 mm for the PTV43.2 as SIB). Future studies on recurrence patterns are warranted to analyze the initial tumor coverage of the boost volume. EP-1123 To contour or not contour hippocampus in stereotactic brain radiotherapy? A dosimetric study. C. Di Carlo 1 , S. Di Biase 1 , L. Caravatta 1 , G. Caponigro 1 , C. Rosa 1 , M. Di Biase 1 , F. Perrotti 1 , M. Trignani 1 , A. Vinciguerra 1 , A. Augurio 1 , S. Giancaterino 1 , A. De Nicola 1 , M.D. Falco 1 , D. Genovesi 1 1 Ospedale Clinicizzato S.S. Annunziata, Radiotherapy, Chieti, Italy

Purpose or Objective To evaluate hippocampal irradiation in patients treated with fractionated stereotactic brain radiotherapy (FSRT). Material and Methods We performed a dosimetric analysis on 22 patients with 1- 4 brain metastases treated with 24 Gy/3 fractions or 20 Gy/4 fractions using volumetric intensity-modulated arc therapy (VMAT). Original plans did not include hippocampus as a structure to avoide in optimization criteria. All cases were then retrospectively replanned for the VMAT planning hippocampus-spared study. Hippocampus was delineated on diagnostic T1-weighted Magnetic Resonance images (MRI) co-registered with planning computed tomography (CT) images. A planning risk volume (PRV) for hippocampus sparing was generated adding an isotropic 5 mm margin. Hippocampus was defined both as a single (Hu) and as pair organ (Hdx, Hsn). Delineation was performed using RTOG atlas as reference than revised by neuroradiologist. Assuming an α/β ratio of 2 Gy, biologically equivalent dose in 2 Gy fractions (EQD2) was calculated. Constraints analyzed were: Dmax<16 Gy, D40%<7.3 Gy, D100%=Dmin<9 Gy. In addition, neurological status (NS) was investigated at baseline and during follow- up and memory or other neurologic deficit were evaluated by CTACE 4.0 scale. Results Among constraints analyzed, Dmax and D40% have been exceeded in 10/22 cases (20 Gy in 6 cases, 24 Gy in 4), whereas D100% was respected in all cases. Hu Dmax ranged between 17-58.9 Gy, with a mean of 31.1 Gy.D40% ranged between 8.9-13.7 Gy and mean D40% was 11.4 Gy. PRV Hu showed a mean Dmax of 33.2 Gy (range 21.5-60.5 Gy) and a mean D40% of 10.8 Gy (7.7-13.7 Gy). When considered as pair organ, Hdx and Hsn respectively, mean Dmax was 33.2 Gy (range 17-58.9 Gy) and 18.4 Gy (range 16.5-20.9 Gy), while mean D40% was 17.5 Gy (7.6-44.2 Gy) and 10.7 Gy (range 8.2-14.3 Gy). PRV Hdx received a mean Dmax of 35.5 Gy (range 23.4-60.5 Gy) and mean D40% was 15.9 Gy (7.5-36.4 Gy); PRV Hsn received a mean Dmax of 22.6 Gy (range 16-28.7 Gy) and a mean D40% of 9.9 Gy (8.7-12.4 Gy). At 3-months follow-up, at least, 14/22 patients were clinically evaluable; NS was investigated in 9/14 patients while missed in 5/14. Neurological deficits occurred in 4/9 patients and 3 of these presented Dmax and D40% exceeding limit. Conclusion Our data showed that hippocampus might be often over- irradiated if not considered in the optimization of the treatment plan in brain FSRT. Hippocampal delineation should be performed especially in case of good life expectation where its saving could be reasonable avoiding relevant damage. EP-1124 PET-MRI prior to re-irradiation of high-grade glioma patients - a planning study D.F. Fleischmann 1,3 , M. Unterrainer 2 , S. Corradini 1 , M. Rottler 1 , P. Bartenstein 2 , C. Belka 1,3 , N.L. Albert 2 , M. Niyazi 1,3 1 LMU Munich, Radiation Oncology, Munich, Germany 2 LMU Munich, Nuclear Medicine, Munich, Germany 3 German Cancer Consortium (DKTK) German Cancer Research Center (DKFZ) Heidelberg, Germany, Purpose or Objective Imaging of positron emission tomography (PET) combined with MRI was conducted prior to re-irradiation for 7 high- grade glioma patients. MRI-based treatment planning of three independent raters was compared with biological tumor volumes (BTVs) automatically generated from PET- MRI data in this prospective phase I clinical trial (NCT01579253). Material and Methods MRI-based treatment plans for 7 high-grade glioma patients with PET-MR imaging preceding re-irradiation were created by three independent raters including all