Abstract Book

S310

ESTRO 37

OC-0593 Dosimetric analysis of local failures in skull- base chordoma/chondrosarcoma following proton therapy L. Basler 1 , R. Poel 2 , A. Bolsi 2 , A.J. Lomax 2 , S. Tanadini- Lang 1 , M. Guckenberger 1 , D.C. Weber 2 1 University Hospital Zürich, Radiation Oncology, Zurich, Switzerland 2 Paul Scherrer Institute, Center for Proton Therapy, Villigen, Switzerland Purpose or Objective Despite combined modality treatment involving surgery and adjuvant radiotherapy, a relevant percentage of chordoma and chondrosarcoma patients develop a local recurrence. In a previous study, we identified optic apparatus and/or brainstem compression, histology and GTV volume as prognostic factors for the risk of local failure. The present study aims to analyze patterns of recurrence and correlate local control with a detailed dosimetric analysis. Material and Methods This study is based on 222 chordoma (151) and chondrosarcoma (71) patients treated with proton radiotherapy at the PSI between 1998 and 2012. All patients underwent surgery with a total of 215 subtotal resections (96.8%) and 7 complete resections (3.2%). Adjuvant proton therapy was administered to a mean dose of 72.5 ± 2.2 Gy RBE . A retrospective pattern of recurrence analysis was performed where local recurrences were contoured on the respective imaging studies, registered with planning imaging and overlap with the initial target structures (GTV, high-dose PTV, low-dose PTV) was calculated. In addition, individual DVH parameters (e.g. Dmin, D98, D95, Dmean, V70) of initial structures and recurrences were calculated and correlated with local failure. Univariable and multivariable cox regression was performed. Results After a mean follow-up of 50 months, 35 (15%) local failures were observed. MRI imaging was available for 27 (77%) of recurred patients. Only one recurrence was located completely outside the initial PTV. The mean proportions of the volume of recurrence inside the initial target structures were 47% (5-86%) for the GTV, 74% (27- 100%) for the high-dose PTV and 82% (16-100%) for the low-dose PTV. On univariable analysis, the following parameters were found to be significantly associated with an increased risk of local recurrence: GTV(D95, p=0.05), GTV(V<66Gy, p=0.01), GTV(volume, p=0.02), PTV(max, p=0.02), PTV(V<66Gy, p=0.03), PTV(V<59Gy, p=0.02), PTV(volume, p=0.01). From multivariable analysis, GTV(volume, p<0.05) and PTV(V<59Gy, p<0.05) were found to be indicators for significantly reduced local control with GTV(volume)>26.5cc differentiating between 78% and 85% rates of local control at 5 years and PTV(V<59Gy)>4.0cc between 73% and 86% at 5 years.

Conclusion This study identified for the first time DVH parameters, which are associated with the risk of local recurrence after proton therapy using pencil beam scanning for patients with skull base chordoma and chondrosarcoma.

Proffered Papers: CL 12: Late-breaking abstracts and practice changing trials

OC-0594 Postoperative [11C]MET-PET predicts radiochemotherapy outcome in glioblastoma: a prospective trial M. Krause 1 , A. Seidlitz 2 , S. Löck 3 , C. Jentsch 3 , I. Platzek 4 , K. Zöphel 5 , J. Petr 6 , J. Van den hoff 6 , J. Steinbach 6 , D. Krex 7 , G. Schackert 7 , M. Falk 2 , M. Baumann 8 , B. Beuthien- Baumann 9 1 university hospital and faculty of medicine c.g. carus- tu dresden- german cancer consortium dktk dresden- german cancer research center dkfz heidelberg- helmholtz-zentrum dresden - rossendorf- national center for tumor diseases dresden, dept. Of radia, 2 university hospital and faculty of medicine c.g. carus- tu dresden- german cancer consortium dktk dresden- german cancer research center dkfz heidelberg- national center for tumor diseases dresden, dept. Of radiation oncology and oncoray, dresden, germ 3 university hospital and faculty of medicine c.g. carus- tu dresden- national center for tumor diseases dresden, dept. Of radiation oncology and oncoray, dresden, germany 4 university hospital and faculty of medicine c.g. carus- tu dresden- national center for tumor diseases dresden, dept. Of radiology, dresden, germany