ESTRO 38 Abstract book

S428 ESTRO 38

Conclusion The bowelbag V15 was the best predictor for acute diarrhea grade ≥ 2 in patients treated with neo-adjuvant chemoradiation for rectal cancer. Further research is needed to assess the value of the bowelbag V15 in modern radiation techniques such as intensity modulated or volumetric arc therapy and in relation to late GI-toxicity. PO-0819 Stereotactic Radiation Therapy in Oligometastatic Colorectal Cancer: 102 patients and 150 lesions M.A. ZERELLA 1,2 , V. Dell'Acqua 1 , A. Surgo 1 , F. Kraja Pupuleku 3 , J. Kobiela 4 , P. Spychalski 4 , C.M. Francia 2 , D. Ciardo 1 , C. Fodor 1 , F. Pansini 5 , S. Vigorito 5 , F. Cattani 5 , M.C. Leonardi 1 , B.A. Jereczek Fossa 1,2 1 IEO European Institute of Oncology- IRCCS, Division of Radiotherapy, Milan, Italy ; 2 University of Milan, Department of Oncology and Hemato-Oncology, Milan, Italy ; 3 University Hospital Centre "Mother Theresa", Department of Oncology, Tirana, Albania ; 4 Medical University of Gdansk, Department of General- Endocrine and Transplant Surgery, Gdansk, Poland ; 5 IEO European Institute of Oncology- IRCCS, Division of Medical Physics, Milan, Italy Purpose or Objective To evaluate local control (LC), progression free survival (PFS), overall survival (OS), toxicity and failure predictors in patients with oligometastatic colorectal cancer (CRC) treated with stereotactic radiation therapy (SRT) in a single institution. Material and Methods Patients with oligometastatic CRC disease (with 1-5 metastases) were analyzed retrospectively. Treatments were performed using VERO® (BrainLab) and CyberKnife® (Accuray) systems. The SRT prescribed dose was dependent on the volume of the lesions and its location. Treatment characteristics are summarized in table 1.

definitions and their association with toxicity in patients with locally advanced rectal cancer (LARC) treated with neo-adjuvant chemoradiotherapy. Material and Methods Data from a historical cohort study including all consecutive patients with LARC treated with chemoradiotherapy in our institute from 2003-2010 was used for this analysis. Patients were treated with a 3D conformal four-field technique. In all patients, bowel structures were delineated using three different techniques; individual small bowel loops (SBL), bowelbag following EMBRACE criteria and the RTOG bowel cavity (Fig 1). Delineations were performed by two observers (JVZ and BO) and checked by a second observer (ER/FP). An experienced radiologist was consulted if needed. Acute toxicity was scored according to CTCAE v4.0. Volumes receiving 5Gy-50Gy with an interval of 5Gy (V5-V50) of the different contours were correlated with occurrence of grade ≥2 acute diarrhea, using a Mann-Whitney U tests.

Results Planning CTs and acute toxicity data were available for 90 patients. Grade ≥2 acute diarrhea occurred in 33 out of 90 patients (36.7%) of which grade 3 in 10 patients (11.1%), no grade 4 or 5 diarrhea occurred. An association was found for all three techniques between irradiated bowel volume and the development of grade >2 acute diarrhea. This association was most pronounced for low dose regions and for SBL and bowelbag compared to the RTOG bowel cavity (Fig 2). The strongest association was observed for the V15 of the bowelbag with a median V15 of 314cc for patients with < grade 2 diarrhea and a median V15 of 410cc in patients with grade ≥2 diarrhea (p=0.04). Interestingly approximately 50% of volume of the bowelbag consisted of small bowel loops, while this was only approximately 13% for the RTOG bowel cavity (Fig 2).

Results 102 consecutive oligometastatic CRC patients (150 lesions overall) were included for this study. They underwent SRT between February 2012 and December 2015. Median prescription dose was 45 Gy (median dose/fraction was 15 Gy delivered in 3 fractions and corresponding BED 10 was 112.5 Gy). Median follow-up was 11.4 months. Acute and late G1 toxicities were observed in 10% and 1.4% of patients, respectively. No patients experienced G3 and G4 toxicity. Considering the radiological response at 3 months and best radiological response, no progression was found in 82% and 85% out of 150 evaluable lesions, respectively. Actuarial 6-month, 1-year, 2-year LC rates were 85%, 67%, 58% (image 1), respectively, whereas OS rates were 95%, 90% and 90%, respectively and PFS rates were 63%, 37% and 27% respectively. Pattern of failure was out-field in 35% and in-field in 31% of patients. Progressive disease was