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oncologists, 2 radiologists and 2 radiotherapists) experienced in the treatment of tumors of the upper abdomen were collected. CbCT were matched in all 3 translational directions and observers were allowed to adjust window width. In this study we will analyze interobserver variability as well as difference between fractions. Results We evaluated a total of 484 cbCTs in 10 patients. Significant variability was seen in the matching of liver lesion with the use of fiducials and without: median cranio-caudal (CC) 4 mm (range, 2-18 mm) versus 13 mm (range 4-33mm), median latero-lateral (LL) 3 mm (range, 2-7) versus 6 mm (range, 3-13 mm), median anterior- posterior (AP) 3,5 mm (range, 2-8 mm) versus 7 mm (range, 1-15 mm). Median variability for kidney tumors was: CC 4 mm (range, 3-7mm), LL 4 mm (range, 3-7), AP 4 mm (range, 3-5mm). Pancreatic tumor was as follows: CC 9 mm (range, 3-10 mm), LL 2 mm (range, 2-4 mm), AP 3 mm (range, 3-4 mm) and for adrenal metastases: CC 10.5 mm (range, 7-20 mm), LL 4 mm (range, 3-6 mm), AP 4 mm (range, 3-4 mm). Conclusion Major variability between observes was seen without the positioning of fiducial markers for liver lesion. Interobserver variability must be taken into account when creating treatment margins. EP-1456 Safety and tolerability of liver reirradiation using high dose SBRT as first and second treatment D. Gabrys 1 , R. Kulik 2 , I. Wzietek 1 , S. Blamek 1 1 Maria Sklodowska-Curie Memorial Cancer Center and Institute of Oncology, Radiotherapy Department, Gliwice, Poland 2 Maria Sklodowska-Curie Memorial Cancer Center and Institute of Oncology, Department of Radiotherapy Planning, Gliwice, Poland Purpose or Objective The liver can tolerate a high dose of radiation if a sufficient volume of healthy liver tissue is spared. The available approaches typically involve stereotactic body radiation therapy (SBRT), however the data on repeat liver irradiation are scarce. In the current paper we examined the safety and tolerability of liver reirradiation and dose volume histograms (DVH) of combined treatment plans. Material and Methods From 46 patients who were treated with SBRT liver tumors for at least two times we selected 11 patients treated with high dose liver irradiation. The patients underwent retreatment for a primary (one patient with HCC) or metastatic (8 colorectal, 1 kidney, 1 ovarian), recurrent or new liver tumour. With the use of Velocity™ software, a treatment planning evaluation system, we integrated imaging scans and treatments information for each patient. As a result, combined dose from all treatments was associated with images for review. Furthermore, we analysed blood parameters and patient performance status. Results Several fractionation schemes were used; for primary therapy, 12-15 Gy per fraction to the total dose of 36-48 Gy, for second course of irradiation 10-15 Gy/fx to the total dose of 30-48 Gy, and third (5 patients) df 10-15 Gy to the total dose 15-50 Gy. Mean cumulative prescribed dose to the liver tumours was 114.1 Gy (range 81-209). 209 Gy in 5 treatments was delivered to one patient over 5 years. Only in one patient the first reirradiation was

administered to previously irradiated area. Five patients after the first reirradiation developed in field recurrence and subsequent liver reirradiation. All patients were treated with photons delivered with linac-based gated radiotherapy (9 patients) or CyberKnife (2 patients). Median interval from initial RT to first retreatment was 7.3 months (range 3.4-50.8), median time from the first radiotherapy to last retreatment was 18.3 months (range 5.9-55.9). Only two patients are still alive although the median time from first radiotherapy to last visit was 31.8 months (range 10.2-59.9). Mean liver volume was 1311.73 cm 3 (range 1095.7-2265.2). The cumulative mean dose to the liver was 22.9 Gy (range 10.2-32.7). Mean volume receiving 10 Gy was 61.5 % (range 36.1-80.2), and 21 Gy was 39.1 % (range 15.1-52.8). The mean maximum dose to the liver volume of 700 cm 3 was 18.7 (range 10.7- 33.7). Median SGOT after retreatments was 38 (20-106), median SGPT was 27 (15-61), median bilirubin was 14.7 (12.4-22.7).The elevation of liver enzymes was related to the progression of the disease within and outside the liver, no patients experienced symptomatic acute or late liver toxicities related to radiotherapy. Conclusion High dose reirradiation with SBRT to the liver tumors is a safe and tolerable option that allows prolongation of survival with advanced liver disease. Prospective studies are needed to establish accurate dose constraints and treatment guidelines. EP-1457 GemOx with low-dose RT and SBRT for locally advanced pancreatic cancer: preliminary safety results B. Meduri 1 , G. Aluisio 1 , E. D'Angelo 1 , C. Tata 1 , F. Gelsomino 2 , A. Spallanzani 2 , R. Ballarin 3 , G. De Marco 1 , G. Luppi 2 , F. Di Benedetto 3 , S. Cascinu 2 , F. Lohr 1 1 Radiation Oncology Unit, Oncology Department, Modena, Italy 2 Medical Oncology Unit, Oncology Department, Modena, Italy 3 Hepato-Pancreato-Biliary and Liver Transplant Unit, Department of Surgery, Modena, Italy Purpose or Objective To evaluate safety of an induction chemotherapy (CHT) regimen combined with low-dose radiotherapy (LDR) used as a chemo-enhancer followed by stereotactic body radiotherapy (SBRT) in locally advanced pancreatic cancer (LAPC) Material and Methods Patients (pts) with non-metastatic inoperable LAPC were enrolled on a prospective single-institution study (NCT02416609). Four CHT cycles with Gemcitabine and Oxaliplatin (day 1-8 of a 21-day cycle) concurrent with LDR were administered; LDR was delivered on days 1 and 2, 8 and 9 of each CHT cycle, using eight doses of 40 cGy each. If no progression was observed after CHT-LDR, pts received 3 fractions of 8, 10 or 12 Gy (total dose 24-36 Gy) of SBRT based on tumor location in relation to stomach and duodenum. 4D-CT with oral and i.v. contrast was used for treatment planning and IGRT-IMRT for delivery. Seven weeks after SBRT tumour re-staging and evaluation for surgery was performed. Toxicity was scored according to CTCAE v4 Results Between February 2014 and January 2017 we enrolled 13 pts. All pts received four CHT cycles, except one because of heart attack. Two pts developed distant metastasis after induction CHT, 11 received SBRT. Total SBRT dose was: 36 Gy (2 pts), 30 Gy (2 pts) and 24 Gy (7 pts). At present 3/11 pts underwent resection without