ESTRO 2021 Abstract Book

S1008

ESTRO 2021

patients. Most of these were mild, but 28% experienced cholangitis. Late toxicity occurred in 14 patients, the most common was cholangitis occurring in 8 patients (25%); other late side effects included ulcus duodeni (1 patient), pancreatitis (1 patient), liver abscess (2 patients), ascites (1 patient) and abdominal pain (4 patients). No deaths due to toxicity were observed.

Conclusion SABR for patients with unresectable CC is a promising treatment option especially in fragile patients with the benefit of short treatment time and acceptable toxicity. Our study adds knowledge to the retrospective evaluation of SABR-treated patients while larger prospective trials are warranted. The impact of chemotherapy and patient reported Quality of Life needs further investigation. PO-1218 Online adaptive MR-guided SBRT for unresectable upper abdominal malignancies using a 1.5T MR-linac L. Daamen 1 , S. de Mol van Otterloo 2 , I. van Goor 2 , H. Eijkelenkamp 2 , B. Erickson 3 , W. Hall 3 , H. Heerkens 2 , G. Meijer 2 , Q. Molenaar 4 , H. van Santvoort 4 , L. Verkooijen 1 , M. Intven 2 1 UMC Utrecht, Imaging Division, Utrecht, The Netherlands; 2 UMC Utrecht, Radiotherapy, Utrecht, The Netherlands; 3 Medical College of Wisconsin, Radiotherapy, Milwaukee, WI, USA; 4 Regional Academic Cancer Center Utrecht, Surgery, Utrecht, The Netherlands Purpose or Objective The introduction of online adaptive MR-guided radiotherapy enables stereotactic body radiation therapy (SBRT) of upper abdominal tumors. The aim of this study was to evaluate the feasibility of MR-guided SBRT on a 1.5T MR-linac in a first group of patients with unresectable malignancies in the upper abdomen. Materials and Methods Patients with unresectable, pathology-proven upper abdominal malignancies who were treated at the UMC Utrecht (April 2019-December 2020) were identified in the prospective ‘Multi-OutcoMe EvaluatioN of radiation Therapy Using the MR-linac’ (MOMENTUM) study. Feasibility of treatment was arbitrarily defined as an on- table time of ≤60 minutes for >75% of delivered fractions and successful completion of >95% of fractions as scheduled. Treatment-related toxicity and tumor stability on CT-imaging were assessed. Results Twenty-five consecutive patients with a median follow-up time of 7 (range 1 to 20) months were treated with 35 Gray (n=4) and 40 Gray (n=21) in five fractions over two weeks. For all fractions, contours were adapted based on the daily anatomy of the tumor and organs-at-risk and delivered within a median on-table time of 47 minutes/fraction (range 30-74). In 98/117 fractions (84%), adapted treatment was completed within one hour. All patients received the scheduled irradiation dose as planned. No acute grade 3 toxicity or higher was reported. Tumor stability is shown in figure 1.