ESTRO 2021 Abstract Book

S1002

ESTRO 2021

studies so far demonstrated higher survival rates and lower incidences of severe toxicity with use of high-dose MR-guided radiotherapy. Conclusion SBRT is an advanced technique that allows delivery of radiation in a precise and time-efficient manner. Standard CT imaging implemented in SBRT limits dose escalation because of its poor soft-tissue contrast. By incorporating MR-guided adaptive techniques with SBRT, the superior imaging enables higher doses to be delivered to the tumour safely whilst limiting radiation to the adjacent healthy tissues. Achieving dose escalation is paramount, as it is the key to making the tumour resectable and thus increasing the chances of cure. PO-1208 Stereotactic MR-guided online adaptive radiation therapy for management of pancreatic malignancies T. Zoto Mustafayev 1 , G. Ugurluer 1 , G. Gungor 1 , B. Atalar 1 , A. Serkizyan 2 , M.U. Abacioglu 1 , F. Agaoglu 1 , M. Sengoz 1 , K. Guven 3 , E. Ozyar 1 1 Acibadem MAA University, Radiation Oncology, Istanbul, Turkey; 2 Acibadem MAA University, Medical Physics, Istanbul, Turkey; 3 Acibadem MAA University, Radiology, Istanbul, Turkey Purpose or Objective MR-guided radiotherapy (MRgRT) is an emerging treatment for irradiation of pancreas. Daily online adaptation with MRgRT has the advantage of dose escalation and OAR protection. We aimed to present our early results of stereotactic and ablative hypofractionated MR-guided online adaptive radiation therapy for locally advanced, recurrent and oligometastatic pancreatic carcinoma. Materials and Methods Thirty-six patients treated with MRgRT between September 2018 and 2020 were evaluated retrospectively and 26 patients with minimum follow-up of 3 months were included in this study. The institutional volumetric dose constraints were used to evaluate the doses to target and OARs. All treatments were planned as step-and- shoot IMRT technique using 6 MV FFF x-rays. Results The median age was 65.5 years (range, 36-83 years) and 57.7% of patients were female. Tumor types included locally advanced pancreatic cancer (46.2%), borderline resectable pancreatic cancer (3.8%), metastatic pancreatic cancer (15.4%), recurrent pancreatic cancer (15.4%) and metastases to pancreas from different primaries (19.2%). All of the initial plans met target and OARs constraints. Among 139 fractions out of 129 fractions (92.1%) were re-optimized. Median GTV volume was 24.6 cc (range, 6.3-132.9 cc) and median PTV volume was 41.2 cc (range, 12.1-193.3 cc). All patients were treated at either end-inhalation or end- exhalation breath hold. The median delivered total dose was 4000 cGy (range, 3000-6750 cGy); with a median fraction number of 5 (range, 5-15 fractions) and the median fraction dose was 800 cGy (range, 450-1000 Gy). Patients were treated every other day except patient who have been treated with 15 fractions. Median follow-up time was 9.7 months. Overall mean survival was 23.1 months, median survival was not reached at the time of this analysis. Estimated 1- and 2-year OS was 85.9% and 75.2%, respectively. The 1- and 2-year local control rates for all patients were 83.5% and 48.7%, respectively. The distant failure rates for 1- and 2- year were 38.7% and 21.5%, respectively. All treatments were very well tolerated. No grade 3 or higher acute or late toxicities were observed. Conclusion Our findings demonstrate high local control rates without any grade 3 and more toxicity. MRgRT has several advantages when compared to other radiotherapy techniques such as; better soft tissue contrast, no need for invasive fiducial markers, online adaptive radiotherapy in every fraction and continous real-time cine MR tumor tracking. Longer follow-up and prospective randomized studies which compare this technique to other modalities are warranted. PO-1209 SIB-SIP SBRT for locally advanced pancreatic cancer: clinical and dosimetric analysis. J. peinado serrano 1,2 , J. Peinado Serrano 3 , P. Romero Pareja 4 , F.A. Derecho Torres 5 , I. Gallego 6 , D. Muñoz Carmona 4 1 University Hospital Virgen del Rocío, Radiation oncology , SEVILLA, Spain; 2 Instituto de Biomedicina de Sevilla, IBIS, Hospital Universitario Virgen del Rocío, Universidad de Sevilla, Consejo Superior de Investigaciones Científicas, Molecular biology of cancer laboratory, SEVILLA, Spain; 3 Instituto de Salud Carlos iii, CIBERONC, madrid, Spain; 4 University Hospital Virgen del Rocío, Radiation Oncology, Sevilla, Spain; 5 Universisty Hospital Virgen del Rocío, Medical Physics, Sevilla, Spain; 6 University Hospital Virgen del Rocío, Medical Oncology, Sevilla, Spain Purpose or Objective 1- Describe the simulation phase, volume delimitation and dosimetric planning with SIB-SIP technique. 2- To analyze the results of local progression-free survival and overall survival (OS) in our cohort of 10 patients Materials and Methods We used a multidampening stereotactic system and a CT-Sim Toshiba Aquilion Big Bore. First, we obtain a slowCT with FOV of 700mm with an acquisition time of 3 seconds over the entire abdominal area to calculate the dose; secondly, a 4DCT acquisition with 240mm FOV focused on the lesion without displacement of the stretcher and with the simultaneous use of the 16 detector crowns available and whose aim is the definition of the ITV ; and lastly, a high-resolution CT (HRCT) with contrast in arterial phase, which is acquired with stopped breathing with FOV of 240mm centered on the lesion for the determination of GTV. We do not use fiducial markers. The risk volumes, PRVs, are created as an expansion of the volumes of the defined OARs (duodenum, stomach, liver, intestinal loops,etc). A new volume, PTV SIP , is defined as the intersection of the PTV with the PRV. The maximum dose compatible with the tolerance of the OAR (between 33 and 40 Gy in 5 fractions) is