ESTRO 2021 Abstract Book

S1235

ESTRO 2021

control (LC), progression free survival (PFS), overall survival (OS), and SBRT-field fractures. LC was defined as no size progression of the treated lesion within 2 years after bone SBRT. Patients were observed for any PFS, death, and fractures with a data cutoff on the 1 st of September 2020. Results The median follow-up time was 16 months. Six patients were lost to follow-up. The number of metastases treated per patient was 1-3. Forty patients had received prior and/or ongoing systemic cancer treatment. 16 patients received systemic treatment due to progression after SBRT treatment. A total of 10 patients had a local relapse. LC at 1 and 2 years were estimated by Kaplan Meier to 91.1% and 63.4%, respectively. 19 patients had disease progression after SBRT. The median PFS estimate was 22.1 months (95% CI 7.2-36.6). PFS at 1 and 2 years was estimated to 72.8% and 45.8%, respectively. Four patients died during follow up. Median overall survival was not reached. Overall survival at 1 year was estimated to 95.6% and at 2 and 3 years both to 87.3%. Two patients experienced a SBRT target site fracture (3.7% of target sites).

Conclusion With a 1-year LC control > 90 %, a high proportion of the metastases responded well to the SBRT treatment. The 2-year LC was lower than expected, when compared with other studies. A lower 2-year LC may be caused by inadequate target dose coverage in the vicinity of critical normal tissue (e.g. thecal sac) and/or indicates a need for a higher target dose. Further dose-response analysis including CTV DVH data will be performed to access factors compromising LC. PO-1508 Nonmalignant skeletal and soft tissue disorders: CT-based contouring atlas for low dose radiotherapy B. Álvarez 1 , A. Montero Luis 1 , R. Alonso 2 , A. Prado 3 , E. Sanchez 1 , R. Ciervide 1 , M. Lopez 1 , J. Garcia 4 , O. Hernando 1 , J. Valero 1 , X. Chen-Zhao 2 , M. Nuñez 1 , K.D. Rossi 1 , M. Izquierdo 1 , M. Garcia-Aranda 1 , P. Fernandez- Leton 3 , C. Rubio 1 1 Hospital Universitario HM Sanchinarro. HM Hospitales., Radiation Oncology, Madrid, Spain; 2 Hospital Universitario HM Puerta del Sur. HM Hospitales., Radiation Oncology, Madrid, Spain; 3 Hospital Universitario HM Sanchinarro. HM Hospitales., Medical Physics, Madrid, Spain; 4 Hospital Universitario HM Puerta del Sur. HM Hospitales., Medical Physics, Madrid, Spain Purpose or Objective The efficacy of low-dose radiotherapy (LD-RT) for osteoarticular and soft tissue non-malignant diseases has been widely reviewed. However, there is no clear consensus for the delimitation of the corresponding planning target volume (PTV), much less using 3D images. The aim of this work is to provide a CT-based contouring tool to those physicians interested in using radiotherapy as a valid and effective treatment of osteoarticular and soft tissue non-malignant diseases.

Materials and Methods Meeting identical quality criteria to those adopted in radiation therapy of cancer, we propose a guideline for