ESTRO 2020 Abstract Book

S657 ESTRO 2020

patients got a hypo-fractionated RT e.g. with a single dose of 2,5 Gy (3 patients), 3 Gy (16 patients) or 4 Gy (5 patients). Mean survival time in these patients after RT was 27 days, median survival time was 17 days. 12 patients terminated RT too early. 8 of these patients got worse under RT, 4 patients decided to quit RT at their own request. In most cases progressive tumor disease was the reason for early death after RT. 13 patients had an immediate benefit in the end of RT (e.g. reduction of pain/tumor bleeding). Conclusion In 29 of 32 patients RT was applied in palliative intent to relieve cancer related symptoms e.g. tumor bleeding or pain. These are well-accepted indications for RT and may explain early death in this group. Fortunately only up to 1% of all patients treated in the department of radiation oncology at the TUM in that year died within 90 days after RT, which reflects the careful patient selection for RT as well as the patient case-mix in a university-based setting. RT is better tolerated due to the use of modern RT and provides effective palliation in certain cases. Furthermore multidisciplinary selection criteria in patients undergoing RT seem to be appropriate. The data are going to be evaluated in a multicenter project. PO-1248 Preliminary safety and survival report of Stereotactic radiotherapy to oligometastases. N. Hanumanthappa 1 , C. Goldsmith 1 , V.M. Mullassery 1 , S.L. Morris 1 , A. Aggarwal 1 , B. Taylor 1 , A. Gaya 1 , D. Smith 1 , E. Dunne 1 , T. GuerreroUrbano 1 , A. Qureshi 1 , V. Staykova 2 , C. Thomas 2 , C. Williams 3 , C. Hartill 4 , L.H. Taylor 4 , V. Harris 1 , C. Edwards 2 , V. Grandi 5 , S. Vivekanandan 1 , C. Sisodia 2 , S. Ahmad 1 1 Guy's and St Thomas's NHS Trust Hospital, Clinical Oncology, LONDON, United Kingdom ; 2 Guy's and St Thomas's NHS Trust Hospital, Medical Physics, London, United Kingdom ; 3 Guy's and St Thomas's NHS Trust Hospital, Radiology, London, United Kingdom ; 4 Guy's and St Thomas's NHS Trust Hospital, Radiotherapy, London, United Kingdom ; 5 St Johns Institute of Dermatology, Dermatology, London, United Kingdom Purpose or Objective To report toxicity and patient/treatment related variables influencing survival outcomes of Stereotactic ablative body radiotherapy (SABR) treatment in patients treated at a single institution for oligometastatic disease due to various primary malignancies. Material and Methods Clinical Data was reviewed for the first 91 consecutive patients treated with LINAC-based SABR for oligometastases from January 2015 to December 2018 within Commissioning through evaluation programme, to evaluate toxicity and factors driving overall survival (OS). Data was prospectively collected at 3 months after treatment, then 6-monthly intervals thereafter. Results The median age of primary diagnosis and metastases detection was 65 and 69. Colorectal (27/91, 30%), Prostate (25/91, 27%) and rest of Gastrointestinal tract (11/91, 12%), were the most common primary malignancies in the cohort. Lymph node (32/91, 35%), Lung (17/91, 19%) and Liver (14/91, 15%) were the most common targets. There were three ≥Grade 3 late toxicities reported during this study period: benign ureteric stricture requiring stent placement and two patients with persistent fatigue. The median Biological equivalent dose (BED) prescribed assuming α/β = 10, was 60 Gy 10 . Most common dose/fractionation used was 30 Gy in 5 fractions with 48 Gy 10 BED (30%). Toxicity of ureteric stricture was observed in patient treated with 60 Gy 10 BED (30Gy in 3 fractions) for a prostate primary with single pelvic lymph node relapse, with planning target volume of 28.7cc the maximum point dose delivered to the ureter was 35 Gy, which was abutting the target . The other 2 patients with

persistent fatigue were treated with 48Gy 10 BED (30 Gy in 5 fractions) for pelvic lymph node relapse due to colorectal primary and 115.5Gy 10 BED (55Gy in 5 fractions) for a liver metastasis due to Anal cancer. The median followup for the whole cohort was 24.83 months, with progression free survival (both local and distant progression) of 56.34% and 34.83% at 12 and 24 mths, respectively and Local control was 80% at 2 years for the whole cohort. The median OS was not reached for the cohort and OS at 12 and 24 mths was 89.8% and 77.5%, respectively. The multivariate analysis for OS was significant with respect to synchronous vs metachronous detection of metastases (HR 3.1, 95% CI 0.003 to 0.26), and local control (HR 2.5, 95% CI 1.26 to 5.05), favouring metachronous metastases and locally controlled disease group. Patients with prostate primary had 100% survival at 2 years, whereas colorectal primary had 88% survival at 2 years. Conclusion Although SABR to oligometastases has been proven to be relatively safe and feasible option of treatment with favourable outcomes in select group of patients, further data with respect to organs at risk (OAR) tolerance doses with hypofractionation is warranted, to safely deliver quality treatment. Our report suggests future studies to be directed towards defining OAR tolerances with common extreme hypofractionated, dose/fractionation regimens. PO-1249 Comparative efficacy of radiotherapy for breast cancer bone metastases. N. Bychkova 1 , E. Khmelevsky 1 1 Gertzen Moscow Research Oncological Institute, Radiotherapy department, Moscow, Russian Federation Purpose or Objective To study the effect of dose escalation during hypofractional radiotherapy for bone metastases of breast cancer in accordance with various molecular subtypes. Material and Methods Out of 768 cases of radiation therapy for bone metastases included in the long-term randomized trial breast cancer was diagnosed in 460 (60%). The molecular subtype of the tumor was defined in 114 patients. They were divided into 3 groups. 64% had hormone-receptor-positive/HER2- negative variant (RE/RP+), 17% - HER2-positive (HER2+) and 8% - triple-negative (Tr-neg) variant. Patient selection criteria for radiotherapy were non-stopping pain, the impossibility of surgical correction of pathological fracture/malignant spinal cord compression, increasing neurologic disfunction. Irradiation volume included one anatomical area of the skeleton with the affected area. The total dose was 13-26 Gy in 2-4 fractions of 6.5 Gy. Results The median follow-up was 70 months. The average age of patients was 56 years. Women with Tr-neg variant turned out to be significantly younger (48.2 years). Lesion length, risk of pathological fracture, initial pain intensity were the same in various groups. The overall effectiveness of EBRT (сomplete and partial pain relief) was 97,8%, the complete response rate (CRR) - 62,5%. The relative pain reduction was 80%, the pain relapse rate - 8%. Significant differences between these indicators in various molecular subtypes of breast cancer could not be found. However, there was a trend to less complete pain relief in HER2-positive patients. The CRR in HER2+ group was 42,1% compared to 61,6% in RE/RP+ group and 66,7% in Tr-neg patients (p=0.09). The relative pain reduction was 72,2%, 79,4% and 80%, respectively (p=0.12). Dose escalation from 13 Gy to 19.5 Gy and 26 Gy in 2, 3, and 4 fractions of 6.5 Gy accompanied by a significant increase in the CRR from 41.4% to 50.3%, and 80.9%, correspondingly (p=0.012). Conclusion The dose escalation from 13 Gy to 26 Gy in 2-4 fractions of 6.5 Gy allows significantly increase the CRR with