ESTRO 2021 Abstract Book

S636

ESTRO 2021

Purpose or Objective To determine the incidence of radionecrosis (RN) and factors related with its appearance in patients with cerebral metastases treated by single fraction (SF-SRS) or multi fraction (MF-SRS) radiosurgery. Materials and Methods 183 patients were treated with SF-SRS or MF-SRS between January/2013-March/2019 and were evaluated with a magnetic resonance 3 months after treatment and then every 6 months. Dose was prescribed to 85-90% isodose line and was delivered by a high-resolution multileaf collimator. Association rates were estimated using Fisher´s, Chi-Squared or Mann-Whitney U test and survival using log- rank test. Results 57% patients were women and 43% men. Mean age was 59. Mean lesion size was 18mm. 49% had received previous local treatment: 67% whole brain radiotherapy (WBRT), 23% surgery and 10% surgery + WBRT. 70% had received systemic treatment.62% received SF-SRS: 88% 15Gy, 10% 18Gy, 2% other dose. 38% received MF-SRS: 62% 35Gy in 7 fractions, 25% 30Gy in 6 fractions, 13% other dose. After a mean follow-up of 4 years (95%CI 2.7-11.6), there were 19% RN cases with a mean appearance of 6 months (95%CI 3.5-16). 82% RN cases were in SF-SRS and 18% in MF-SRS group (p=0.01). Cumulative incidence of RN in SF-SRS group was 31% at a year, 78% at 3 years and 89% at 5 years; in MF-SRS group was 28%, 60% and 72% respectively, with statistical association of developing RN at 5 years in SF-SRS group (HR 2.7 95%CI 1.1-6.5). About RN cases, mean PTV volume was 4cc (95%CI 1.5-10.7), mean conformity index 1.7 (95%CI 1.4-1.8), mean dose to isocenter 18Gy (95%CI 17-20) and mean marginal dose 14Gy (95%CI 0-15.3). 44% RN occurred in lesions between 10-20mm, 32% in larger than 20mm and 24% in those under 10mm. 53% RN were symptomatic, being 83% RN of MF-SRS group symptomatic vs 46% of SF-SRS group. Univariate analysis showed that SF-SRS (OR 3.5 95%CI 1.4-9) and previous WBRT (OR 5.2 95%CI 1.1-24.1) were predictive factors of developing RN. Although there was no significant association, patients who developed RN had higher V10-V12 mean; V10 10cc (95%CI 5-22) vs 8cc (95%CI 5-22) and V12 7cc (95%CI 3-15) vs 6cc (95%CI 3-15). Mean overall survival RN free was 2.8 years (95%CI 1.8-5.4). Conclusion RN is related with size, SF-SRS and previous WBRT. The more lesion size, the more risk of develop symptomatic RN. More experience is needed to determine the relation between V10-12Gy and the RN appearance. PD-0803 LINAC-Hypofractionated Stereotactic Radiation Therapy in Five Fractions for Vestibular Schwannoma A. Gawish 1 , A. Mpotsaris 2 , I. Sandalcioglu 3 , K.P. Stein 4 , T. Brunner 5 , B. Röllich 5 1 1Medical Faculty University Hospital Magdeburg, University Clinic for Radiation Therapy, Magdeburg, Germany; 2 Medical Faculty University Hospital Magdeburg, University Clinic of Neuroradiology, Magdeburg, Germany; 3 Medical Faculty University Hospital Magdeburg, University Clinic of Neurosurgery, Magdeburg, Germany; 4 Medical Faculty University Hospital Magdeburg,, Clinic for Neurosurgery, Magdeburg, Germany; 5 Medical Faculty University Hospital Magdeburg, University Clinic for Radiation Therapy, Magdeburg, Germany Purpose or Objective To analyze the long-term results of LINAC hypofractionated stereotactic radiation therapy (SFRT) applied in five fractions for vestibular schwannomas (VS) Materials and Methods One hundred eleven patients with vestibular schwannomas underwent medical treatment of hypofractionated stereotactic radiation therapy. The median follow-up time interval was 52 months (range 6–121 months). All patients had a prescribed dose of 22 Gy in five fractions to D90. Re-staging was performed by thin-slice contrast-enhanced T1-MRI. Progression was defined as ≥2 mm post-treatment tumor enlargement. Progression or death for any reason was counted as an event in progression-free survival rates. Acute toxicity was defined as adverse events occurring within 3 months of HSRT, and long-term toxicity was defined as such events occurring after 3 months. Results In 58/111 patients (52%) the SFRT resulted in partial or complete remission, mean time interval for response in 50 % of them was 4 years, while in 42 patients (38%) VS failed to show any remission, resulting in stable disease. 4/111 patients (3.6%) showed marked progressive VS after treatment in the first 3 years, 2 of them received conventionally fractionated radiotherapy. Seven were lost to follow-up. The median planning target volume was 2.1ml (range 0.78- 8.66) The 5 and 7-year progression-free survival rates were 95 and 69%, respectively. No patient reported a marked improvement or deterioration with hearing ability. Post-radiotherapy MRI showed variability in edema collection, but no patient suffered from radio-necrosis. Grade 2 temporary facial nerve disorders were observed in 5 patients (4.5%) 3-9 months after SFRT.