ESTRO 2021 Abstract Book

S988

ESTRO 2021

Conclusion iRT should be administered with caution in patients undergoing treatment to PTVs localized in the thorax due to the higher risk of lung toxicity. The abscopal effect is rare and difficult to detect. However, it should be pursued during follow-up. PO-1192 Safety analysis of Stereotactic Body Radiotherapy 8 fractions of 7,5 Gy for Ultracentral Lung Tumors E. Baena 1 , A. Mañes 1 , M. Villalobos 1 , B. Gutiérrez 1 , A. Arellano 1 , S. Moreno 1 , E. Luguera 2 , J. Molero 2 , A. Melero 2 , S. Villà 1 , J. Jové 1 1 Catalan Institute of Oncology, Radiation Oncology, Badalona (Barcelona), Spain; 2 Catalan Institute of Oncology, Medical Physicist, Badalona (Barcelona), Spain Purpose or Objective Concept of ultracentral lung tumors (UCLT) has recently appeared to define a higher risk subgroup of central lung tumors (CLT) whose planned target volume (PTV) is overlapping with proximal mediastinal structures (as defined in the ongoing phase I SUNSET trial). Optimal dose fractionation for stereotactic body radiotherapy (SBRT) to UCLT is still not clear at SBRT guidelines. Our center's protocol recommends a more protracted regimen of 60 Gy in 8 fractions of 7.5 Gy for CLT and UCLT with no restrictions regarding Organs at Risk (OAR) which could compromise target volume coverage. Our objective is to describe clinical outcomes and toxicity in this subgroup of patients treated at our institution. Materials and Methods We have retrospectively reviewed outcomes of 41 patients (pts), 9 women and 32 men, with a median age of 75 years (range 56-89), treated with SBRT for CLT (defined as tumors located within 2 cm of the proximal bronchial tree) and UCLT (if its PTV overlaps with a mediastinal structure). Patients were treated between July 2013 and July 2020. Thirty-six pts (87.8%) were smokers or ever-smokers with compromised respiratory function (median FEV1: 59%). Histological diagnosis was unknown for 20 pts (48.7 %) and 6 (14.6%) were metastases of another origin. Total dose (TD) was 60 Gy en 8 fractions of 7.5 Gy in all cases. Median ITV volume was 11.1 cc (range 0.8-112 cc). Optimal PTV coverage was achieved in 40 pts with median D95 of 100% TD and median D2 of 108.23% TD. Acute and late toxicities were graded according the common terminology criteria for adverse events (CTCAE v 5.0). Local progression was defined as confirmed growth of irradiated lesion from the date of end SBRT. Results Forty-one lesions were treated, 17 (41.4%) CLT and 24 (58.5%) UCLT. The median follow up was 23 months. Median Overall Survival was 35 months. Local control (LC) rates at 1 year, 2 years and 3 years were 97.1, 75.7 and 68.1% respectively. No statistically significant differences in LC between CLT and UCLT were found. Only 2 pts developed significant toxicity, a grade 5 esophageal perforation and a grade 3 hemoptysis. Conclusion SBRT in 8 fractions of 7.5 Gy for UCLT is safe even without compromising PTV dose coverage and achieves the expected good results of LC. Nevertheless, there is no significant evidence, but it seems to be some tendency to worse local control in UCLT (*see chart 2)