ESTRO 2021 Abstract Book

S992

ESTRO 2021

Purpose or Objective Regarding the widespread use of high-resolution brain imaging, there has been some controversy over the efficacy of prophylactic cranial irradiation (PCI) in small cell lung cancer (SCLC). This study evaluated the differential prognostic impact of PCI according to limited and extensive-stage groups without intracranial

metastases at baseline. Materials and Methods

We analyzed the retrospective multicenter cohort data of the Korean Association for Lung Cancer Registry, which included approximately 10% of newly diagnosed lung cancer cases from 52 institutions in Korea. Limited or extensive-stage SCLC cases with negative pretreatment brain imaging were identified from the cohort patients registered in 2014 ‒ 2015, and clinical follow-up continued until December of 2018. A total of 229 SCLC patients were analyzed. Results Of the 107 and 122 patients with limited and extensive-stage disease, 26 (23%) and 17 (10%) patients received PCI, respectively. The two-year rates of overall survival (OS) with and without PCI were 50% and 29% in the limited-stage group ( P = 0.018), and 12% and 13% in the extensive-stage group ( P = 0.336), respectively. After adjusting for other covariates, PCI administration was associated with improved OS in the limited-stage group (hazard ratio 2.33; 95% confidence interval 1.24 ‒ 4.39). In time-course hazard rate function plots of all-cause mortality of the limited-stage group, the patient subset without PCI showed a greater short-term risk increment over a one-year follow-up period. Conclusion Without brain metastases at baseline, patients with limited-stage SCLC would benefit from PCI. However, routine administration of PCI is unnecessary in extensive-stage SCLC. This study provides insights into the role of PCI in the real-world clinical setting. PO-1197 Plan complexity as an independent outcome predictor of lung cancer patients treated with SBRT C. Li 1 , J. Shen 1 , A. Shi 1 , Y. Zhang (Corresponding Author) 1 1 Key Laboratory of Carcinogenesis and Translational Research (Ministry of Education/Beijing), Department of Radiation Oncology, Peking University Cancer Hospital & Institute, Beijing 100142, China, Radiation Oncology, Beijing, China Purpose or Objective Respiratory motion and high dose rate using flattening filter free (FFF) beams for lung stereotactic body radiation therapy (SBRT) may increase MLC interplay effect and delivery uncertainties especially for complex plans involving small fields. However, potential impact of plan complexity on the patient outcomes was not proved by clinical data, which is the purpose of this study. Materials and Methods Table 1 shows clinical details of 132 plans from 122 patients that were retrospectively included. High- complexity (HC) and low-complexity (LC) groups were statistically stratified according to plan averaged beam modulation (PM) and plan averaged beam irregularity (PI). PM and PI were defined in Figure 1: Eq. (1-2) and Eq. (3-5) respectively. The cutoffs were calculated using R-3.6.1 package based on the significance of correlation with survival time. Patient outcomes were evaluated using local recurrence-free survival (LRFS). Propensity-score-matched (PSM) pairs were generated to reduce bias. Random Survival Forest (RSF) was used to evaluate the importance of plan complexity on survival predictions. Table 1 Clinical characteristics of the cohort SBRT plans (n=131) Age 68 (62-77) Gender Male 76 (58%) Female 55 (42%) Pathology SCC 22 (16.8%) AD 92 (70.2%) NSCLC 11 (8.4%) Others 6 (4.6%) Stage Ⅰ 100 (76.3%) Ⅱ 4 (3.1%) Ⅳ 27 (20.6%) Treatment duration (days) 10 (7-11) Treatment fractions 8 (5-8) PTV mean dose (Gy) 60.4 (55.1-64.5) PTV volume (cm3) 32.5 (21-50.2) BED10 116 (110.7-120) Tumor’s location Central 13 (9.9%)