ESTRO 2021 Abstract Book

S571

ESTRO 2021

classification to oligometastatic patients treated with stereotactic body radiotherapy (SBRT), to determine the prevalence of the proposed OMD stages and to investigate their prognostic value. Materials and Methods This retrospective study was approved by the institutional ethics board and analyzed patients treated between 01/2014 and 12/2019 at the University Hospital Zurich. Inclusion criteria were solid-organ malignancies, 1-5 metastatic lesions, age ≥18 years, and treatment of all progressive or persistent metastases with SBRT. Patients were categorized according to the OMD classification system and grouped into de-novo, repeat and induced, OMD. Time-to-event curves were generated using the Kaplan-Meier method. Pairwise log-rank tests were used to determine significant differences. All statistical analyses were performed in R (R version 4.03.0) Results 386 patients with 555 metastases were included into this study. The most frequent primary malignancies were lung cancer (n=134; 35%), prostate cancer (n=39; 10%) and malignant melanoma (n=33; 9%). All patients were successfully categorized according to the OMD classification system: patients were classified as de-novo, repeat or induced OMD in n=187 (48%), n=102 (26%) and n=97 (25%) patients, respectively. Table 1 summarizes the patient characteristics (Table 1). After a median follow-up time of 24.0 months, the median overall survival (OS) was 39.9 months for all patients. The OMD classification system was significantly associated with OS: median OS was significantly shorter for patients with induced OMD (27.1 months) compared to patients with de-novo OMD (46.3 months) (p=0.006) and compared to patients with repeat OMD (50.3 months) (p=0.0057) (Fig. 1A). OS was not different between patients with de-novo and repeat OMD (p=0.587). Median progression-free survival (PFS) in patients with induced OMD of 4.3 months was significantly shorter than 5.4 months in patients with repeat OMD (p=0.012) and 8.4 months in those with de-novo OMD (p>0.001) (Fig. 1B). Patients with de-novo OMD had significantly longer PFS than those with repeat OMD (p=0.020)