Evaluation of the prognostic value of the ESTRO EORTC classification of oligometastatic disease in patients treated with stereotactic body radiotherapy: A retrospective single center study

Purpose: To explore the prognostic value of the oligometastatic disease (OMD) states as proposed by the European Society for Radiotherapy and Oncology (ESTRO) European Organisation for Research and Treatment of Cancer (EORTC) classification system.

Materials and methods: This retrospective single-institution study included patients with 1-5 extracranial metastases from any solid malignancy treated with SBRT to all metastases. OMD states were defined according to the ESTRO EORTC classification. Overall survival (OS) and progression-free survival (PFS) were analyzed using the Kaplan-Meier method. Discriminatory strength of the classification was assessed by Gönen & Heller's concordance probability estimate (CPE). Univariable and multivariable Cox regression models were used to assess predictors of OS and PFS.

Results: In total, 385 patients were included. The median follow-up was 24.1 months. The most frequent OMD states were metachronous oligorecurrence (23.6%) and induced oligoprogression (18.7%). Induced OMD patients had significantly shorter median OS (28.1 months) compared with de-novo (46.3 months, p = 0.002) and repeat OMD (50.3 months, p = 0.002). Median PFS in de-novo OMD patients (8.8 months) was significantly longer than in repeat (5.4 months, p = 0.002) and induced OMD patients (4.3 months, p < 0.001). The classification system had moderate discriminatory strength for OS and PFS. Multivariable analyses confirmed that compared with induced OMD, de-novo OMD was associated with longer PFS and repeat with longer OS.

Conclusion: All patients were successfully categorized according to the ESTRO EORTC classification system. The discriminatory strength of the classification was confirmed for OMD patients treated with metastases-directed SBRT. Larger multicenter trials are needed to validate the prognostic power for OMD patients irrespective of primary tumor and treatment approach.