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Oligorecurrent disease (ORD) refers to the development of oligometastatic disease (OMD) in a systemic therapy-free interval [2]. For patients with ORD, systemic therapy-free survival (STFS), i.e. deferral of systemic therapy after local ablative therapies such as stereotactic body radiotherapy (SBRT), has been recognized as a clinically meaningful endpoint [3]. Here, we evaluate patterns of STFS in ORD patients after SBRT.

This retrospective study included all patients treated with SBRT only (no systemic therapy 1 months before diagnosis of ORD and minimum 1 month after SBRT) for 1-5 extracranial oligorecurrent metastases from any solid malignancy at the University Hospital Zurich between 01/2014 and 12/2019. OMD states were defined according to the ESTRO EORTC classification [2]. STFS was defined as the time from SBRT to the start of any systemic therapy or death. STFS was analysed using the Kaplan-Meier method. Pairwise log-rank tests were used to determine significant differences. Univariable and multivariable analyses were performed to assess predictors of STFS.

Among 585 screened OMD patients who were treated with SBRT, 167 (29%) presented with ORD and 141 (84%) of these did not receive systemic therapy during or after SBRT. Fifty-four percent (n=76) presented with metachronous oligorecurrence, 38% (n=53) with repeat and 9% (n=12) with induced oligorecurrence. Lung cancer was the most common primary tumor (n=47, 33%), followed by prostate cancer. (n=17, 12%). Patient characteristics are outlined in Table 1.

After a median follow up time of 26 months, 56% (n=79) patients had started systemic therapy. The median STFS was 23.9 months (95%CI 18.6-35.2) (Fig. 1A). Median STFS differed significantly between induced (9.9mo, 95%CI 3.7-NR), metachronous (23.5mo, 95%CI 19.1-36.2) and repeat ORD (37.0mo, 95%CI 13.8-NR) (Fig. 1B). In multivariable analysis, both repeat and metachronous ORD were significantly associated with longer STFS compared with induced ORD (repeat ORD: HR 0.32, 95%CI 0.15-0.70, p=0.004; metachronous OPD: HR 0.45, 95%CI 0.21-0.94, p=0.03). The number of previous lines of systemic therapy (0 vs. 1 or more) and involved organs (single vs. multiple) were also significantly associated with STFS (Tab. 2).

Table 1: Patient characteristics.

Patients treated with SBRT with ORD can achieve long-term STFS. SBRT for ORD might be an alternative treatment in patients who either cannot receive systemic therapy due to comorbidities or to defer systemic therapy in patients with a perceived low cancer activity. However, the impact on overall survival remains uncertain. Prospective randomized trials are needed to determine if a holiday from systemic therapy after SBRT is safe and feasible for ORD patients.