Clinical Evaluation of Stereotactic Ablative Radiotherapy for Oligometastases From Rare Primary Cancers

Stereotactic ablative body radiotherapy (SABR) has emerged as a promising treatment modality for oligometastatic disease from primary breast, prostate, lung, and colorectal cancers. However, there is a paucity of information on clinical outcomes of SABR to oligometastases from rare primary cancers (RPCs). This study aimed to report the treatment outcomes and to investigate what factors are prognostic in terms of overall survival (OS) and progression-free survival (PFS) in patients receiving SABR for oligometastases from RPCs.

All patients with oligometastases from any RPCs were included in this retrospective review of patients treated with SABR at one single institution. This cohort excluded breast, prostate, lung, colon, and rectum primary cancer. OS and PFS were calculated using Kaplan-Meier statistics and post-SABR toxicities were scored following the Common Terminology Criteria for Adverse Events (CTCAE) v. 4.0. An analysis of prognostic factors for OS and PFS was performed based on log-rank tests which were used for the analysis of prognostic factors for OS and PFS based on the site of primary cancer, previous radiotherapy status, previous systemic therapy status, the number of oligometastases, SABR treatment site, biological equivalent dose, total size of gross tumour volume, and planning target volume (PTV).

A total of 114 patients with 126 metachronous oligometastatic lesions from RPC receiving SABR were included. The median patient age when they received SABR was 66.7 years (range: 22.3-91.8 years), with the median follow-up of the cohort being 21.7 months (range: 2.8-75.8 months). The estimated median OS was 40.1 months (95% confidence interval [CI]: 27.5-52.6 months), and the estimated median PFS was 14.2 months (95% CI: 11.0-17.5 months). The treatment was well tolerated, with the majority of patients experiencing only grade 1 fatigue as the most common acute toxicity. The previous radiotherapy status (P = 0.04) and cumulative PTV (P = 0.01) were identified as statistically significant independent predictors of OS. For PFS, SABR treatment site (P = 0.03) was the only statistically significant independent predictor.

There are limited studies published on the efficacy and post-treatment toxicities of using SABR in the management of oligometastases from RPC. This study confirmed that SABR was a safe, noninvasive treatment option for patients with extracranial oligometastases originated from RPC in terms of the favourable post-treatment toxicities.