Stereotactic ablative radiotherapy (SABR) for liver metastases: where we currently stand

Stereotactic ablative radiotherapy (SABR), which is already used as the standard-of-care treatment in other cancers such as lung, is being increasingly used in the treatment of liver metastases in the oligometastatic setting in unresectable, pre-treated patients. Various phase I/II trials and retrospective series have demonstrated its efficacy in terms of local control (LC), progression-free and overall survival. Tumour characteristics of <6 cm, <3 tumours and absence of extrahepatic disease, and favourable histologies including colorectal, breast and renal primaries have been associated with favourable outcomes. Radiation dose is also important, with a biological effective dose ≥120 Gy giving optimal LC. To be determined in SABR is the exact dose and fractionation employed, which is variable without evidence from randomised controlled trials to date. The aforementioned trials have established that SABR is well-tolerated. In the short-term, it can cause gastrointestinal disturbances, transaminitis and radiation-induced liver disease. In the longer-term, gastrointestinal bleeding and bone fractures have been rarely reported, though follow-up periods for studies were limited. Other treatments such as radiofrequency ablation (RFA) and surgery are also standard-of-care. There have been no prospective randomised trials to date comparing these treatments. Smaller trials demonstrated no survival differences between SABR and RFA, though these studies favour better LC in SABR for larger tumours (>3 cm). To summarise, SABR for liver metastases leads to good LC and survival and is well-tolerated, especially with favourable tumour characteristics. Prospective studies will need to help establish its exact role in comparison with other localised therapies.