Stereotactic ablative body radiotherapy for lung cancer

Lung stereotactic ablative radiotherapy (SABR) is a method of delivering high 'ablative' doses of radiotherapy to tumours in the lung. It was developed at the Karolinska Institute in the early 1990s using the methods established in cranial radiosurgery with multiple beams prescribed to an isodose and using a custom designed stereotactic body frame for immobilisation. Since then, aligned with the advances in radiotherapy technology and techniques (e.g. four-dimensional computed tomography simulation and image-guided radiotherapy), there has been a rapid increase in the use of lung SABR for both early stage lung cancer and lung metastases. For peripheral primary lung cancers less than 5 cm in diameter, high rates of local control and low levels of acute and late toxicity are consistently reported in the published literature. Compared with historical control rates of stage I lung cancers treated with conventionally fractionated radiotherapy, SABR seems to offer higher rates of local control, lower levels of acute toxicity and a better quality of life after treatment. However, the full results of randomised controlled trials of SABR versus conventionally fractionated are awaited and will provide higher-level evidence. For central lung tumours, very high SABR doses can be associated with significant toxicity. Dose-adapted fractionation schedules seem to have much lower rates of toxicity and prospective trials, including the completed RTOG 0813 study and the on-going EORTC LUNGTEC study, should provide further evidence of safety and establish organ at risk tolerances. SABR can also be used for tumours metastatic to the lung with high rates of local control and is a reasonable alternative to surgery in selected patients. Going forward, prospective trials are underway to establish the safety and efficacy of SABR in oligometastatic disease. Population-based outcomes will be crucial in supporting/establishing SABR as the treatment of choice in medically inoperable patients with peripheral stage I lung cancers. Randomised phase III trials will hopefully extend the evidence base and show the safety and the utility of SABR in early central tumours and oligometastatic disease.