Response assessment and surveillance following stereotactic ablative radiotherapy for lung cancer

Lung stereotactic ablative radiotherapy (SABR) is almost invariably associated with radiation-induced parenchymal injury within 3 years of treatment completion. While distinguishing normal fibrotic changes from local recurrence is challenging, accurate detection of local relapse has become increasingly crucial in the context of rapidly growing SABR patients’ population and multiplying salvage therapy options. Knowledge of the natural history of radiation-induced lung injury as well as recognition of the risk factors associated with recurrence are essential to assist timely diagnosis of recurrence while avoiding unnecessary investigations. In this review, we discuss the patterns of recurrence after SABR, the expected post-treatment radiological changes as well as the prevailing and evolving strategies to differentiate recurrence from radiation-induced lung injury. The limitations of the current response assessment methods and the promising avenues of functional imaging and radiomics are discussed. Finally, current general consensus guidelines for surveillance post-SABR for early-stage non-small cell lung cancer are summarized.