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Radiotherapy (RT) is a highly effective and potentially curative treatment used in many solid cancers for local control and for palliative purposes. However, a body of research has emerged over the last few decades, across numerous cancer types that point to the potential for RT to promote metastatic spread. Numerous mechanisms have been implicated in this response including selective pressure on surviving cancer cells, increase of circulating tumour cells (CTCs), modulation of inflammatory and stress signalling, epithelial to mesenchymal transition, induction of actin-myosin contractility and promotion of an invasion-permissive tumour stroma.

Despite the growing array of preclinical evidence, meaningful interrogation of the impact of RT on metastasis in the clinic is sparse, and the subject remains a contentious issue. The lack of clinical data to support RT induced metastasis potentially invokes a false sense of security- we don’t see it, so why does it matter? Of course gathering data to support or refute this view point is inherently difficult, considering the ubiquitous use of RT as standard of care in many cancers plus the different delivery modalities and varying treatment protocols used. This lecture will explore the biology behind the pro-metastatic effects of RT, whether the weight of pre-clinical evidence is enough to indicate clinical relevance, and finally the potential new therapeutic window that it presents.