New treatments are usually thoroughly evaluated before they enter clinical practice, but much of what doctors do is based on experience rather than evidence. However, just because a practice is widely accepted within the profession does not guarantee that it is effective; there are many historical examples, such as the practice of copious and repeated blood letting, which persisted from antiquity well into the mid-19th century.1 In recognition of this, last year the UK National Institute for Health and Clinical Excellence (NICE) launched an initiative to identify interventions delivered by the NHS that do not benefit patients.2

Of course, to reject the experience and insight of generations of clinicians as “low grade evidence” and subject everything we do to randomised controlled trials would be hugely wasteful and impractical. Many existing practices should be retained and new ideas introduced because the benefits are large and evident without further study.

The question that arises is what standard of evidence we are willing to accept to persist with a particular practice. Anyone proposing a trial or a cost effectiveness study of treatment of broken legs or cataracts would quite rightly have their calculator and clipboard taken from them. However, for conditions such as cancer, the benefit is less clear cut: practice is justified in the belief that the dying die more slowly than would have been the case and that a handful may live to die of something else. In some areas of our practice we have little evidence on which to base this belief. We discuss one example, resection of lung metastases in patients who have had surgery to remove …