GTV, CTV and PTV (target volumes)

Imaging the prostate before every session is now standard, and this trial supports it for rectal side effects and PSA control. It also shows how hard it is to turn a physical gain in accuracy into a survival result, and that a randomised trial can throw up an unexplained difference that should be reported rather than smoothed over.

Variation between clinicians in drawing the target is one of the largest errors in radiotherapy and one of the least measured in terms of what it does to patients. The field has studied whether outlines differ far more than whether the differences matter.

With image guidance, a larger safety margin around a brain metastasis did not buy better tumour control and added healthy brain to the high-dose region. It is one of the few randomised tests of a margin itself.

In the recipe a systematic error, one repeated at every session, costs about three and a half times as much margin as a random error of the same size, so reducing systematic error is the larger lever for shrinking the margin.