Stereotactic Ablative Body Radiotherapy (SABR) has become widely used radiotherapy treatment modality for many indications ranging from early stage disease to an oligometastatic setting. All this combines the use of virtually all technological advances that characterise modern radiotherapy from small conformal fields to intensity modulation and explicit accounting for motion. However, given its novelty, clinical evidence for SABR is still emerging and many clinical trails include SABR as the intervention to define (eg in dose or fractionation defining studies) or to test against other treatment modalities. In any case, physical quality assurance (QA) must be an essential aspect of the trial to ensure that SABR is delivered accurately and in a comparable fashion in radiotherapy centres participating in the trial. The present paper tries to explore the balance between the QA cost and benefit for trial conduct in terms of patient numbers and timely completion of the trial.