In its Report 24 on ‘Determination of Absorbed Dose in a Patient Irradiated by Beams of X or Gamma Rays in Radiotherapy Procedures’, the International Commission on Radiation Units and Measurements (ICRU)  concluded that “although it is too early to generalize, the available evidence for certain types of tumour points to the need for an accuracy of ±5% in the delivery of an absorbed dose to a target volume if the eradication of the primary tumour is sought”. The ICRU continues, “Some clinicians have requested even closer limits such as ±2%, but at the present time (in 1976) it is virtually impossible to achieve such a standard”. These statements were made in a context where uncertainties were estimated at the 95% confidence level, and have been interpreted as if they correspond to approximately two standard deviations. Thus the requirement for an accuracy of 5% in the delivery of absorbed dose would correspond to a combined uncertainty of 2.5% at the level of one standard deviation. Today it is considered that a goal in dose delivery to the patient based on such an accuracy requirement is too strict and the figure should be increased to about one standard deviation of 5%, but there are no definite recommendations in this respect.1 The requirement for an accuracy of ±5% could, on the other hand, also be interpreted as a tolerance of the deviation between the prescribed dose and the dose delivered to the target volume. Modern radiotherapy has confirmed, in any case, the need for high accuracy in dose delivery if new techniques, including dose escalation in 3-D conformal radiotherapy, are to be applied. Emerging technologies in radiotherapy, for example modern diagnostic tools for the determination of the target volume, 3-D commercial treatment planning systems and advanced accelerators for irradiation, can only be fully utilized if there is high accuracy in dose determination and delivery.
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