Statistically significant differences have been reported in prostate plans at four cancer centres that followed the prostate fractionated irradiation trial protocol. When each centre was compared with the other three, significant differences in plan quality metrics were observed for the bladder wall, the left and right femoral heads and clinical target volume. However, analysis of tumour control probability as well as rectal wall and bladder wall normal tissue complication probabilities suggested that the clinical significance of these differences is minimal. Local planning practices were partly responsible for the observed differences. Thus, these results could be used as a reference for the acceptable degree of variation between centres when a common protocol is adopted.