The requirement of the Ionising Radiation (Medical Exposure) Regulation 2000 [IR(ME)R] of justifying all exposures to ionizing radiation includes those from radiotherapy double exposure portal images resulting in exposure to normal tissues outside the treatment volume. Typical effective doses were calculated for a range of common sites using CT data to outline those parts of specific organs subject to concomitant radiation and generate dose-volume histograms. The product of the mean dose and the relative probability of inducing a fatal cancer in specific organs was used to determine a representative total effective dose in mSv per monitor unit for each site. A table of representative effective doses, ranging from 0.32 mSv to 2.56 mSv per monitor unit, was produced, which may be used to monitor cumulative effective doses of individual patients from double exposure portal images, in addition to those received from localization procedures.