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3D conformal external beam radiotherapy (3DCRT) involves volume-based radiotherapy planning which allows for better tumour targeting. Efficacy of treatment is related to dose, the limiting factor of which is normal tissue toxicity. In thisprospective study the anorectal toxicity of high dose 3DCRT was assessed.Seventen consecutive men with localised or locally advancedprostate cancer treated with 74 Gy of 3DCRT wereprospectively studied. Pre and post treatment anorectal physiology studies included anal canal manometry, measurement of rectal distension threshold, mucosal electrosensitivity, laser doppler flowmetry, barostat studies and endoanal ultrasound.The median age and follow up of the subjects were 70 (50-79) years and 17 (14-40) weeks. Nine of 17 subjects developed new symptoms. Significant changes in anorectal physiology included an increase in mean (SD) rectal mucosal electrosensitivity threshold [from 22.9 (9.1) to 34.8 (9.1) mA (P = 0.0027)] and an increase in mean (SD) rectal elastance [from 0.0537(0.028) to 0.0781(0.039) mmHg/ml (P = 0.0181)].Over 50% of patients develop some degree of anal incontinence after treatment with 3DCRT. Physiological data suggest these symptoms are secondary to rectal toxicity with the anal sphincter complex relatively undamaged.