Background and purpose: We evaluated dose distributions in the anorectum and its relation to acute gastrointestinal toxicities using dose surface maps in an image-guided (IG) IMRT and 3D-conformal radiotherapy (3D-CRT) population. Material and methods: For patients treated to 78 Gy with IG-IMRT (n = 260) or 3D-CRT (n = 215), for whom acute toxicity data were available, three types of surface maps were calculated: (1) total anorectum using regular intervals along a central axis with perpendicular slices, (2) the rectum next to the prostate, and (3) the anal canal (horizontal slicing). For each toxicity, an average dose map was calculated for patients with and without the toxicity and subsequently dose difference maps were constructed, 3D-CRT and IG-IMRT separately. P-values were based on permutation tests. Results: Dose distributions in patients with grade >= 2 acute proctitis were significantly different from dose distributions in patients without toxicity, for IG-IMRT and 3D-CRT. At the cranial and posterior rectal site, in areas receiving moderate dose levels (approximate to 25-50 Gy), dose differences up to 10 Gy were identified for IG-IMRT. For pain, cramps, incontinence, diarrhea and mucus loss significant differences were found as well. Conclusions: We demonstrated significant relationships between acute rectal toxicity and local dose distributions. This may serve as a basis for subsequent dose-effect modeling in IG-IMRT, and improved dose constraints in current clinical practice. (C) 2015 Elsevier Ireland Ltd. All rights reserved.