Background and purpose: To predict the lowest achievable rectum 035 for quality assurance of IMRT plans of prostate cancer patients. Materials and methods: For each of 24 patients from a database of 47 previously treated patients, the anatomy was compared to the anatomies of the other 46 to predict the minimal achievable rectum D35. The 24 patients were then replanned to obtain maximally reduced rectum D35. Next, the newly derived plans were added to the database to replace the original clinical plans, and new predictions of the lowest achievable rectum D35 were made. Results: After replanning, the rectum D35 reduced by 9.3 Gy +/- 6.1 (average +/- 1 SD; p < 0.001) compared to the original plan. The first predictions of the rectum D35 were 4.8 Gy +/- 4.2 (average +/- 1 SD; p < 0.001) too high when evaluated with the new plans. After updating the database, the replanned and newly predicted rectum D35 agreed within 0.1 Gy +/- 2.8 (average +/- 1 SD; p = 0.89). The doses to the bladder, anus and femoral heads did not increase compared to the original plans. Conclusions: For individual prostate patients, the lowest achievable rectum D35 in IMRT planning can be accurately predicted from dose distributions of previously treated patients by quantitative comparison of patient anatomies. These predictions can be used to quantitatively assess the quality of IMRT plans. (c) 2013 Elsevier Ireland Ltd. All rights reserved.