Background: In order to improve the evaluation of possible rectal toxicity based on the rectal normal tissue complication probability (NTCP), we consider the fractional dependence of the NTCP on the wall thickness (t_W) and rectal displacement (R_M). Materials and Methods: The two-dimensional NTCP model (NTCP_2D) was developed using radiotherapy plans of ten randomly selected patients with prostate cancer. The clinical rectal structures were substituted with rectal walls of cylindrical shape. To simulate full, partially-full and empty state of the rectum, three tW were generated under the conditions of same length of the rectum and same volume of the rectal wall. A threshold iso-line, NTCP_TR, was used to split the NTCP_2D field into areas: a lower risk area and a higher risk area for rectal toxicity. Two factors are introduced to help with the estimation of NTCP: a volume factor k₁ which is the ratio between the volumes of the rectal wall and the intersection of the rectal wall with the planning target volume and a probability factor k₂, which is the ratio between the area of low risk to the entire area of the NTCP_2D. Results: A correlation > 0.9 between factors k1 and k₂ was found. Conclusion: The NTCP_2D field and the ratios k₁ and k₂ can be used as a patient-specific parameters to evaluate the probability of rectal toxicity.