A New Approach to Computing Normal Tissue Complication Probability of an Intensity-Modulated Radiotherapy Treatment with Stereotactic Radiotherapy Boost of Nasopharyngeal Carcinoma: A Case Study

We attempted to develop a method to compute the normal tissue complication probability (NTCP) of various critical organs from combined intensity-modulated radiotherapy (IMRT) and stereotactic radiotherapy (SRT) boost treatment of nasopharyngeal carcinoma (NPC), with the aid of a nonlinear image registration method. The SRT's planning computed tomography (CT) of a NPC patient treated with IMRT was warped to the IMRT's planning CT using a nonlinear image registration. Because CT and dose were inherently in-register, the entire dose distribution could be deformed using the same deformation field derived from the two CT sets. Using the biologically effective dose concept and the linear-quadratic model, physical doses of IMRT and SRT were converted to a 2Gy-per-fraction equivalent dose to facilitate dose summation. The variation of organs' maximum doses of the combined treatments between traditional maximum dose sum and the proposed method was 1.5Gy ± 1.7Gy. After the correction of the effect of fractionation and dose heterogeneity within each organ, NTCP of each organ of interest was computed for the combined treatments. Based on the results of this case study, it is believed that dose registration could be a method for the NTCP computation of various critical organs when different fractionation schemes of radiation therapy treatment are instituted.