A variable critical-volume model for normal tissue complication probability

Predicting late-term normal-tissue complication probability (NTCP) after radiotherapy is an important factor in the optimization of conformal radiotherapy. We propose a new NTCP model, based on the properties of the high dose region. The principal assumption of the new model is that a whole-organ complication will occur when the radiation damage to a normal organ volume (a portion of the total organ) exceeds a threshold value. The dose threshold for complications varies with the size of the volume (percent of the total organ). We hypothesize that a complication occurs if the complication threshold is exceeded for any organ volume. We used the average dose to a volume as a measure of radiation damage to that volume. Also, we used the power law to scale the average dose to various organ volumes to a whole-organ equivalent dose, and to identify the volume with the most harmful dose-size combination-the critical volume. We used a logistic distribution to calculate the probability that the patient will develop a complication, given the dose delivered to the critical volume. We used a maximum likelihood fit to estimate the model parameters for late-term rectal complications in a set of patients treated for prostate carcinoma with external photon beam radiotherapy (EBRT). Good correspondence was found between the experimental data and the model predictions. (C) 2001 American Association of Physicists in Medicine.