Influence of Dose Distribution in the Target on the Values of Integral Radiobiological Criteria Based on Synthetic Dose-Volume Histograms

Abstract

Purpose: To investigate the influence of absorbed dose distribution in the tumor volume on the values of
 integral radiobiological criteria, such as the probability of local tumor control (Tumor Control Probability, TCP) and normal tissue complication probability (NTCP), based on synthetic dose-volume histograms (DVH), which enables to assess the treatment effectiveness and probability of complications
 from healthy organs and tissues.
 Material and methods: Synthetic dose-volume histograms were considered theoretically possible to analyze the dependence of equivalent uniform dose (EUD) on values of Nimirko model parameters and absorbed dose distribution for both tumor and critical organs. The calculation was carried out in Wolfram
 Mathematica program based on the algorithm, where an array of absorbed dose values per fraction of
 irradiation volume with step by absorbed dose was calculated for the given values of absorbed dose and
 the parameter determining the width of distribution. The resulting array was differentiated to obtain
 the differential DVH. The EUD values were obtained from the normalized differential DVHs.
 Results: During analysis of EUD value dependence on parameter a values determining degree of nonlinearity of absorbed dose summation in sub-volumes of irradiation and on absorbed dose distribution
 the following was revealed: for critical organs at high values of parameter a the essential role belongs to
 high and average values of absorbed dose, and for tumors at low values of parameter a the main role belongs to the minimum value of absorbed dose. Thus, the value of parameter a should be negative for a
 tumor, and positive for critical organs, and should be specifically defined for specific structures.
 Conclusion: To improve the predictive value (prediction) of the effect of radiotherapy using the
 TCP/NTCP Nimirko model, an adequate definition of the parameter a, the value of which can only be determined from absorbed dose distributions over the volume of anatomical structures within clinical trials, is necessary.