ESTIMATION OF THE SPATIAL-DISTRIBUTION OF TARGET-CELLS FOR RADIATION PNEUMONITIS IN MOUSE LUNG

Purpose: Recent studies of Liao et al, and Travis et al, have demonstr ated that irradiation of cross-sectional partial volumes contiguous to the base of mouse lung produces a higher incidence of pneumonitis tha n irradiation of equally sized subvolumes contiguous to the apex, One interpretation of this finding is that the critical target cells for p neumonitis are not distributed uniformly throughout the lung. The purp ose of the present study was to test this interpretation by obtaining an estimate of the spatial distribution of the critical target cells f or pneumonitis in mouse lung, based on the partial-volume data, Method s and Materials: A mathematical model was derived describing the proba bility. of radiation pneumonitis as a function of dose, volume of lung irradiated, and location in the lung of the irradiated subvolume, The model includes a nonparametric description of the spatial target-cell distribution in lung, to be estimated from data. The model was fitted to the lethality data of Liao et al, and Travis et al. to obtain esti mates of the proportion of target cells contained in each of various s ubvolumes of the lung, Results: The results indicate that the critical target cells in mouse lung are concentrated in the base and, to a som ewhat lesser extent, in the apex of the lung, with fewer cells in the middle region, The estimated spatial distribution of target cells in m ouse lung agrees well with the distribution of alveoli, whose concentr ation in the midlung is limited by the presence of the major conductin g airways, Conclusion: Heterogeneity in the spatial distribution of cr itical target cells in normal tissue implies that the complication pro bability (NTCP) depends on the location in the organ of the irradiated subvolume, as web as on radiation dose and subvolume size, Calculatio ns using an NTCP model for mouse lung indicate that irradiation of equ al subvolumes of lung with the same dose can lead to complication prob abilities covering the full range from 0 to 100%, depending only on th e location in the lung of the irradiated subvolume. (C) 1997 Elsevier Science Inc.