In-field and out-of-field effects in partial volume lung irradiation in rodents: possible correlation between early dna damage and functional endpoints.

PURPOSE: Recent observations have shown that there are regional variations in radiation response in mouse lung as measured by functional assays. Furthermore, there are both in-field and out-of-field effects in radiation-induced lung damage as observed by DNA assay in rats. The purpose of this work is: (a) to examine mice lethality data following partial volume lung irradiation to assess the possibility of directional or regional effects, (b) to evaluate the correlation between mice lethality data and DNA damage assayed by micronuclei production in rat lung, and (c) to re-interpret mice lethality considering the existence of directional effects in lung cellular response to partial volume irradiation. METHODS AND MATERIALS: The lethality data for mice, generated at the M. D. Anderson Cancer Center, Houston, and micronuclei yield data for rats obtained at Princess Margaret Hospital, Toronto, were used. A radiobiological model that allows for out-of-field and in-field effects for lung cell damage and lung response was developed. This model is based on the observation of DNA damage in shielded parts of rat lung that was assumed relevant to cell lethality and consequently overall lung response.
RESULTS: While the experimental data indicated directional or regional volume effects, the applicability of dose and volume as sole predictors of lung response to radiation was found to be unreliable for lower lung (base) irradiation in mice. This conforms well to rat lung response where micronuclei were observed in shielded apical parts of lung following base irradiation. The radiobiological model, which was specifically developed to account for the lung response outside of primary irradiated volume, provides a good fit to mice lethality data, using parameters inferred from rat micronuclei data.
CONCLUSION: Response to lung irradiation in rodents, in particular, elevated sensitivity to base irradiation, can be interpreted with a hypothesis of in-field and out-of-field effects for cellular response. If the existence of these effects for lung is subsequently proven in humans, it will require the incorporation of geometrical and directional information in normal tissue complication probability calculations for lung. These considerations are ignored in present approaches based only on conventional dose-volume histograms.