Purpose: To investigate whether irradiating small lung volumes with a large dose or irradiating large lung volumes with a small dose, given the same mean lung dose (MLD), has a different effect on pulmonary function in laboratory animals.
Methods and materials: WAG/Rij/MCW male rats were exposed to single fractions of 300 kVp X-rays. Four treatments, in decreasing order of irradiated lung volume, were administered: (1) whole lung irradiation, (2) right lung irradiation, (3) left lung irradiation, and (4) irradiation of a small lung volume with four narrow beams. The irradiation times were chosen to accumulate the same MLD of 10, 12.5, or 15 Gy with each irradiated lung volume. The development of radiation-induced lung injury for < or =20 weeks was evaluated as increased breathing frequency, mortality, and histopathologic changes in the irradiated and control rats.
Results: A significant elevation of respiratory rate, which correlated with the lung volume exposed to single small doses (> or =5 Gy), but not with the MLD, was observed. The survival of the rats in the whole-lung-irradiated group was MLD dependent, with all events occurring between 4.5 and 9 weeks after irradiation. No mortality was observed in the partial-volume irradiated rats.
Conclusions: The lung volume irradiated to small doses might be the dominant factor influencing the loss of pulmonary function in the rat model of radiation-induced lung injury. Caution should be used when new radiotherapy techniques that result in irradiation of large volumes of normal tissue are used for the treatment of lung cancer and other tumors in the thorax.