The effects of fractionated hemithorax irradiation on normal lung tissue were examined by measuring changes in the vascular permeability surface area product (PS) and relative lung blood flow in Sprague-Dawley rats. The rats received five daily fractions per week of either 3.0 or 4.0 Gy for 4 weeks to the left lung. Between 3 and 5 weeks after the start of irradiation, the average PS was approximately 50% above normal for the group of rats that received 3.0 Gy/day and 200-300% above normal in the group of rats that received 4.0 Gy/day. Treatment with cyproheptadine, indomethacin, or theophylline had no effect, but treatment with dexamethasone significantly reduced PS to near normal levels. Left-to-right blood flow ratios in the group of rats that received 3.0 Gy/day decreased to 66% of normal levels by 4 weeks. In the group of rats that received 4.0 Gy/day, blood flow decreased to 46% of normal levels by 4 weeks. Treatment with dexamethasone maintained normal blood flow until the drug dose was reduced. These results agree with earlier studies using single-dose irradiation and indicate that the methods used to measure PS and blood flow are sensitive at low doses.

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