In order to achieve practical application for the treatment planning of radiotherapy, lung density was assessed by means of CT. Cork phantoms were measured 360 times to test the reliability of CT densitometry. Densities were corrected by the CT value of pure water measured at the same time. The mean value of cork phantoms as measured by CT was 0.287±0.11g/cm3, the true value of which was 0.320g/cm3. These errors were within an allowable limit for radiotherapy. Lung density was studied in 50 patients with a mean age of 50 years, ranging from 46 to 76. The values obtained of the upper, middle, and lower lung were 0.123±0.46 g/cm3, 0.121±0.033, and 0.154±0.057 g/cm3 during inspiration and were 0.215±0.058 g/cm3, 0.228±0.066, and 0.260±0.078g/cm3 during expiration, respectively. The dorsal, central, and ventral portions of the lung were measured simultaneously with a patient on supine position. In order to hold the variation of radiation dose within 2%, the variation must be kept within 30 HU of CT value. As for the upper lung, the difference of which between both subjects and sites was smallest during inspiration in this results, that was 20%; the difference of 60 HU CT value existed between the dorsal and ventral lungs. Variations in CT number between subjects were more than 60 HU in one third of this series. Before starting radiotherapy to the lung field, lung densitometry is warranted in order to irradiate the exact dose.
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