Comparison of computerized tomographic and radionuclide methods in determining intracranial cystic tumor volumes.

The dosimetry of radioactive phosphorus (32P) for intracavitary treatment of cystic brain tumors is dependent upon accurate determination of the cyst volume. The authors have used both high-resolution computerized tomography (CT) scanning and an isotope dilution technique with technetium-99m (99mTc) sulfur colloid to determine cyst volumes in an experimental model and in six patients with intracerebral cystic tumors. In six separate comparisons using an experimental phantom "cyst," no significant differences between CT and 99mTc values were detected. In six patients with cystic neoplasms varying from 3 to 6.7 ml in volume, the percent differences between values obtained by CT and isotope dilution ranged from -17.26% to +13.13%. In individual patients, these differences proved to be of little significance for planning dosimetry. In both experimental and clinical trials comparing isotope dilution and CT measurement techniques, the CT technique using the standard software of the scanner and 5-mm thick slices proved to be a highly accurate method of determining cyst volume. The authors now base dosimetric calculations for intracavitary radiation with 32P on CT technique alone.