Pathophysiological aspects of malignant brain tumors studied with positron emission tomography

Abstract

To further understand the control of brain tumor fluid balance and pH, the following studies were undertaken. The transport of a water soluble molecule across the brain and tumor capillary endothelium was studied during glucocorticoid and radiation treatment. The brain and brain-tumor acidity (pH) was evaluated as a single measurement in patients receiving a low maintenance dose of glucocorticoid. Transport changes and pH were measured in 61 patients with cerebral tumors using 82Rubidium (82Rb) and 11C-Dimethyloxa-zolidindione (11C-DMO), respectively, and Positron Emission Tomography (PET). Supplementary studies of tumor and contralateral brain blood flow and blood volume using the C15O2/PET and C15O/PET technique, respectively, were included to validate the 82Rb/PET model and obtain further information. A total of 125 PET scans were performed. Supplementary studies were undertaken to estimate delay of blood registration and form distribution of arterial blood isotope activity curves. Blood-to-tumor barrier transport was outlined at baseline and at 6 and 24 hours after the start of glucocorticoid treatment, finding a significant decrease in the transport. Radiation treatment (2-6 gray) did not alter the blood-to-tumor barrier transport when restudied within one hour in patients receiving glucocorticoid. In accordance with others, we observed pH values in gray and white matter in the range of 6.74-7.09 and 6.77-7.03 respectively. The pH in brain tumors was as high as 6.88-7.26, suggesting that tumors are more alkalotic than the normal brain. The permeability surface area product and the permeability coefficient were determined from the 82Rb/PET transport and C15O2/PET flow studies. Baseline permeability values were comparable to the literature values both for 82Rb and potassium. No difference in tissue blood volume was seen between 82Rb/PET and C15O/PET models and was of the same magnitude in the tumor and the contralateral tissue. The pH and fluid control in human brain tumors are perceived as metabolically controlled rather than, as previously believed, a result of simple passive exchange of alkalotic or osmotic active molecules between plasma and tumor interstitial space. Aspects of tumor alkalosis, tumor edema production, glucocorticoid edema clearance, and relationship between the anti-edema effect of glucocorticoid and the shown transport changes to 82Rb will be reviewed in the light of metabolic control mechanisms.