Edema and tumor perfusion: characterization by quantitative 1H MR imaging.

Tumor perfusion is of central importance to the clinical oncologist because it has a direct effect on the success of cancer therapy. Yet determining whether a tumor is well or poorly perfused is difficult without the use of invasive techniques, because the variables that affect tumor perfusion are poorly understood. Quantitative MR imaging of tumor edema may provide a means of characterizing tumor perfusion, of studying heterogeneity of perfusion within the tumor mass, or of monitoring changes in tumor perfusion after therapy. A combination of factors often results in production of a large amount of edema within cranial or extracranial tumors. Any tumor that is encapsulated, whether by a fibrous tumor capsule or by a structure such as the cranium, will have an elevation in interstitial fluid pressure because dissipation of fluid is hindered. Elevated pressure of interstitial fluid acts to occlude tumor capillaries, so edema can cause a striking reduction of tumor perfusion. Because MR imaging can potentially be used for quantitative imaging of tumor edema, it may provide a means of indirectly measuring tumor perfusion. A review of the literature suggests that diffusion-weighted MR imaging may be better than T1- or T2-weighted MR imaging for quantitative imaging of tumor edema. I do not propose that diffusion-weighted imaging can measure perfusion directly; rather I hypothesize that a diffusion-weighted image can be correlated with tumor edema. Because edema indirectly regulates perfusion through the mechanism of interstitial fluid pressure, I propose an indirect correlation between the diffusion-weighted image and regional tissue perfusion. If the relationship between tumor perfusion and the pharmacokinetics of chemotherapeutic agents is better understood, MR imaging of tumor edema may even aid in predicting the delivery of drugs to a tumor.