Simple methods for determining the accuracy of tumor blood flow measurements using radioactive microspheres in rats

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Two simple methods are presented which allow positive identification of the accuracy and precision of the microsphere technique and a quick verification of sphere entrapment in tumor vessels. A known flow of Ringer's solution from a motor-driven syringe is perfused through the rat's isolated systemic circulation from left ventricle to right atrium and collected in a funnel. Using this preparation, total blood flow in rats measured with radioactive microspheres injected into the left ventricle was 97% of actual flow. The coefficient of variation (standard deviation/mean) of the microsphere measurements was 0.22. In the same preparation, nonentrapment of microspheres in subcutaneous tumor nodules grown on a hind limb could be measured from the difference in counts collected in venous effluent before and after placement of a tourniquet proximal to the tumor. For example, in two types of transplantable carcinoma, we found nonentrapment of less than 0.1% of the injected microspheres. Such a shunt would correspond to less than 10% of microspheres entering a typical tumor nodule and, in turn, less than 10% underestimation of true flow to the tumor. These two techniques may be helpful to other investigators in testing the accuracy of microsphere methods in various small animal tumor models.