In order to reduce side effects of high-dose chemotherapy the penetration of the cytostatic substances into the main circulation should be prevented or limited. An efficient procedure for checking the leakage is generally based on the addition of a radionuclide to the perfusate. By scintillation probes the distribution of this radionuclide in the body of the patient is checked. A method is presented which continuously calculates the actual volume of the fluid which is penetrating into the main circulation during the perfusion. The values of momentary and cumulative volume crossover enables the surgical oncologist to detect a leakage readily and reliably and may be the basis for measures of confining it.