Kinetic Analysis with Probability Mapping in Radioimmunoscintigraphy

Radioimmunoscintigraphy, RIS, is currently undertaken with, by nuclear medicine standards, a poor radiopharmaceutical since usually less than one per cent of the administered activity is taken up by the tumor. This may be compared with over 70% of the administered Tc-99m labeled tin colloid taken up by the liver during a liver scan. The requirement is to detect small tumor recurrences, firstly since large tumors are either clinically or radiologically apparent so the extra test using RIS does not contribute to clinical management, and secondly since radioimmunotherapy appears efficacious only in patients whose tumor recurrences are under 2 cm in size (see Kalofonos and Epenetos, This book, Part VIII). Some of the physical problems that arise when attempting to image small tumors have been outlined by Britton and Granowska (This book, Part II). The prime importance of a good signal is stressed so the Poisson noise inherent in the signal is reduced. However, for a given signal, tissue and blood background activity due to the radiopharmaceutical contributes a further source of noise. While reduction of this noise in the tumor environment may be obtained by more rapid clearance of unbound antibody through the use of F(ab′)2 fragments or second antibody clearance techniques, and/or by the use of single photon emission tomography, these approaches also have inherent problems and sources of error. An alternative is to reduce the effect of tissue background activity by some form of subtraction technique.