**10.3 Background radioactivity**

A high background level of radiation due to the presence of radioactivity in normal tissues reduces the tumor/background ratio which reduces the success of diagnosis and therapy. Tumor/background ratio may be diminished by the following factors: (1) the relative long circulation time of nonlocalized murine immunoglobulin in human beings; (2) binding of radiolabeled antibodies to antigens, released by the tumor in the blood pool; (3) the presence of free radionuclides and the subsequent accumulation in kidneys, bladder and other tissues, (4) non-specific uptake antibody by binding Fc fragment of antibody to normal tissues; (5) phagocytosis of murine immunoglobulin especially in the liver; (6) the presence of immune complexes due to the reaction of labelled antibody with Fc cell surface receptors, if non-fragmented immunoglobulins are used and (7) unconjugation of radio-isotope from antibody in the patient body (Sergides et al., 1999).
