USE OF BREMSSTRAHLUNG RADIATION TO MONITOR Y-90 TUMOR AND WHOLE-BODY ACTIVITIES DURING EXPERIMENTAL RADIOIMMUNOTHERAPY IN MICE

Background. Large differences in uptake between tumors, even for the s ame size, frequently observed in clinical and experimental radioimmuno therapy (RAIT), make monitoring of uptake in individual tumors imperat ive in comparing protocols. Y-90, widely-used for RAIT, emits no gamma radiation and absorption of the beta particle in tissue makes its det ection unsuitable for in vivo monitoring. We tested whether bremsstrah lung radiation, produced when betas are decelerated by nuclei, could b e used to monitor tumor uptake. Methods. Subcutaneous human LS174T col on carcinoma tumors were grown in the upper thigh of nude mice and lab eled antibody injected intracardially. With the tumor placed in the 2 cm-diameter aperture in a lead collimator, photons with energies from 100 to 200 keV transmitted through plastic, which absorbed the beta pa rticles, were counted to maximize shielding from the rest of the body. The contribution of the normal tissues was subtracted by counting the non-tumor-bearing leg in the same position. Excretion was calculated from whole body activity determined by removing the collimator, placin g the mouse in a syringe surrounded by tissue-equivalent material 10 c m from the detector, and counting photons between 200 and 740 keV to m inimize the effect of tissue attenuation. Results. For tumors larger t han 0.14 gm, a good correlation was obtained between the in vivo brems strahlung measurements and the measurements on excised tumors in a cal ibrated well counter. Similar excretion rates observed in all the anim als suggested that the whole body counting was accurate. Conclusions. Bremstrahlung detection appears feasible and reliable for monitoring b oth tumor and whole body activities.