RELATIONSHIPS BETWEEN TUMOR SIZE AND CURABILITY FOR UNIFORMLY TARGETED THERAPY WITH BETA-EMITTING RADIONUCLIDES

Targeted radionuclide therapy is a new form of radiotherapy that diffe rs in some important respects from external beam irradiation. One of t he most important differences is due to the finite range of ionizing b eta particles emitted as a result of radionuclide disintegration. The effects of particle range have important implications for the curabili ty of tumors. Methods: We used a mathematical model to examine tumor c urability and its relationship to tumor size for 22 beta-emitting radi onuclides that may have therapeutic potential. The model assumed a uni form distribution of radionuclide throughout. Results: For targeted ra dionuclide therapy, the relationship between tumor curability and tumo r size is different from that for conventional external beam radiother apy. With targeted radionuclides, there is an optimal tumor size for c ure. Tumors smaller than the optimal size are less vulnerable to irrad iation from radionuclides because a substantial proportion of the disi ntegration energy escapes and is deposited outside the tumor volume. C onclusion: We found an optimal tumor size for radiocurability by each of the 22 radionuclides considered. Optimal cure diameters range from less than 1 mm for short-range emitters such as Au-199 and P-33 to sev eral centimeters for long-range emitters such as Y-90 and Re-188. The energy emitted per disintegration may be used to predict optimal cure size for uniform distributions of radionuclide.