Relationships between tumor size and curability for uniformly targeted therapy with beta-emitting radionuclides

Targeted radionuclide therapy is a new form of radiotherapy that differs in some important respects from external beam irradiation. One of the most important differences is due to the finite range of ionizing beta particles emitted as a result of radionuclide disintegration. The effects of particle range have important implications for the curability of tumors.

We used a mathematical model to examine tumor curability and its relationship to tumor size for 22 beta-emitting radionuclides that may have therapeutic potential. The model assumed a uniform distribution of radionuclide throughout.

For targeted radionuclide therapy, the relationship between tumor curability and tumor size is different from that for conventional external beam radiotherapy. With targeted radionuclides, there is an optimal tumor size for cure. Tumors smaller than the optimal size are less vulnerable to irradiation from radionuclides because a substantial proportion of the disintegration energy escapes and is deposited outside the tumor volume.

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 199Au and 33P to several centimeters for long-range emitters such as 90Y and 188Re. The energy emitted per disintegration may be used to predict optimal cure size for uniform distributions of radionuclide.