Radioisotope Choice

In considering the radioisotope for attachment to the pharmaceutical or biologic agent, the match between target residence time and biodistribution with physical half-life is important. As was mentioned earlier, residence times of the radiophar-maceutical and half-life should bear a relationship which results in maximum treatment efficacy. Often, the radiopharmaceutical will remain in the target tissues until decayed. This would be the case in thyroid treatment with I-131 sodium iodine and radiosynovectomy with P-32 colloid.

Another seemingly overlooked consideration for radionuclide choice for therapy is the underlying pathology of the disease. Different disease processes have variable sensitivity to the types and rates of radiation delivered. While many processes are highly radiosensitive, many other diseases are highly radio-resistant. They may present as solid masses such as tumors or may be distributed in thin cell layers such as over bony trabecula in the marrow, or along the meninges. These large differences in target type point to geometrical considerations and problems for dosimetry estimation. They also pose challenges for treatment efficacy and toxicity from differences in dose homogeneity.

Radioisotope selection is extremely important in radionuclide therapy planning. Strategies that are most effective have the pathological basis of the disease process as a primary consideration.

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