How are radiopharmaceutical doses determined?
Several methods of determining prescribed dose exist and are used depending on the radiopharmaceutical in question and the disease being treated. These include:
- • Flat doses : For example, ablation of the thyroid remnant in a patient who had a thyroidectomy for thyroid cancer is sometimes treated with a flat dose of 1.1 GBq (30 mCi) 131 I. Although this dose has been shown to be effective, it was initially largely chosen because, for a long period of time in the US, it was the largest dose that could be given to an outpatient in most states. Thus, dose selection is based not only on safety and efficacy but also on regulatory considerations. Furthermore, flat doses may be tiered and chosen based on patient demographics and risk factors.
- • Weight-based doses : These can be based on the weight of the patient, for example with 131 I ibritumomab tiuxetan for non-Hodgkin lymphoma, or on the weight of the organ being treated, for example with yttrium-90 ( 90 Y) microsphere liver-directed therapy or 131 I therapy for benign thyroid disease. In the latter case, the dose is calculated based on organ weight but is also corrected for the amount of uptake of the radiopharmaceutical.
- • Dosimetric doses : Dosimetry refers to the measurement of dose delivered and can apply to any combination of dose delivery to the tumor, individual organs, or whole body. Dosimetry allows precision dosing based on the individual patient’s unique radiopharmaceutical kinetics. Generally, a small radiopharmaceutical dose is administered and serial measurements are made to calculate kinetics generally with some combination of blood (and/or body fluid) sampling, imaging, and whole-body radiation measurements.