Radionuclide Therapy and Clinical Management
Therapeutic principles
Radionuclide therapy uses unsealed sources to deliver absorbed dose to target tissue while limiting normal-organ toxicity. Understand the differences among I-131 for hyperthyroidism and differentiated thyroid cancer, Lu-177 dotatate for somatostatin-receptor tumors, Lu-177 PSMA for prostate cancer, Y-90 microspheres for hepatic tumors, and Ra-223 for bone metastases. Dose-limiting organs vary: bone marrow, kidneys, salivary glands, lungs, and liver. Alpha emitters have high linear energy transfer and short range, so they spare surrounding tissue but require careful handling.
Clinical management and traps
Before I-131, assess pregnancy and breastfeeding, withdraw antithyroid drugs appropriately, and use a low-iodine diet. “Stunning” refers to reduced subsequent iodine uptake after a diagnostic dose; it is a reason to minimize diagnostic activity before therapy. For PRRT, renal protection with amino acids and monitoring for myelosuppression and carcinoid crisis are important. After therapy, counsel on radiation safety, close contact, and body-fluid precautions; release criteria depend on measured dose rate. Traps include treating hyperthyroidism without considering gland size and uptake, missing pregnancy, and ignoring renal function before nephrotoxic agents.
Worked example
If a patient has a 24-hour thyroid uptake of 30% and a 20-g gland, a simplified Marinelli calculation can estimate the I-131 activity needed for a desired absorbed dose; in practice, adjust for uptake, gland size, and retention. Rule of thumb: prescribe by target dose and patient-specific dosimetry when available, not by a single fixed activity for every patient.