Radioisotope therapy (RIT), also referred to as targeted radionuclide therapy, is a specialized form of nuclear medicine that uses therapeutic radiopharmaceuticals to treat disease. This approach involves introducing a small amount of radioactive material into the body to deliver radiation directly to diseased cells. RIT aims to destroy specific target cells, such as cancer cells, while minimizing the radiation dose received by healthy tissues. Unlike traditional external beam radiation, RIT delivers the treatment systemically through the bloodstream. This makes it a viable option for treating metastatic disease, where cancer has spread throughout the body.
The Science of Targeted Cell Destruction
Radioisotope therapy relies on a two-part system designed for high-precision targeting. The first component is the radioisotope, the radioactive substance responsible for cell destruction. The second component is a carrier or targeting molecule, such as a peptide, antibody, or small molecule, which guides the radioisotope to the diseased cells. This carrier is engineered to bind specifically to receptors or antigens overexpressed on the surface of the target cells.
Once administered, the targeting molecule locks onto the cancer cell surface, concentrating the radioactive payload at the disease site. Radioisotopes like Lutetium-177 or Yttrium-90 emit short-range radiation, such as beta or alpha particles. These particles travel only a short distance, often less than a millimeter, after being emitted.
This short travel distance provides RIT’s high precision. The emitted particles deposit their energy directly into the target cell, causing lethal damage to the cell’s DNA. Since the radiation is confined to the immediate vicinity of the binding site, surrounding healthy tissues receive a significantly lower dose of radiation.
Medical Conditions Treated with RIT
RIT is applied to treat conditions where target cells possess unique uptake mechanisms or specific surface receptors. A long-established treatment is the use of Iodine-131 (I-131) for thyroid conditions, including hyperthyroidism and thyroid cancer. Since the thyroid gland naturally absorbs iodine, I-131 is readily taken up by thyroid tissue, delivering a destructive dose of radiation to the abnormal cells.
Peptide receptor radionuclide therapy (PRRT) is a targeted treatment for neuroendocrine tumors (NETs). This therapy uses a radioisotope like Lutetium-177 (Lu-177) attached to a peptide that targets somatostatin receptors, which are abundant on NET cells.
Another approach uses Radium-223 (Ra-223) to manage pain and prolong life in men with metastatic prostate cancer that has spread to the bones. Ra-223 mimics calcium and is preferentially incorporated into areas of high bone turnover characteristic of bone metastases.
Radioligand therapy (RLT) is also used for advanced prostate cancer by targeting the prostate-specific membrane antigen (PSMA) protein. Lu-177 is often attached to a molecule that binds to PSMA, allowing the radioisotope to be delivered directly to the cancer cells throughout the body.
Navigating the Treatment Process
The RIT procedure begins with pre-treatment planning, often involving diagnostic scans using a small dose of a similar radiopharmaceutical. These scans confirm that the target cells are taking up the agent, indicating the therapy will be effective. Treatment administration varies, such as swallowing a capsule for I-131 therapy or receiving an intravenous (IV) infusion for agents like Lutetium-177.
Following administration, the patient must follow specific safety protocols. For high-dose treatments, patients often stay in a specialized single hospital room for several days to limit radiation exposure to others. The body naturally eliminates the radioisotope over time, primarily through urine, sweat, and saliva.
Patients are encouraged to drink plenty of fluids and urinate often to help flush the radioactive material more quickly. Before discharge, a radiation safety officer measures the remaining radioactivity, and the patient is released once the level drops below a safe threshold. Post-discharge instructions include temporary precautions, such as maintaining distance from pregnant women and small children for a period of time.
Managing Adverse Reactions
Although RIT is highly targeted, adverse reactions can occur as the radioisotope passes through or temporarily accumulates in non-target organs. Common short-term side effects include fatigue, mild nausea, and temporary dry mouth (xerostomia). Xerostomia can occur with therapies like I-131 because salivary glands may absorb the radioactive iodine. Medications, such as anti-nausea drugs, are often provided to manage gastrointestinal discomfort.
A more significant, though less frequent, reaction is myelosuppression, a temporary decrease in blood cell counts caused by circulating radiation affecting the bone marrow. Patients undergoing RIT are closely monitored with regular blood tests to track cell counts. In some cases, a protective amino acid infusion is given before and during the radiopharmaceutical infusion to safeguard the kidneys from radiation exposure, particularly during treatments for neuroendocrine tumors.