Radiation therapy is a common cancer treatment that uses high-energy radiation to destroy malignant cells. Radioisotopes are atoms with unstable nuclei that release this energy as radiation particles or waves. Harnessing radioisotopes allows medical professionals to deliver a targeted dose of energy directly to diseased tissue. This approach minimizes damage to surrounding healthy structures.
How Radiation Kills Cancer Cells
The goal of radiation therapy is to inflict lethal damage upon the cancer cell’s deoxyribonucleic acid (DNA). High-energy radiation, consisting of alpha particles, beta particles, or gamma waves, causes ionization within the cellular environment. This ionization creates the biological damage necessary to kill the cell.
The process of cell death occurs through two main routes: direct and indirect DNA damage. Direct damage happens when radiation particles physically strike the DNA double helix, causing immediate strand breaks. Indirect damage is more common and occurs when radiation interacts with water molecules within the cell. This interaction produces highly reactive chemical species known as free radicals or reactive oxygen species.
These free radicals then chemically attack the DNA, leading to strand breaks and other lesions. The most devastating form of damage is the double-strand break, which is difficult for a cell to repair. Cancer cells are susceptible to this damage because they are often rapidly dividing and possess impaired DNA repair mechanisms compared to healthy cells. The irreparable damage prevents the cancer cell from successfully dividing and ultimately leads to its demise.
Systemic Delivery of Radioisotopes
Systemic radioisotope therapy, also known as targeted radionuclide therapy, involves administering a radioactive substance that travels through the bloodstream to find cancer cells throughout the body. This method relies on attaching the radioisotope to a specific molecule known as a “targeting agent.” This agent is designed to bind selectively to receptors that are overexpressed on the surface of cancer cells.
The targeting agent guides the radioactive payload directly to the tumor site, allowing the radiation dose to concentrate in the malignant tissue. The radioisotope is typically given intravenously or sometimes taken orally. This approach effectively treats widespread disease or small metastatic sites difficult to target with external radiation beams.
Examples of Systemic Therapy
For thyroid cancer, the thyroid gland naturally takes up iodine. Administration of Iodine-131 (\(^{131}\)I) allows the radioisotope to concentrate and destroy residual cancer cells. Lutetium-177 (\(^{177}\)Lu) is used for certain neuroendocrine tumors in Peptide Receptor Radionuclide Therapy (PRRT). Lutetium-177 is attached to a peptide that specifically binds to somatostatin receptors on the tumor cell surface, delivering a high dose of beta-particle radiation directly into the tumor.
Localized Placement (Brachytherapy)
Brachytherapy, meaning “short distance,” is a form of internal radiation therapy where a sealed source of radioisotopes is placed directly inside or immediately next to the tumor. This technique takes advantage of the principle that radiation dose intensity drops off rapidly with distance from the source. Placing the source right at the tumor allows a very high dose to be delivered to the cancer while sparing surrounding healthy tissue.
Temporary Implants
Temporary brachytherapy, often delivered as high-dose-rate (HDR) treatment, involves placing a highly radioactive source like Iridium-192 (\(^{192}\)Ir) into the tumor area for a few minutes before it is removed. This method is common for treating cervical, breast, and some head and neck cancers. It requires specialized remote afterloading equipment in a shielded room.
Permanent Implants
Permanent brachytherapy is a low-dose-rate (LDR) treatment involving implanting tiny radioactive “seeds” directly into the tumor, where they remain permanently. These seeds, which often contain radioisotopes such as Iodine-125 (\(^{125}\)I) or Palladium-103 (\(^{103}\)Pd), release their radiation slowly over weeks to months as they decay. This technique is used for treating early-stage prostate cancer, where the seeds deliver a continuous, focused dose to the gland.
Patient Safety and Specialized Handling
The use of radioisotopes necessitates strict safety protocols to protect patients, staff, and the public from unintended radiation exposure. For patients undergoing systemic therapy, temporary isolation is often required in specialized, shielded treatment rooms. This isolation lasts until the patient’s emitted radiation levels drop to safe, predetermined levels, a duration dictated by the radioisotope’s half-life.
Hospital staff must adhere to stringent guidelines, including the use of protective gear and continuous monitoring, to minimize exposure during administration and patient care. After systemic treatment, bodily fluids can be temporarily radioactive, requiring specific instructions for handling waste and personal hygiene. Patients are advised to take precautions upon discharge, such as limiting close contact with children and pregnant women for a specified period.