A radioisotope is a form of an element with an unstable atomic nucleus that decays, releasing energy as radiation. In nuclear medicine, these radioactive atoms are carefully selected and utilized for both diagnosis and treatment, forming the basis of targeted therapy. The decay process emits particles, such as alpha or beta particles, or electromagnetic energy like gamma rays, which interact precisely with biological tissue. This controlled release of energy allows medical professionals to deliver a destructive dose directly to malignant cells while minimizing exposure to healthy surrounding tissue.
How Radiation Damages Cancer Cells
The goal of using radioisotopes in cancer therapy is to induce irreparable damage to the genetic material within the tumor cells. Radiation acts through two main pathways. Direct damage occurs when high-energy particles strike the DNA molecule itself, immediately causing breaks in one or both of the DNA strands. Double-strand breaks are particularly devastating because they are difficult for the cell to repair, often leading to genomic instability.
The second, more common mechanism is indirect damage, which involves the ionization of water molecules that make up the majority of the cell’s mass. This process generates highly reactive chemical species known as free radicals, notably hydroxyl radicals. These free radicals then chemically attack the DNA and other cellular components, leading to widespread molecular disruption. Cancer cells are especially vulnerable because their rapid division rate leaves them less time to activate DNA repair mechanisms before they attempt to replicate. The accumulating damage ultimately triggers cell death, either through apoptosis or through a catastrophic failure during division called mitotic catastrophe.
Categorizing Radioisotope Delivery Methods
Radioisotopes are deployed in cancer treatment using several methods designed to optimize the delivery of radiation energy to the tumor site.
Systemic or Targeted Therapy
One approach is systemic or targeted therapy, often referred to as radiopharmaceutical therapy. A radioactive atom is chemically attached to a targeting molecule, such as a peptide or antibody, and administered intravenously or orally. This combined agent travels through the bloodstream and seeks out specific molecular receptors that are overexpressed on the surface of cancer cells. This delivers the radiation payload directly to the tumor cells throughout the body.
Brachytherapy
Brachytherapy, which translates to “short-distance therapy,” involves placing a sealed radioactive source directly into or immediately next to the cancerous tissue. Brachytherapy is effective because the radiation dose intensity falls off rapidly with distance. This means a very high dose can be delivered to the tumor while sparing adjacent organs. The sources, which can be temporary wires or permanent seeds, leverage this localized effect to treat tumors in areas like the prostate, cervix, or breast.
External Beam Radiation Therapy
The third category is external beam radiation therapy, also known as teletherapy, where a beam of radiation is projected onto the tumor from outside the body. While modern external beam treatments typically use linear accelerators, radioisotopes have historically been used, such as Cobalt-60, which emits a high-energy gamma ray beam. The external beam is carefully shaped and angled from multiple directions to ensure the radiation dose converges and maximizes at the deep-seated tumor volume.
Key Isotopes and Their Targeted Cancers
The choice of radioisotope depends on the type of cancer, as each element possesses unique physical and chemical properties that dictate its clinical application.
Iodine-131 (I-131)
Iodine-131 (I-131) is used for treating thyroid cancer. The thyroid gland naturally absorbs iodine to produce hormones, and the radioactive form mimics this behavior, concentrating the I-131 within the remaining thyroid tissue and any metastatic cancer cells. This destroys them with its beta emissions.
Prostate Cancer Isotopes
For advanced prostate cancer, two radioisotopes are prominent in targeted systemic therapy. Lutetium-177 (Lu-177) is combined with a molecule that targets Prostate-Specific Membrane Antigen (PSMA), which is highly expressed on prostate cancer cells. This beta-emitting radiopharmaceutical is used to treat metastatic castration-resistant prostate cancer. Lutetium-177 is also used in treating neuroendocrine tumors by linking it to a somatostatin analog, allowing it to target receptors on those specific cancer cell surfaces.
A different approach for prostate cancer that has spread to the bone involves Radium-223 (Ra-223). Ra-223 is chemically similar to calcium and is preferentially incorporated into areas of new bone growth, a process common in bone metastases. Radium-223 is an alpha-emitter, releasing high-energy particles that travel only a very short distance. This allows it to destroy tumor cells within the bone matrix with minimal damage to the adjacent bone marrow.
Yttrium-90 (Y-90) for Liver Cancer
For liver cancers, particularly those that have not spread widely, Yttrium-90 (Y-90) is utilized in a procedure called radioembolization. This treatment involves injecting millions of microscopic glass or resin spheres containing the Y-90 isotope directly into the blood vessels supplying the liver tumor. The small spheres become lodged in the tumor’s capillaries, delivering a high dose of localized beta radiation to the malignant tissue.
Brachytherapy Isotopes
In brachytherapy applications, Iridium-192 (Ir-192) is frequently used for temporary, high-dose-rate treatments for cancers of the cervix, breast, and prostate. The source is temporarily maneuvered into the tumor area to deliver a powerful, fractionated dose before being safely removed. Alternatively, Cesium-131 (Cs-131) is often used as a permanent seed implant for low-to-intermediate-risk prostate cancer. Its short half-life allows for a rapid delivery of the therapeutic dose over a manageable timeframe.