Radioisotopes are powerful tools in treating cancer and other diseases. They deliver targeted radiation to destroy harmful cells while minimizing damage to healthy tissue. From radioimmunotherapy to brachytherapy, these techniques offer hope for patients with various types of cancer.

Understanding radiation dosimetry is crucial for safe and effective treatment. Factors like radiation dose, fractionation, and radiosensitivity help doctors tailor treatments to each patient's needs. These concepts ensure the best possible outcomes while minimizing side effects.

Radiotherapy Techniques

Radioimmunotherapy

• Radioimmunotherapy combines radiation therapy with immunotherapy to target and destroy cancer cells
• Involves using monoclonal antibodies labeled with radioactive isotopes (iodine-131, yttrium-90) that specifically bind to cancer cells
• Antibodies deliver radiation directly to the targeted cancer cells while minimizing damage to healthy tissue
• Can be used to treat various types of cancer (non-Hodgkin's lymphoma, prostate cancer)
• Administered intravenously and the antibodies circulate throughout the body to locate and bind to cancer cells
• Side effects may include low blood cell counts, fatigue, and nausea

Brachytherapy

• Brachytherapy involves placing radioactive sources directly inside or near the tumor site
• Allows for high doses of radiation to be delivered precisely to the tumor while sparing surrounding healthy tissue
• Can be used to treat various types of cancer (prostate, cervical, breast)
• Radioactive sources can be placed temporarily or permanently depending on the type of brachytherapy
  • Temporary brachytherapy involves placing the sources for a set period and then removing them
  • Permanent brachytherapy involves implanting small radioactive seeds that remain in place and gradually decay over time
• Brachytherapy can be used alone or in combination with external beam radiation therapy

Targeted Radionuclide Therapy

• Targeted radionuclide therapy uses radioactive substances that are designed to target specific molecules or receptors on cancer cells
• Involves using small molecules or peptides labeled with radioactive isotopes (lutetium-177, radium-223) that bind to specific targets on cancer cells
• Allows for the selective delivery of radiation to cancer cells while minimizing exposure to healthy tissue
• Can be used to treat various types of cancer (neuroendocrine tumors, prostate cancer)
• Administered intravenously and the radioactive substances travel through the bloodstream to locate and bind to cancer cells
• Side effects may include nausea, vomiting, and low blood cell counts

Therapeutic Radioisotopes

Iodine-131

• Iodine-131 is a radioactive isotope of iodine used in the treatment of thyroid cancer and hyperthyroidism
• Emits beta particles and gamma rays, which can penetrate and destroy thyroid tissue
• Administered orally in liquid or capsule form and is absorbed by the thyroid gland
• Concentrates in thyroid tissue due to the gland's natural uptake of iodine
• Can also be used in radioimmunotherapy by labeling antibodies with iodine-131 to target specific cancer cells
• Side effects may include neck pain, swelling, and temporary loss of taste or smell

Yttrium-90

• Yttrium-90 is a radioactive isotope used in the treatment of liver cancer and rheumatoid arthritis
• Emits beta particles, which have a short range in tissue and can deliver high doses of radiation to targeted areas
• Can be incorporated into microspheres or attached to monoclonal antibodies for targeted delivery to tumor sites
• Used in a procedure called selective internal radiation therapy (SIRT) for the treatment of liver tumors
  • SIRT involves injecting yttrium-90 microspheres into the hepatic artery, which supplies blood to the liver tumors
  • The microspheres become lodged in the tumor's blood vessels and deliver high doses of radiation directly to the tumor
• Side effects may include abdominal pain, nausea, and fatigue

Lutetium-177

• Lutetium-177 is a radioactive isotope used in targeted radionuclide therapy for the treatment of neuroendocrine tumors and prostate cancer
• Emits beta particles and gamma rays, which can penetrate and destroy cancer cells
• Can be attached to small molecules or peptides that bind to specific receptors on cancer cells (somatostatin receptors, PSMA)
• Allows for the selective delivery of radiation to cancer cells while minimizing exposure to healthy tissue
• Used in a procedure called peptide receptor radionuclide therapy (PRRT) for the treatment of neuroendocrine tumors
  • PRRT involves injecting lutetium-177 labeled peptides that bind to somatostatin receptors on neuroendocrine tumor cells
  • The peptides deliver radiation directly to the tumor cells, causing cell death and tumor shrinkage
• Side effects may include nausea, vomiting, and low blood cell counts

Radiation Dosimetry

Radiation Dose

• Radiation dose refers to the amount of energy absorbed by tissue from ionizing radiation
• Measured in units of gray (Gy) or sieverts (Sv)
  • Gray is a unit of absorbed dose and represents the amount of energy absorbed per unit mass of tissue
  • Sievert is a unit of equivalent dose and takes into account the biological effects of different types of radiation
• Radiation dose determines the biological effects on tissue and the likelihood of causing damage or cell death
• Factors that influence radiation dose include the type and energy of radiation, exposure time, and distance from the source
• Radiation dose can be calculated using dosimetry techniques such as thermoluminescent dosimeters (TLDs) or optically stimulated luminescence (OSL) dosimeters

Fractionation

• Fractionation involves dividing the total radiation dose into smaller doses delivered over multiple treatment sessions
• Allows for the repair of sublethal damage in normal tissues between treatment sessions
• Exploits the differences in radiation sensitivity between tumor cells and normal cells
  • Tumor cells are generally more sensitive to radiation and have a reduced capacity for repair compared to normal cells
  • Fractionation allows normal cells to recover while still delivering a high cumulative dose to the tumor
• Conventional fractionation involves delivering small doses (1.8-2 Gy) daily over several weeks
• Hypofractionation involves delivering larger doses (>2 Gy) per fraction over a shorter overall treatment time
• Hyperfractionation involves delivering smaller doses (<1.8 Gy) multiple times per day with shorter intervals between fractions

Radiosensitivity

• Radiosensitivity refers to the susceptibility of cells or tissues to damage from ionizing radiation
• Varies among different cell types and tissues depending on factors such as cell cycle phase, oxygenation, and DNA repair capacity
• Highly radiosensitive tissues include bone marrow, lymphoid organs, and intestinal epithelium
  • These tissues have a high proportion of rapidly dividing cells and are more susceptible to radiation-induced damage
• Radioresistant tissues include muscle, bone, and nervous tissue
  • These tissues have a lower proportion of dividing cells and are less sensitive to radiation
• Tumor cells are generally more radiosensitive than normal cells due to their rapid proliferation and reduced DNA repair capacity
• Radiosensitivity can be modified by various agents such as radiosensitizers (oxygen, chemotherapy drugs) or radioprotectors (amifostine)