Introduction to Radioactivity
Radioactivity is a physical phenomenon where unstable atomic nuclei release energy by emitting radiation. This process is called radioactive decay, and it occurs in a variety of materials, particularly in certain isotopes. In medical applications, radioactivity is harnessed for diagnostic and therapeutic purposes, particularly in nuclear medicine and brachytherapy.
Types of Radiation Emitted
When a radioactive substance decays, it emits one or more types of radiation. The main types of radiation are:
- Alpha Radiation (α): Consists of two protons and two neutrons (helium nucleus). It has low penetration and can be stopped by a sheet of paper or skin.
- Beta Radiation (β): High-energy electrons (or positrons) emitted from the nucleus. Beta particles are more penetrating than alpha particles but can be stopped by materials like plastic or glass.
- Gamma Radiation (γ): High-energy electromagnetic radiation with very high penetration power. Gamma radiation is used in many medical treatments, including cancer therapy.
Radioactive Decay and Half-Life
The process of radioactive decay is governed by the principle of half-life, which is the time it takes for half of the atoms in a sample to decay. The rate of decay follows the exponential decay law:
N(t) = N₀ * e^(-λt)
Where:
- N(t) is the number of radioactive atoms at time t,
- N₀ is the initial number of radioactive atoms,
- λ is the decay constant (unique for each isotope),
- e is the base of the natural logarithm.
Example: If a sample has a half-life of 5 years, then after 5 years, half of the atoms in the sample will have decayed. If the initial count was 1000 atoms, only 500 atoms will remain after 5 years.
Applications of Radioactivity in Medicine
Radioactive materials have a range of uses in the field of medicine. They are used for diagnostic imaging, cancer treatment, and therapy. Let’s explore some of the key applications:
Brachytherapy
Brachytherapy is a form of cancer treatment in which a radioactive source is placed directly inside or very close to the tumor. This treatment minimizes the exposure to surrounding healthy tissue while delivering a concentrated dose of radiation to the cancerous cells.
Commonly used isotopes in brachytherapy include:
- Iridium-192 (Ir-192): Often used for prostate cancer treatment. Its half-life is about 73.8 days, and it emits both gamma and beta radiation.
- Iodine-125 (I-125): Used in the treatment of prostate cancer. It has a half-life of 59.4 days and emits low-energy gamma radiation.
- Cesium-137 (Cs-137): Used in the treatment of cervical, uterine, and prostate cancers. It has a half-life of about 30 years and emits gamma radiation.
Activity = A₀ * e^(-λt)
This equation gives the activity of the radioactive source at any given time t. The activity (A) is the number of decays per unit time.
Example: In prostate brachytherapy, a small implant of Iodine-125 is placed directly into the tumor. The decay of Iodine-125 emits low-energy gamma radiation, effectively killing cancer cells while limiting damage to healthy tissues.
Nuclear Medicine
Nuclear medicine uses small amounts of radioactive materials (radiopharmaceuticals) to diagnose and treat diseases. These materials are usually injected, swallowed, or inhaled by the patient, and they emit gamma rays that can be detected by special cameras to provide images of the internal organs.
Some common radioactive isotopes used in nuclear medicine include:
- Technetium-99m (Tc-99m): The most widely used radioisotope in diagnostic imaging. It has a half-life of 6 hours and emits gamma rays, making it ideal for single photon emission computed tomography (SPECT).
- Fluorine-18 (F-18): Used in positron emission tomography (PET) scans. It has a half-life of 110 minutes and is commonly used for detecting cancer, heart disease, and brain disorders.
- Iodine-131 (I-131): Used both for the diagnosis and treatment of thyroid conditions, such as thyroid cancer or hyperthyroidism. It has a half-life of 8 days and emits both beta and gamma radiation.
Example: In a typical PET scan, a patient is injected with a radiopharmaceutical like F-18, which accumulates in the tissues of interest (e.g., tumors). The gamma rays emitted by the F-18 are detected by the PET scanner to create detailed images of the internal organs and tissues.
Radiation Safety and Protection
When working with radioactive materials in medical applications, it is crucial to follow proper safety guidelines to minimize the risks to both patients and healthcare workers. Key principles of radiation protection include:
- Time: Reducing the time of exposure to radiation limits the amount of energy absorbed by the body.
- Distance: Increasing the distance from the radiation source reduces exposure due to the inverse square law.
- Shielding: Using lead or other dense materials to shield against radiation, particularly gamma and X-rays, which are highly penetrating.