Medical radiation refers to the controlled use of high-energy waves or particles in healthcare settings. This energy, typically ionizing radiation, possesses enough power to remove electrons from atoms, altering the chemical structure of matter it passes through. The applications are diverse, serving as tools for both visualizing internal anatomy and treating various diseases. By carefully regulating the dose and focus, medical professionals harness these properties to achieve specific diagnostic or therapeutic outcomes.
Diagnostic and Therapeutic Applications
Medical radiation is broadly categorized by its purpose: diagnostic information or therapeutic treatment. Diagnostic applications utilize relatively low doses of radiation to create images of the body’s interior, helping clinicians identify disease or injury. Procedures like X-rays and Computed Tomography (CT) scans pass external beams through the patient to capture anatomical details, relying on the differential absorption of energy by various tissues.
Other diagnostic methods, falling under nuclear medicine, involve introducing small amounts of radioactive substances, known as radiopharmaceuticals, into the body. These substances are chemically designed to accumulate in specific organs or tissues, emitting radiation that is detected externally. This technique, used in Positron Emission Tomography (PET) scans, provides physiological information about organ function and metabolism.
Conversely, therapeutic applications, primarily in radiation oncology, involve the use of much higher doses of radiation. The goal is to kill or control the growth of diseased cells, most commonly those found in tumors. This is achieved by aiming highly energetic beams, often generated by linear accelerators, directly at the target tissue.
A precise method called external beam radiation therapy uses multiple intersecting beams to concentrate the maximum dose at the tumor site while minimizing exposure to surrounding healthy tissue. Another technique, brachytherapy, places a sealed radioactive source directly inside or next to the treatment area, offering a highly localized dose distribution. Therapeutic radiation can also be delivered internally using radiopharmaceuticals that target and destroy cancerous cells throughout the body.
How Radiation Affects Biological Tissues
The mechanism by which medical radiation interacts with living tissue centers on ionization, the removal of electrons from atoms within cells. Ionizing radiation exerts its biological effect through two main pathways: direct or indirect damage. Direct damage occurs when radiation particles physically strike and break chemical bonds within DNA molecules. This compromises the cell’s genetic integrity, resulting in DNA strand breaks, base alterations, or crosslinking.
The indirect effect is often the predominant mechanism, especially for X-rays and gamma rays, since cells are approximately 75% water. When radiation interacts with water molecules, it causes radiolysis, generating highly reactive molecules called free radicals. A particularly destructive product is the hydroxyl radical (\(\text{OH}^{\cdot}\)), which can diffuse a short distance from its origin.
These free radicals then chemically attack cellular components, including the cell membrane, proteins, and the DNA itself. The resulting DNA damage includes base damage, single-strand breaks, and the more severe double-strand breaks. Cells possess sophisticated repair mechanisms to correct this damage, but if the damage is too extensive or incorrectly repaired, the cell’s future is compromised.
In diagnostic imaging, the radiation dose is low enough that cells typically repair the transient damage without major consequence. Conversely, therapeutic radiation is delivered at doses intended to overwhelm the cancer cell’s repair capacity. This massive, irreparable damage triggers cell death, which is the desired outcome for tumor eradication.
Measuring and Safeguarding Against Exposure
The effects of medical radiation are quantified using specialized units to manage patient and occupational exposure. The absorbed dose, measured in Gray (Gy), represents the amount of energy deposited by the radiation per unit mass of tissue. Because different radiation types cause varying degrees of biological harm, a second unit accounts for this difference in risk.
The Sievert (Sv) is the unit for effective dose, calculated by weighting the absorbed dose based on the type of radiation and the sensitivity of the exposed organs. This unit provides a standardized measure of the potential biological harm or risk to the whole body from a given exposure. Regulatory bodies use these units to monitor and restrict exposure levels for patients and medical personnel.
Safety protocols are guided by the principle known as ALARA, which stands for “As Low As Reasonably Achievable.” This concept acknowledges that even small radiation doses carry a theoretical risk, promoting efforts to keep exposures below regulatory limits. ALARA is implemented through three fundamental protective measures: time, distance, and shielding.
Minimizing the time spent near a radiation source directly reduces the total dose received. Maximizing the distance from the source is effective, as radiation intensity decreases rapidly with separation. Shielding involves placing absorbing materials, such as lead barriers or aprons, between the source and the person to attenuate the radiation. These practices ensure that the benefit of the medical procedure outweighs the exposure risk.