Ionizing radiation is an indispensable tool in modern medicine. Every day it is used to diagnose disease, guide medical procedures, sterilize equipment, and treat cancer. At the same time, excessive exposure to ionizing radiation can damage living cells and increase the risk of adverse health effects. Understanding both the benefits and the risks of radiation is therefore essential for anyone working in healthcare.

The biological effects of radiation originate at the microscopic level. As ionizing radiation passes through tissue, it removes electrons from atoms and molecules, producing ions and highly reactive chemical species. These interactions can damage proteins, cell membranes, and, most importantly, DNA. Although cells possess remarkable repair mechanisms, severe or unrepaired DNA damage can interfere with normal cell function, trigger programmed cell death, or occasionally lead to mutations that contribute to cancer.

The previous chapter introduced the biological consequences of radiation exposure in the context of nuclear accidents and nuclear weapons. In this chapter, we focus instead on the practical aspects of radiation used in medicine: how radiation dose is measured, why different types of radiation produce different biological effects, and how healthcare professionals minimize radiation exposure while obtaining the diagnostic or therapeutic benefits of ionizing radiation.

Radiation Dose: Measuring the Energy Deposited in Tissue

One of the most common misconceptions about radiation is that the strength of a radioactive source is the same as the amount of radiation received by a person. In reality, several different quantities are used to describe radiation, and each answers a different question.

These quantities are closely related but describe different aspects of radiation exposure. A highly radioactive source does not necessarily deliver a large dose to a patient, and two patients receiving the same absorbed dose may experience different biological effects depending on the type of radiation and the tissues exposed.

Absorbed Dose

The most fundamental measure of radiation exposure is the absorbed dose, defined as the amount of ionizing energy deposited in a unit mass of tissue.

The SI unit of absorbed dose is the gray (Gy).

Historically, radiation dose was measured using the rad, where

Although the gray is now the standard SI unit, the rad may still appear in older publications or technical documents.

Why Absorbed Dose Is Not Enough

Suppose two patients each receive an absorbed dose of 1 Gy. One is exposed to x-rays during radiation therapy, while the other receives the same absorbed dose from alpha particles inside the lungs after inhaling radioactive material.

Although both patients receive the same amount of deposited energy, the biological effects are very different. Alpha particles deposit their energy over extremely short distances, producing dense ionization tracks that cause complex DNA damage. X-rays, in contrast, deposit their energy more sparsely throughout the tissue.

For this reason, absorbed dose alone cannot fully describe the biological effects of radiation. We must also account for the type of radiation responsible for the dose.

Equivalent Dose: Accounting for the Type of Radiation

Different types of ionizing radiation do not produce the same amount of biological damage, even when they deposit the same amount of energy. For example, alpha particles lose their energy over a very short distance, producing dense tracks of ionization that are difficult for cells to repair. X-rays and gamma rays, on the other hand, spread their energy over much longer distances, producing less concentrated damage.

To account for these differences, radiation protection uses the concept of equivalent dose. The equivalent dose combines the absorbed dose with a radiation weighting factor, sometimes called the relative biological effectiveness (RBE) or quality factor (QF), which reflects how damaging a particular type of radiation is to living tissue.

where w_R is the radiation weighting factor.

The SI unit of equivalent dose is the sievert (Sv).

Historically, the unit rem (roentgen equivalent man) was used instead of the sievert.

For x-rays, gamma rays, and most beta particles, the weighting factor is 1. As a result, the numerical values of absorbed dose (Gy) and equivalent dose (Sv) are the same. However, for alpha particles, the equivalent dose is approximately twenty times larger than the absorbed dose because alpha particles produce much greater biological damage.

Effective Dose: Accounting for the Tissue Exposed

Equivalent dose accounts for the type of radiation but does not consider where in the body the radiation is absorbed. Different organs have different sensitivities to radiation. For example, the lungs, thyroid, bone marrow, and breast tissue are generally more sensitive to radiation-induced cancer than muscle or skin.

To compare the overall health risk from different medical procedures, radiation protection introduces the concept of effective dose. Effective dose combines the equivalent dose delivered to each organ with a tissue weighting factor that reflects the relative sensitivity of that tissue.

Although the full calculation can become quite complex, the important idea is straightforward: the same equivalent dose delivered to different organs does not necessarily produce the same overall health risk.

It is important to remember that effective dose is a tool for estimating long-term population risk. It is not intended to predict the health outcome for an individual patient, whose risk depends on many additional factors such as age, sex, and overall health.

Biological Effects of Ionizing Radiation

The biological effects of ionizing radiation begin at the atomic level. As radiation passes through tissue, it ionizes atoms and molecules, producing charged particles and highly reactive free radicals. These species can damage important cellular components, particularly DNA.

Most DNA damage is successfully repaired by the cell's natural repair mechanisms. In fact, cells experience thousands of DNA lesions each day from normal metabolic processes, ultraviolet radiation, and naturally occurring background radiation. However, if the damage is extensive or repaired incorrectly, the affected cell may respond in one of several ways:

• Successful repair: The DNA damage is repaired, and the cell continues to function normally.
• Programmed cell death (apoptosis): If the damage is too severe, the cell destroys itself to prevent abnormal growth.
• Loss of reproductive ability: The cell remains alive but can no longer divide.
• Mutation: Occasionally, damaged DNA is repaired incorrectly, potentially leading to uncontrolled cell division and, over many years, cancer.

Whether a cell survives radiation exposure depends on many factors, including the radiation dose, the type of radiation, the rate at which the dose is delivered, and the type of tissue involved.

Which Tissues Are Most Sensitive?

Not all tissues respond to radiation in the same way. Cells that divide rapidly are generally the most radiosensitive because DNA damage is more likely to interfere with cell division before repair can occur. In contrast, cells that divide infrequently tend to be more radioresistant.

This difference in radiosensitivity explains many of the early symptoms observed after large radiation exposures. Bone marrow damage reduces the production of blood cells, increasing the risk of infection and bleeding. Damage to the digestive tract can produce nausea and vomiting, while injury to hair follicles may result in temporary hair loss.

Radiation Dose and Biological Risk

The biological response to radiation depends not only on the total dose received but also on how quickly that dose is delivered. A dose received over a few minutes generally produces greater biological effects than the same dose spread over weeks or months because cells have less time to repair radiation-induced damage.

For very large radiation doses, tissue damage appears soon after exposure and is referred to as a deterministic effect. Examples include skin burns, cataracts, and acute radiation syndrome. These effects occur only after a threshold dose has been exceeded, and their severity increases as the dose increases.

At lower doses, the primary concern is the increased probability of developing cancer many years later. These are known as stochastic effects. Unlike deterministic effects, stochastic effects have no known threshold; instead, the probability of occurrence increases with dose, although the severity of the disease does not.

For diagnostic imaging, the radiation doses are typically far below the threshold for deterministic effects. Consequently, radiation protection in medicine focuses primarily on minimizing stochastic risk while ensuring that patients receive the diagnostic or therapeutic benefits of the procedure.

Radiation Protection Principles

Because ionizing radiation has the potential to damage living tissue, healthcare professionals follow well-established safety practices whenever radiation is used. These practices are designed to keep radiation exposures as low as reasonably achievable while still obtaining the medical information or treatment needed. This philosophy is known as the ALARA principle, which stands for As Low As Reasonably Achievable.

ALARA does not mean avoiding radiation at all costs. Instead, it recognizes that radiation often provides substantial medical benefits. The goal is to minimize unnecessary exposure while ensuring that patients receive the diagnostic or therapeutic information required for appropriate care.

Time

The radiation dose received is directly proportional to the amount of time spent near a radiation source. Reducing the exposure time decreases the total dose received.

This principle is particularly important for healthcare workers who routinely perform fluoroscopic procedures, prepare radiopharmaceuticals, or work near patients who have received radioactive tracers.

Distance

Increasing the distance from a radiation source is one of the simplest and most effective methods of reducing exposure. As radiation spreads outward from a small source, its intensity decreases rapidly with distance according to the inverse-square law.

where I is the radiation intensity and r is the distance from the source.

Doubling the distance from a point source reduces the radiation intensity to approximately one-quarter of its original value.

Shielding

Shielding places absorbing material between the radiation source and the individual. The most effective shielding material depends on the type of radiation involved.

In medical imaging departments, structural shielding is incorporated into walls, doors, windows, and equipment. Staff members may also use mobile lead barriers or syringe shields when handling radioactive materials.

Monitoring Radiation Exposure

Healthcare workers who routinely work with ionizing radiation wear personal dosimeters that continuously monitor their occupational exposure. These devices do not protect against radiation; instead, they record the accumulated dose so that exposure can be tracked over time.

Depending on the workplace, dosimeters may be worn on the chest, at the collar outside a lead apron, or on the wrist or hand when radioactive materials are handled directly. The recorded doses are reviewed regularly to ensure that occupational exposures remain well below regulatory limits.

Special precautions are also taken for pregnant workers because the developing fetus is more sensitive to ionizing radiation than most adult tissues. Healthcare facilities may modify work assignments or provide additional monitoring to ensure that fetal radiation exposure remains well below recommended limits.

Radiation Dose in Medical Imaging

Patients are exposed to ionizing radiation during many medical imaging procedures, including x-ray radiography, computed tomography (CT), fluoroscopy, nuclear medicine, and positron emission tomography (PET). The amount of radiation received depends on the type of examination, the body part being imaged, the patient's size, and the imaging equipment used.

Radiation dose from diagnostic imaging is typically reported as effective dose, measured in millisieverts (mSv). Effective dose provides a way to compare the overall biological risk associated with different imaging procedures, even though different organs receive different amounts of radiation.

These values are only approximate and vary depending on the imaging protocol, patient characteristics, and equipment. Advances in detector technology and image reconstruction algorithms continue to reduce radiation doses while maintaining excellent diagnostic image quality.

Balancing Risk and Benefit

Although ionizing radiation has the potential to damage living tissue, medical imaging is performed only when the expected clinical benefit outweighs the potential risk. For most diagnostic procedures, the radiation dose is low, and the information obtained can significantly improve patient care by allowing earlier diagnosis, more accurate treatment planning, or the avoidance of unnecessary surgery.

Radiation therapy provides an even clearer example of this balance. In radiation therapy, doses that are hundreds or even thousands of times larger than those used for diagnostic imaging are intentionally delivered to tumors in order to destroy cancer cells. Sophisticated treatment planning allows these high doses to be concentrated within the tumor while minimizing exposure to surrounding healthy tissues.

Modern medicine relies on the thoughtful use of ionizing radiation. By understanding how radiation dose is measured, how biological effects occur, and how exposure can be minimized through appropriate safety practices, healthcare professionals can safely use one of the most valuable diagnostic and therapeutic tools available.