Ionizing Radiation in Medicine

Function

Electromagnetic and particulate radiation can produce ion pairs through interaction with matter. Ionizing radiation enables the acquisition of high-quality diagnostic images to support clinical diagnosis. Clinically relevant ionizing radiation types include γ rays, x-rays, α particles, neutrons, β particles, charged nuclei, and positron emission radiation.

Life on Earth has persisted for approximately 4 billion years in the presence of natural ionizing radiation, which has likely influenced biological evolution and continues to contribute to present-day biological processes. Natural background radiation remains low, typically a few millisieverts per year.[1]

Natural sources of ionizing radiation include cosmic rays, terrestrial radiation from radioactive elements such as uranium and thorium, and internally deposited radionuclides, including potassium-40. Natural radiation constitutes the primary source of human exposure to ionizing radiation, particularly in regions with elevated background levels, such as Guarapari, Brazil; Kerala, India; Ramsar, Iran; and Yangjiang, China.[2] Ionizing radiation is also present in consumer products, including smoke detectors containing americium-241, and may result from radioactive substances released through improper disposal of radioactive waste.[3]

Radiation constitutes an essential component of modern medicine, with widespread application in diagnostic radiology, including x-ray imaging, CT, and fluoroscopy for interventional radiology procedures. Radiation is also utilized in the treatment of selected malignancies through radiotherapy.

Earlier estimates suggested that up to 46% of ionizing radiation exposure in the US originated from medical sources. More recent data indicate that medical exposure accounts for approximately 20% to 30% of total cumulative population exposure, attributed to reductions in CT dose and increased utilization of nonionizing modalities, such as magnetic resonance imaging and ultrasound, with substantial variation across countries and healthcare systems. Dosimetry of ionizing radiation is a well-established branch of the physical sciences.[4] Dosimetry provides the calculation of safe and effective patient doses during radiotherapy and the monitoring of chronic occupational exposure among clinicians. Radiation dose is measured in gray (Gy, absorbed dose) and sievert (Sv, equivalent and effective dose), with historical use of rad (1 rad = 0.01 Gy). Determinants of the impact of radiation exposure include exposure duration, distance from the radioactive source, and intensity of radioactivity or rate of energy emission.

Issues of Concern

The lifetime cancer mortality risk associated with pediatric CT exposure exceeds that observed in adults. A study following more than 3.7 million children over a mean duration of 10.1 years demonstrated a clear dose–response relationship between medical imaging radiation exposure and hematologic malignancy risk.[5] Recent evidence indicates that pediatric CT is associated with increased incidence of solid tumors, leukemia, lymphoma, and myelodysplasia, particularly with multiple high-dose examinations. Absolute risk remains low, and the benefits of clinically indicated CT typically outweigh associated risks.

Extrapolation of epidemiologic data from survivors of atomic bomb radiation exposure has estimated a risk of approximately 1 fatal cancer per 1,000 CT scans performed in young children.[6] Another projection estimated that CT examinations performed in 2023 may result in approximately 103,000 future cancer cases over the lifetime of exposed patients, with CT-related malignancies potentially accounting for 5% of annual new cancer diagnoses if current utilization patterns persist.[7]

Radiation risk discussion in imaging must account for the clinically recognized misdiagnosis rate, estimated at approximately 3% to 4%. Balanced assessment of potential risks and benefits of imaging procedures remains essential to address patient concerns regarding radiation exposure.[8] Extensive imaging, including whole-body CT in trauma patients, has demonstrated incidental findings in approximately 1 out of 3 examinations. Less than 1% of patients required urgent medical attention, and less than 3% underwent follow-up. 

Overutilization of CT imaging significantly increases healthcare costs. Frequent and unnecessary CT examinations incur direct per-scan expenses, with additional downstream costs from repeat imaging, follow-up procedures, overdiagnosis, and potential overtreatment, leading to increased healthcare utilization.[9]

Nonionizing radiation sources include cellular phones, AM and FM radio, microwaves, sun exposure, and visible light. Sun exposure involves ultraviolet radiation, which is nonionizing but capable of inducing DNA damage through photochemical mechanisms. Nonionizing radiation lacks sufficient energy to ionize atoms and does not produce the same biological effects as ionizing radiation at typical exposure levels. However, long-term ultraviolet exposure from sunbathing is associated with skin cancer secondary to cumulative photochemical injury.

Clinical Significance

Acute radiation sickness occurs following high-dose exposure, typically in the range of 1 to 12 Gy.[10] Acute radiation syndrome is a multisystem clinical condition involving 4 principal organ systems, with development of distinct subsyndromes, as follows:

• Hematopoietic syndrome
• Gastrointestinal subsyndrome
• Neurovascular subsyndrome
• Cutaneous subsyndrome

Potentially beneficial countermeasures include the following:

• Cytokines
• Hematopoietic stem cell transplantation
• Fluoroquinolones
• Bowel decontamination
• Serotonin receptor antagonists
• Loperamide
• Enteral nutrition
• Topical corticosteroids
• Antihistamines
• Antibiotics
• Surgical excision or grafting for radiation-induced skin injury [11][12]

Chronic effects may persist for several years following exposure. Ionizing radiation exposure is associated with increased risk of malignancy, cardiovascular disease, age-related disorders, and neurodegenerative conditions.[13][14] Additional reports have described associations between increased ionizing radiation exposure and tuberculosis, viral infections, digestive system diseases, particularly chronic liver disease, cataracts, and chromosomal disorders, including Down syndrome.[15]

Increasing evidence over the past several decades has supported the carcinogenic risk associated with exposure to ionizing radiation. Multiple studies utilizing data from nuclear disasters have attempted to estimate cancer risk according to specific absorbed radiation doses. Reduction of radiation dose during hospital-based imaging is an important strategy to limit unnecessary patient exposure to ionizing radiation. Pediatric patients are at increased risk of receiving higher effective doses relative to body size when adult imaging protocols are applied. Modern pediatric protocols substantially mitigate this risk. Fetal radiation exposure requires consideration prior to imaging in pregnant patients. Appropriateness of imaging should be evaluated to minimize patient harm.[16]

The biological effects of ionizing radiation occur primarily through cellular damage. Cellular repair mechanisms restore function in most cases. However, damage to DNA and repair pathways may result in loss of normal cellular function. Cellular death may occur, with subsequent mucosal sloughing in affected tissues. A high radiation dose delivered over a short interval and administered in fractionated doses is considered more conducive to cellular repair than prolonged low-dose exposure. Teratogenic effects are associated with high-dose ionizing radiation during organogenesis (2–8 weeks postconception). Fetal doses below 50 to 100 mGy are not associated with increased teratogenic risk, and most diagnostic imaging procedures, including CT, remain well below this threshold when appropriately performed.

Other Issues

Ionizing radiation is increasingly recognized not only as a carcinogenic risk factor but also as a contributor to elevated healthcare costs when overutilized.[17] Clinical emphasis has shifted toward greater reliance on diagnostic reasoning and physical examination skills rather than dependence on imaging modalities like CT for primary diagnosis. 

The linear no-threshold model, which assumes cancer risk increases linearly with dose without a threshold, has served as the foundation of radiation protection for decades. Increasing criticism of this model has emerged in recent years. Statistically validated data describing radiation effects are primarily available for doses above 100 mGy per year, with linear extrapolation to lower doses remaining an assumption.[18] Radiobiologic and epidemiologic evidence has strengthened understanding of low-dose cancer risk, and the linear no-threshold model does not appear to overestimate risk substantially. No alternative model has demonstrated superior utility for radiologic protection.[19] Alternative hypotheses, including threshold models and radiation hormesis, propose potential beneficial effects at low doses and continue to generate scientific debate regarding dose–response relationships in low-dose exposure.[20]

A safety framework in healthcare, ALARA (as low as reasonably achievable), limits ionizing radiation exposure while maintaining diagnostic image quality. Occupational exposure among healthcare workers is higher than that of the general population in settings involving fluoroscopy or nuclear medicine. However, most personnel in diagnostic radiography and CT receive low cumulative occupational doses. Effective dose has historically been expressed in roentgen equivalent man (rem). Average annual occupational exposure approximates 5 rem/year for healthcare workers, compared with approximately 0.1 rem/year for members of the public. Limitation of exposure time is a key protective measure in clinical practice. Appropriate shielding and maximization of distance from radiation sources are essential when time limitation is not feasible during imaging procedures.

Ionizing radiation also has established therapeutic benefits in healthcare. Radiation therapy reduces cancer recurrence rates and may decrease the extent of surgical resection, thereby minimizing functional and cosmetic morbidity, including in breast cancer management.[21] Radiation therapy also improves survival outcomes across multiple malignancies, including breast, prostate, gastric, esophageal, testicular, and pancreatic cancers.

Enhancing Healthcare Team Outcomes

The International Commission on Radiological Protection is dedicated to protecting people and the environment from the adverse effects of ionizing radiation exposure. Protection strategies involve effective management of radiation dose, requiring a comprehensive understanding of dose quantities. For more than 90 years, the system of radiological protection has evolved in parallel with scientific advances in radiation exposure knowledge and the development of radiation imaging modalities. Primary objectives include prevention of harmful tissue reactions (deterministic effects) by maintaining organ and tissue doses below established thresholds and control of the probability of stochastic effects.

The system relies on 3 dose quantities: absorbed dose, equivalent dose, and effective dose. Absorbed dose is the fundamental physical quantity used to limit deterministic tissue reactions. Effective dose combines equivalent doses across tissues and is used for protection against stochastic effects. The International Commission on Radiological Protection now designates absorbed dose as the most appropriate quantity for limiting tissue reaction thresholds, while stochastic risk limits are expressed using effective dose.[22]

Clinicians should aim to reduce ionizing radiation exposure to both patients and healthcare personnel, particularly in populations at increased risk. Imaging justification should be guided by clinical relevance, with consideration of whether additional imaging is expected to alter diagnosis or management. Appropriate utilization with radiation-minimizing strategies is indicated when additional imaging is likely to influence patient outcomes. Imaging is not indicated when no impact on diagnosis or treatment is anticipated.

With the rise of interventional radiology and the increasing number of procedures performed under fluoroscopic guidance, assessment of interventional radiologists’ attitudes toward personal radiation protection and the use of radiation protection devices is important. Results from a survey of 504 members of the Society of Interventional Radiology indicate that, although many radiologists use standard radiation safety devices, such as lead aprons and thyroid shields, certain tools, including leaded eyeglasses, ceiling-suspended shields, and rolling shields, are less frequently utilized. Reported reasons for avoidance include comfort, ease of use, and availability. The study suggests further investigation into barriers to device use and access to protective tools in interventional radiology practice.[23]

Implementation of protocol-based healthcare, including standardized approaches to reduce ionizing radiation dose, is an effective strategy to improve patient safety and enhance team performance. Current advancements in artificial intelligence, including machine learning algorithms, enable more precise dose optimization and continuous monitoring, reducing radiation exposure for both patients and operators during medical procedures.[24]