Deterministic effects of radiation: Rad Tech

Deterministic effects of radiation occur in predictable ways when the absorbed dose is high enough. These responses have an approximate threshold: below it, the effect is not expected; above it, the severity increases as dose increases. Deterministic effects can range from localized tissue injury to acute radiation syndromes after very large whole-body exposures. As a radiologic technologist, understanding deterministic effects is essential to providing safe patient care, optimizing imaging procedures, monitoring dose during higher-dose procedures, and applying radiation-protection practices that help prevent tissue injury.

Deterministic radiation effects, also called tissue reactions, occur only after the absorbed radiation dose to a tissue exceeds a specific threshold. Below that threshold, the effect is not expected. Above it, the severity of tissue injury increases as dose increases.

Deterministic effects differ from stochastic effects. Deterministic effects have a threshold and a nonlinear dose-response relationship. In contrast, stochastic effects have no established threshold: the dose increases the probability of an effect, but not its severity.

Now acute radiation syndromes can develop after a very large, short-term exposure to most or all of the body. Systemic syndromes include hematopoietic, gastrointestinal, and cerebrovascular syndromes. A cutaneous syndrome involving skin injury may occur at the same time.

Hematopoietic syndrome, also called hematologic syndrome, occurs after whole-body doses of approximately 2 to 10 Gy. Radiation damages bone marrow stem cells, reducing red blood cells, white blood cells, and platelets. Illnesses that result may include infection, fever, anemia, and bleeding. If the exposure is lethal, the mean survival time is about 10 to 60 days.

Gastrointestinal syndrome can occur after a whole-body dose of approximately 10 to 50 Gy. Severe injury to rapidly dividing cells in the intestinal lining causes fluid loss, infection, and electrolyte imbalance. When lethal, death typically occurs within about 4 to 10 days.

At whole-body doses above about 50 Gy, cerebrovascular syndrome, also called central nervous system syndrome, causes death within hours to about 3 days. Findings can include severe nausea and vomiting, confusion, loss of coordination, seizures, respiratory distress, coma, and increased intracranial pressure from cerebral edema.

Acute radiation syndrome generally progresses through a prodromal phase, a latent phase, and manifest illness, followed by recovery or death.

The prodromal phase occurs first, usually after a whole-body dose above about 1 Gy. It can begin within minutes to hours and may last from hours to a day or two. Common findings include nausea, vomiting, and diarrhea. Blood tests may show an early decrease in white blood cells, called leukopenia. Earlier onset and greater severity generally indicate a higher absorbed dose.

After the prodromal phase, patients experience a latent phase. Signs and symptoms temporarily lessen or disappear, so they may appear to improve even though tissue injury continues. This phase can last from hours to weeks, depending on the absorbed dose and the syndrome involved. It becomes shorter as dose increases and may disappear after extremely high exposures.

Lastly, during manifest illness, clinical manifestations return and reflect the syndrome that has developed. The outcome may be recovery or death, depending on the dose, distribution of exposure, and extent of injury. Sometimes, recovery or death is categorized as the fourth phase.

The lethal dose 50/60, or LD50/60, means the whole-body absorbed dose expected to cause death in 50 percent of an exposed population within 60 days. Mean survival time is the average interval between a lethal exposure and death. In general, mean survival time decreases as absorbed dose increases. However, LD50/60 is a population-level measure, not a prediction for one person.

Now, unlike whole-body acute radiation syndrome, local tissue injury occurs when a high dose is delivered to a limited area. Significant cell death can cause tissue shrinkage, called atrophy, and impaired tissue function.