Early versus late radiation effects

Radiation toxicity can be divided into early and late effects. Early effects of radiation are seen after large doses of radiation are delivered over short periods of time. Early toxicity is generally seen in rapidly dividing, self-renewal organs including skin, bone marrow, and gut epithelium. Early radiation effects are responsible for clinical syndromes that may be encountered in the emergency department following a massive exposure.

Late toxicities are generally seen in organs with slowly dividing or quiescent, terminally differentiated cells. Organs in which late toxicities are common include the central nervous system, kidneys, and liver. Many late radiation effects are attributable to a combination of parenchymal cell death and microvascular disease.

The most well-known delayed complication of radiation exposure is malignancy. Exposures to the Chernobyl accident and atomic bomb testing in the Marshall Islands led to high incidences of thyroid malignancies. Atomic bomb survivors have been shown to have an increased risk of leukemia, and young women with Hodgkin disease treated with radiation therapy have been shown to have an increased risk for breast cancer.

The absorbed dose of radiation is the amount of energy absorbed by biologic tissue. Radiation dose is measured in Gray (Gy) or radiation absorbed dose (rad). Gray is the SI unit for dose and is expressed as J/kg. One gray is equal to 100 rad. Since the biologic effects of different types of radiation (eg, gamma vs alpha vs neutron) vary significantly, expressing radiation exposure in terms of equivalent dose is sometimes useful. By assigning a weighting factor to each type of radiation (gamma = 1, alpha = 20), the equivalent dose can be calculated by multiplying the absorbed dose (Gy) by the radiation weighting factor. Equivalent dose is then expressed in sieverts (Sv).
Localized cutaneous injury

Localized exposure to high doses of radiation may cause cutaneous injury similar to burns. Blistering, erythema, desquamation, and ulceration often present about 12-20 days after irradiation with the onset and severity related to the magnitude of exposure.

A local exposure of 3 Gy leads to epilation within 1-2 weeks. A local exposure of 6 Gy may cause immediate signs of burn. Greater exposures of 10-15 Gy lead to dry desquamation, and exposures of 20-50 Gy lead to wet desquamation (partial-thickness burn) within 2-3 weeks. For doses greater than 50 Gy, a cutaneous syndrome of necrosis and ulceration may occur from damage to endothelial cells and small blood vessels. Vascular complications may present months to years after exposure.

Date: 2015-01-12; view: 920