5 Effects of Radiation Exposure

Objectives Define the key words. Explain the difference between the direct and indirect theories of biological damage. Determine the relative radiosensitivity or radioresistance of various kinds of cells in the body. Explain the difference between somatic and genetic effects.

Objectives Explain the difference between a threshold dose–response curve and a non-threshold dose–response curve. Identify the factors that determine radiation injuries. List the sequence of events that may follow exposure to radiation.

Objectives Explain the difference between deterministic and stochastic effects. List the possible short- and long-term effects of irradiation. Identify critical tissues for dental radiography in the head and neck region

Objectives Discuss the risks versus benefits of dental radiographs. Utilize effective dose equivalent to make radiation exposure comparisons. Adopt an ethical responsibility to follow ALARA.

Key Words Acute radiation syndrome (ARS) ALARA (as low as reasonably achievable) Cumulative effect Deterministic effect Direct theory Dose–response curve

Key Words Genetic cells Genetic effect Genetic mutations Indirect theory Ionization Irradiation Irreparable injury Latent period

Key Words Law of B and T Lethal dose (LD) Non-threshold dose–response curve Period of injury Radiolysis of water Radioresistant

Key Words Radiosensitive Recovery period Risk Somatic cells Somatic effect Stochastic effect Threshold dose–response curve

Introduction Patients are often concerned with the safety of dental x-ray procedures. Oral health care professionals share the same concerns.

Introduction Because even the experts cannot always predict a specific outcome from an amount of radiation exposure, the radiation protection community conservatively assumes that any amount of radiation may pose a risk.

Theories of Biological Effect Mechanisms Ionization Direct theory Indirect theory (radiolysis of water)

Figure 5-1 Direct theory and indirect theory Figure 5-1 Direct theory and indirect theory. In the direct theory, x-ray photons collide with large molecules and break them apart by ionization. The indirect theory is based on the assumption that radiation can cause chemical damage to the cell by ionizing the water within it.

Figure 5-2 Indirect theory Figure 5-2 Indirect theory. X-rays ionize water, resulting in the formation of free radicals, which recombine to form toxins.

Cell Sensitivity to Radiation Exposure Radiosensitive Radioresistant Law of B and T Somatic effect Genetic effect Cumulative effect

Cell Sensitivity to Radiation Exposure White blood cells (lymphocytes) Red blood cells (erythrocytes) Immature reproductive cells Epithelial cells Endothelial cells Connective tissue cells Bone cells Nerve cells Brain cells Muscle cells High sensitivity Low sensitivity

The Dose-Response Curve Threshold dose-response curve Non-threshold dose-response curve

Figure 5-3 Diagram of dose–response curve Figure 5-3 Diagram of dose–response curve. (A) A typical “threshold” curve. The point at which the curve intersects the base line (horizontal line) is the threshold dose that is the dose below which there is no response. If an easily observable radiation effect, such as erythema (reddening of the skin) is taken as “response,” then this type of curve is applicable. (B) A linear “non-threshold” curve, in which the curve intersects the base line at its origin. Here it is assumed that any dose, no matter how small, causes some response.

The Dose-Response Curve ALARA “As Low As Reasonably Achievable”

Factors that Determine Radiation Injury Five outcomes: Nothing, the cell is unaffected by the exposure Cell is injured or damaged but repairs itself and functions at pre-exposure levels Cell dies, but is replaced through normal biological processes

Factors that Determine Radiation Injury Five outcomes: Cell is injured or damaged, repairs itself, but now functions at a reduced level Cell is injured or damaged, and repairs itself incorrectly or abnormally, resulting in a biophysical change (tumor or malignancy)

Factors that Determine Radiation Injury Outcomes depend on: Total dose Dose rate Area exposed Variation in species Individual sensitivity Variation in cell sensitivity Variation in tissue sensitivity Age

Sequence of Events Following Radiation Exposure Latent period Period of injury Recovery period Assumption: dose received was non-lethal

Figure 5-4 Concept of accumulated irreparable injury Figure 5-4 Concept of accumulated irreparable injury. After exposure to radiation cell recovery can take place. However, there may be a certain amount of damage from which no recovery occurs, and it is this irreparable injury that can give rise to later long-term effects.

Radiation Effects on Tissues of the Body Deterministic (non-stochastic) Effect — when the severity of the change is dependent on the dose Stochastic Effect — when a biological response is based on the probability of occurrence rather then the severity of the change (i.e., cancer)

Short- and Long-term Effects of Radiation Short-term effects of radiation are those seen minutes, days, or months after exposure. Acute Radiation Syndrome (ARS) symptoms include erythema, nausea, vomiting, diarrhea, hemorrhage, and hair loss.

Short- and Long-term Effects of Radiation Long-term effects of radiation are those that are seen years after the original exposure. The long-term effects observed are somatic damage, which may result in an increased incidence of cancer, embryological defects, low birth weight, cataracts, and genetic mutations.

Figure 5-5 Ulcerated lesion Figure 5-5 Ulcerated lesion. Early carcinoma on the finger of a dentist who admitted holding films in the patient’s oral cavity during exposure.

Figure 5-6 Radiation injury on the finger of a dentist caused by holding films in the patient’s oral cavity during exposure. A lesion of this type would be likely to result in squamous cell carcinoma (cancer).

Risk Estimates A risk may be defined as the likelihood of injury or death from some hazard. The primary risk from dental radiography is radiation-induced cancer and, possibly, the potential to affect pregnancy outcomes. Risk estimates vary, depending on several factors, such as speed of film, collimation, and the technique used.

Risk Estimates In dental radiography, the most critical tissues of the head and neck are the mandible (red bone marrow), the lens of the eye, the thyroid gland, and possibly the hypothalamus-pituitary-thyroid combination.

Table 5-1 Critical Organs and Doses for Dental Radiograpy

Table 5-2 One in One Million Fatality Risk

Table 5-3 Effective Dose Equivalent

Review: Chapter Summary Ionizing radiation has the potential to produce biological damage. There are two generally accepted theories on how radiation may cause damage to cellular tissues: direct theory and indirect theory.

Review: Chapter Summary The terms radiosensitive and radioresistant are used to describe the degree of susceptibility of various cells and body tissues to radiation. Biological changes or damage that occur in somatic cells will affect the irradiated individual but will not passed down.

Review: Chapter Summary Biological changes or damage that do not affect the irradiated individual but are passed to future generations are called genetic effects.

Review: Chapter Summary The dose–response curve is a method used to plot the dosage of radiation administered with the response produced to establish responsible levels of radiation exposure. ALARA — as low as reasonably achievable. Every dose of radiation should be kept to a minimum.

Review: Chapter Summary Factors that influence a biological response to irradiation: dose amount, dose rate, area exposed, species exposed, individual sensitivity, cell sensitivity, tissue sensitivity, and age. Sequence of events following radiation: latent period, a period of injury, and recovery period.

Review: Chapter Summary Deterministic-tissue response that is directly related to the radiation amount. Stochastic effect is tissue response that is based on the probability of occurrence rather than severity.

Review: Chapter Summary Short- or long-term effects: Short — erythema and general discomfort Long — an increased incidence of cancer, embryological defects, poor pregnancy outcomes, cataracts, and genetic mutations. The potential benefits of dental radiographs outweigh the risk.

Recall: Study Questions General Chapter Review

Reflect: Case Study Retaking a radiograph because of a technique or processing error causes an increase in radiation exposure for the patient. Discuss ways a retake radiograph affects the factors that determine radiation injury.

Relate: Laboratory Application Proceed to Chapter 5, Laboratory Application, to complete this activity.