Radiation Safety

Absorbed Dose

The absorbed dose is a measure of the amount of energy deposited in a medium by ionizing radiation per unit mass of matter and is equal to the amount of heat generated by the radiation per tissue weight in a specified material. The International System of Units (SI) measure for absorbed dose is the gray (Gy), named after Louis Harold Gray, a British physicist. One gray equals 1 joule (J) of energy absorbed per kilogram (J/kg). Historically, the unit most commonly used was the radiation absorbed dose (rad); 1 Gy = 100 rad. The absorbed dose is not an accurate indicator of biologic effect given that different types of ionizing radiation produce different degrees of tissue damage.

Equivalent Dose

The equivalent dose, a measure of the radiation dose to tissue, takes into account the different degrees of damage by different types of radiation by introducing a radiation weighting factor (W _R ). The SI unit of equivalent dose is the sievert (Sv), named after Rolf Sievert, a Swedish physicist. Thus, equivalent dose = absorbed dose × W _R . Another unit, the roentgen equivalent man, or rem, is still sometimes used; 1 Sv = 100 rem.

W _R is calculated based on the type of radiation, using 1 for X-rays and gamma rays and 3–10 for protons and neutrons. The sievert better describes the biologic effect of radiation and is commonly used when risk from ionizing radiation is assessed. It also allows quantification of risk and comparison to other commonly encountered modes of exposure. For fluoroscopic interventions in vascular surgery, the sievert and gray are roughly equal; absorbed dose = equivalent dose.

Effective Dose

Different tissues and organs have different sensitivity to radiation, therefore the concept of effective dose is introduced to take into account the part of the body irradiated and the volume and time over which the radiation dose is applied. The effective dose, measured in sieverts, is calculated by weighting the equivalent dose by a tissue weighting factor (W _T ). This calculation takes into consideration the distribution of radiation as well as the radiosensitivity of various organs or tissues. To avoid confusion, W _R and W _T are sometimes grouped together into one single weighting factor (W).

Effective dose may be evaluated prospectively for planning and optimization of radiation protection, as well as retrospectively for assessment of the radiation-associated risk incurred. It is mainly used as a protective and regulatory quantity and not for epidemiologic study of populations. It does not provide a precise indicator of an individual patient’s risk, as there is no consideration of patient age, gender, or other confounding factors.

Because grays and sieverts quantify relatively large amounts of radiation, in medical use, radiation is typically described using milligrays (mGy) or millisieverts (mSv).

Deterministic Effects

Deterministic effects of radiation are dose-dependent and result in cell death, impacting hair follicles, skin, subcutaneous tissues, and the lens of the eye. These acute events occur when a threshold level of radiation has been exceeded, and the higher the dose, the greater the injury ( Fig. 26.1 ). The threshold is not absolute and can vary among individuals. Table 26.2 shows some threshold levels of human organs with corresponding deterministic effects. Doses required to produce deterministic effects are often large and exceed 1 to 2 Sv. Symptoms arise when a significant proportion of cells are killed by radiation, and subsequent inflammation or fibrosis may produce additional damage to the organ. Examples of deterministic effects include radiation-induced dermatitis, cataracts, infertility, and organ atrophy or fibrosis ( Figs. 26.2 and 26.3 ). Given increasing concerns about the late manifestation of cataracts from low doses of ionizing radiation, the recommended threshold of the lens is 0.5 Sv.

Whole-body exposure to 10- to 20-Gy of high-energy radiation, delivered at one time, can be fatal to humans. For acute whole-body equivalent doses, 0.5 to 1Sv may produce light radiation sickness; 1 Sv causes slight blood changes; and 2 to 3 Sv causes nausea, hair loss, and hemorrhage. An acute dose of 3 Sv causes death in 50% of individuals within 30 days, and with doses higher than 6 Sv, survival is unlikely.

Stochastic Effects

Stochastic, or probabilistic, effects of radiation cause DNA damage to single cells, which results in mutation. This is an all-or-none phenomenon, with the probability of occurrence increasing as the cumulative radiation exposure increases without an established threshold level ( Fig. 26.4 ). The severity of the effect of mutation is unrelated to the dose. Mutations may lead to cancer and heritable genetic defects. Theoretically, stochastic effects can occur even at low doses, but it is assumed that with radiation doses of less than 100mSv/year, the probability of stochastic effects is very low. At such low doses, it is assumed that the probability of incurring cancer or heritable effects will be directly proportional to the equivalent dose. This is known as the linear-nonthreshold model of incremental risk.

Leukemia and other cancers have been shown to be associated with radiation exposure. It is estimated that the probability of fatal cancer developing as a result of radiation exposure is 4% per 1Sv of lifetime dose equivalent (or 0.004% per mSv). For reference, the background risk of spontaneous fatal cancer is approximately 20%. The nominal nonfatal cancer risk has been estimated at 0.8% per sievert. Radiation-induced cancer risks from relatively low-dose exposure are difficult to estimate. A statistically significant increase in cancer risk has not been demonstrated in populations exposed to doses of less than 100 mSv. Studies of atomic bomb survivors showed an increased incidence of leukemia and other tumors of the lung, thyroid, breast, skin, and gastrointestinal tract. There is usually a latent period of 2 to 5 years for the leukemia, 5 years for thyroid cancer, and 10 or more years for other cancers to manifest. The risk of radiation-induced malignancy with prenatal exposure or for children and adolescents is slightly higher than that seen in adults by a factor of 2 to 3. Fetal effects are discussed in the “Radiation and Pregnancy” section, later in this chapter.

Hereditary effects from radiation exposure have not been observed in humans in studies of the offspring of atomic bomb survivors. Based on extrapolation from animal studies, the ICRP proposed a nominal risk of heritable effects of 0.1% per sievert.

Background Radiation

We are constantly exposed to radiation through naturally occurring radioactive materials, as well as by cosmic radiation and human activities. The average annual natural background radiation, which varies depending on the geographic location, is around 3mSv/year in the continental United States. Living at higher altitude is associated with a higher dose. The greatest source of domestic radiation is radon gas (about 2mSv/year). Radon arises from the decay of radium; it seeps out of the soil and may concentrate in poorly ventilated concrete homes because of its high density. Radon is the second most frequent cause of lung cancer after cigarette smoking. Other manmade radiation sources include building materials, fuel, televisions, smoke detectors, and various fluorescent devices.