Disclaimer: This newsletter, provided by ITIS, is funded by a grant from the Illinois Department of Public Health and supported by Northwestern Memorial Hospital and Northwestern University Medical School. It is for educational purposes only and is meant to summarize the information available at the time of its creation. It should be construed neither as medical advice nor opinion on any specific clinical situation. For more information on a specific clinical situation,or updated information, please consult your health care provider.

RADIATION AND PREGNANCY

Vol. 2#2 December, 1993

Eugene Pergament, MD, PhD; Amy K. Stein, MS, CGC; Judith L. Miller, MS

Radiation exposure during pregnancy often prompts many physicians and patients to contact the Illinois Teratogen Information Service. This RISK||NEWSLETTER is intended to provide a useful framework for evaluation of radiation exposure during pregnancy.

Ionizing radiation can be produced by electromagnetic waves consisting of uncharged photons such as x-rays and gamma rays. The terms x-ray and gamma rays refer to the origin of radiation; x-rays are generated by electrons whereas gamma-rays are emitted by nuclei involved in radioactive decay. Charged particles such as alpha and beta particles emitted by radionuclides can also produce ionization. All radiation is characterized by its energy, so no matter what the source, a radiation exposure can be expressed in the same units, either rad or rem.

The amount of radiation can be measured in two ways: ionizations produced in air by x-rays or gamma radiation from any source are measured in roentgens ®, and the radiation dose absorbed by the body tissue is measured in rad or Gray (Gy) (1 Gy = 100 rad, cGy = rad, mGy = 0.1 rad). Exposure of typical tissues to 1R yields approximately 1 rad of absorbed dose. Different types of ionizing radiation, however, can deposit the same total energy but produce different degrees of biological damage. The dose equivalent, expressed in Seivert (Sv) or rem (1 Sv = 100 rem, 1 mSv = 0.1 rem), reflects the differing biological effects of specific types of radiation. For those types of radiation typically used in hospitals, rem and rad are interchangeable. However, with alpha or neutron radiation (such as from plutonium or radium) the dose in rem would be a multiple of the rad. Radiation protection standards generally are expressed in rems; experimental studies often report radiation exposures only in R; dosimetry calculated for medical irradiation may be given in rad or rem (Jankowski 1986, Brent et al. 1993).

The following simplification of the relationship may be helpful: a dose of approximately 1 rem or an absorbed dose of approximately 1 rad is imparted to human tissue at a location where a survey instrument indicated an exposure of 1 roentgen.

Radiation, the transmission of energy from one body or source to another, can be broadly classified into ionizing and non-ionizing radiation. Ionizing radiation is any form of radiation with sufficient energy to displace orbital electrons, resulting in the formation of electrically charged ions in matter. Examples of non-ionizing radiation are radar, microwaves, and magnetic resonance imaging.

The use of the term radiation to describe ultrasound is both confusing and erroneous. Ultrasound is a form of mechanical energy that produces oscillations in an elastic medium (water, tissue) at frequencies above the threshold for human hearing. Although there are still ongoing studies investigating the effects of ultrasound, this form of energy appears to be relatively safe (Brent 1989, 1993).

The biological effects of ionizing and non-ionizing radiation vary considerably. Exposure to non-ionizing radiation presents no measurable risk to the embryo or fetus (Brent et al. 1993). This RISK||NEWSLETTER therefore will consider only the ionizing forms of radiation.

Most information on the effects of acute exposure to ionizing radiation has come from epidemiological studies of children born to Japanese Atomic bomb survivors. One limitation of these studies is that they are examining the effects of a single, relatively high dose exposure and not of intermittent or continuous low dose exposures typically experienced in medical, occupational, or environmental situations (Boice 1990). In addition to animal studies, other sources of information have been patients treated with a radiotherapeutic procedure before or during pregnancy.

As with any teratogen, the effects of prenatal exposure to radiation will differ depending upon the timing of the exposure. Preconceptional radiation effects on the germ cells, expressed by reduced fertility after radiation, has been well documented. The most critical target for ionizing radiation is DNA, so the possibility of radiation induced mutations in the parental germ cells exists. The most radiosensitive period is during rapid cell division in the testis. This occurs during spermatogenesis and may involve a period of up to six months prior to ejaculation. It has been suggested that the oocyte resting in suspended meiosis I is resistant to radiation induced mutations (Russell 1986). However, during the six or seven weeks prior to ovulation, when the first meiotic division starts, the oocyte becomes sensitive to damage by irradiation. But even during these periods of increased sensitivity, the magnitude of the genetic risk due to any kind of preconception radiation exposure appears to be insignificant (Brent et al. 1993, Russell 1986).

In order to assess the teratogenic risk of radiation exposure after conception, the stage of embryogenesis at the time of the exposure has to be considered. During the preimplantation period (approximately the first 14 days after conception), it is believed that radiation exposure will either be lethal or have no effect ("all or none") (Jankowski 1986). The period of organogenesis (from the end of the second week to the eighth week post-conception) is usually considered the most sensitive period for malformations. The central nervous system (CNS) is the most sensitive organ system as radiation induced malformations of other structures are uncommon (Mole 1991). The fetal period (from eight weeks until the end of gestation) is a time of decreasing sensitivity to most environmental agents; nevertheless, the CNS continues to develop throughout gestation, retaining sensitivity to radiation. Studies on Japanese A-bomb survivors who were exposed in utero have suggested the following critical periods for developing severe mental retardation (SMR) (Otake and Schull 1984). The probability for radiation related SMR is essentially zero with exposure before 8 weeks post-conception, is at a maximum with irradiation between 8 and 15 weeks, and decreases between 16 and 25 weeks. After 25 weeks gestation (and for exposures of less than 100 rad), no cases of SMR have been reported.

There is general agreement that fetal exposure of less than 5 rads is not considered teratogenic (Bentur 1991, Brent et al. 1993). It cannot be stated that there are no risks associated with lower doses, however. It is important to stress that the risks are determined by the level of the fetal exposure, which is likely to be much less than the maternal exposure. On the other hand, it may be difficult to determine fetal dose accurately, especially for radioisotope exposure.

One of the most frequent radiation exposures during pregnancy is a diagnostic x-ray from CT scan or fluoroscopy. To determine the fetal dose, factors such as patient size, equipment used, and technical considerations have to be taken into account. Russell (1986) stated that the gonad dose for similar examinations can vary between hospitals by two orders of magnitude. Nevertheless, there are few diagnostic studies, which result in a dose greater than 1 rem to the uterus. Certainly, diagnostic x-ray doses exceeding 5 rads to the fetus would be exceptional even when fluoroscopic diagnostic studies are involved.

The estimated fetal dose from a dental x-ray is 0.00001 rad and from a chest x-ray, 0.06 rad (Reprotox) or <0.01 rem (Jankowski 1986). Two diagnostic x-ray procedures, which may expose the fetus to higher levels, are a barium enema (0.8 rad; [Jankowski 1986, Reprotox 1993) and lumbosacral spine x-ray (0.64 rem [Jankowski 1986). A CT scan of the abdominal area may give an ovarian dose of 0.5 to 1.1 rems, depending on the circumference of the patient and technical factors.