Radiation Safety in Premature and Critically-Ill Neonates

KEY POINTS

• 1.

  Digital radiography is a frequently used imaging technique in neonatal intensive care units (NICUs), with many infants having multiple radiographs during their stays.

• 2.

  High-energy photons in ionizing radiation can damage DNA, particularly with repeated natural background exposures and diagnostic medical imaging.

• 3.

  Radiation dose is a measure of the amount of exposure of the tissues of the body to radiation; it is determined by the energy, duration, and area of beam exposure.

• 4.

  In NICUs, the highest reported total radiation doses are fortunately less than the annual natural background effective dose. However, caregivers need to remain cognizant of the risks. The significance of scatter or secondary radiation in the vicinity of a radiographic exposure also needs to be considered. Several guidelines have been established to minimize these risks.

• 5.

  A key concept that guides imaging use in the NICU is the As Low as Reasonably Achievable (ALARA) principle. The ALARA principle is the offspring of the linear no-threshold hypothesis, which implies that there is no safe dose of ionizing radiation.

Introduction

The benefit of medical imaging in saving lives and improving treatment for the premature and critically-ill neonate is well established. Digital radiography is the most frequently used imaging technique in the neonatal intensive care unit (NICU), with many infants having multiple radiographs during their stays. These images, largely of the chest and abdomen, provide critical information about life-support device placement, acute life-threatening conditions such as tension pneumothorax and bowel perforation, and disease response to medical and surgical interventions. To generate these important diagnostic images, digital radiography—along with fluoroscopy and computed tomography (CT)—use low doses of ionizing radiation.

Substantial scientific data show that high-energy photons from ionizing radiation can damage DNA when delivered at high doses; however, these effects occur at many multiples of the doses received in diagnostic medical imaging and natural background exposure. All humans are exposed to low doses of ionizing radiation from ubiquitous sources such as cosmic and terrestrial radiation; the degree of exposure depends on variables such as altitude and home ventilation. Although children are more radiosensitive than adults (i.e., the cancer risk per unit dose of radiation is higher) and have longer lifetimes over which to express a radiation-induced genetic mutation, adaptive DNA repair mechanisms appear to function well in children exposed to low doses of radiation. We know that delivering doses at increments rather than all at once allows tissues to recover, and the risk from multiple serial exams is not cumulative. Most important, the radiation doses to which even the most frequently imaged critically-ill neonates are exposed remain a fraction of the annual natural background radiation. Because ionizing radiation is one of the potential iatrogenic harms to which neonates and their parents and caregivers may be exposed, it is important to understand how dose is measured and optimized in neonatal radiographic, fluoroscopic, and CT imaging.

Quantifying Radiation Exposure

Radiation dose is a measure of the amount of exposure of the tissues of the body to radiation. The dose of radiation to which a patient is exposed is determined by three factors: the energy of the x-ray beam, the duration of beam exposure, and the area over which the beam is applied. There are several methods used to quantify radiation dose; the most useful are calculations of absorbed dose, equivalent dose, and effective dose. Absorbed dose, expressed in milligrays (mGy), is the amount of energy deposited by radiation into an absorbing medium; the mass can be anything—people, water, air, or a rock. Equivalent dose, measured in millisieverts (mSv), is calculated for individual organs. Effective dose, also measured in mSv, is calculated for the whole body; it is the summation of equivalent doses to all organs, each adjusted according to the organ sensitivity to radiation. Effective dose is the preferred metric for reporting radiation dose and is widely used in the scientific community to compare different techniques for dose optimization. It is important to recognize that the International Commission on Radiological Protection introduced the use of effective dose for the targeted purpose of setting limits for radiation protection—not to predict cancer risk among exposed persons. The formula for calculating effective dose incorporates weighting factors for radiation quality and organ sensitivity across all ages and both sexes; as a result, this formula does not apply to any specific individual or radiosensitive subpopulations such as children.

The average annual effective dose of natural background ionizing radiation in the United States is 3 mSv, with a range from 1 to 20 mSv. At these background doses, there is no direct evidence of harm. A person might accumulate an effective dose from natural background of about 50 mSv in the first 17 years of life and about 250 mSv during an average 80-year lifetime. For healthcare workers with occupational exposure, the International Commission on Radiological Protection recommends a dose limit of 20 mSv/year. In the United States, the National Council on Radiation Protection and Measurements sets a dose limit of 50 mSv/year for these individuals.

Estimation of exposure to medical radiation in the NICU has been reported using modern digital radiographic techniques. These studies report effective dose ranges from 0.012 to 0.016 mSv per radiograph. Total effective doses for some highly imaged infants has been reported to be as high as 1.5 mSv for patients with chronic lung disease or necrotizing enterocolitis. Even the highest reported total radiation doses are less than the annual natural background effective dose. In order to help caregivers better understand dose metrics, use of a relative risk descriptor comparing medical radiation dose to natural background dose is helpful. Because of the complexity of scientific dose metrics, the background equivalent radiation time was designed to educate the general public about radiation dose without complex concepts or terminology. Using the background equivalent radiation time relative risk descriptor, a single portable radiograph of the chest would be the equivalent of approximately 1 day of natural background radiation ( Table 6.1 ).

Another concern that arises in the NICU is the significance of scatter or secondary radiation in the vicinity of a radiographic exposure. ^, Scatter radiation occurs when the beam intercepts an object—most frequently the patient’s body—that causes the x-rays to be scattered. The radiation dose from scatter is a very small fraction of the dose received from the primary x-ray beam. An important principle of dose reduction is the inverse square law; this property of physics states that if one doubles the distance from the primary radiation source, dose is reduced by a factor of four. To minimize the amount of scatter radiation dose to caregivers, parents, and neighboring patients in the NICU, these individuals should be positioned at least 1 meter from the irradiated field; at this distance, the primary scatter radiation is estimated to represent approximately 0.1% to 0.2% of the incident radiation. If healthcare personnel are required to be within 1 meter of the radiation field, they should wear a lead apron.

Another key concept that guides imaging use in the NICU is the As Low as Reasonably Achievable (ALARA) principle. The ALARA principle is the offspring of the linear no-threshold hypothesis, which implies that there is no safe dose of ionizing radiation. The ALARA principle is based on the assumption that low doses of radiation might be harmful and therefore should be minimized for medical imaging procedures.