In a provocative article, Finkelstein argued that the occurrence of two brain tumors in Ontario cardiologists in one year is statistically unlikely to the point of suggesting an occupational risk such as radiation exposure related to fluoroscopically guided invasive procedures (3). Although it is unlikely that a cardiologist would reach the threshold of deterministic radiation exposure illness such as radiation sickness, skin breakdown, or immunosuppression, there is another category of health risk that is related to stochastic events. Unlike deterministic events, stochastic events are not related to reaching a threshold of radiation exposure but rather to low-frequency, unpredictable events associated with low-dose radiation exposure. These stochastic events result in an incidence of neoplastic disease that is predictable in populations exposed to low-dose radiation (5,000 deaths per year in the U.S. population due to natural background radiation) (9). The incidence of cancer deaths due to low-dose radiation is 250 per million or 0.025 percent (12). The merits and nuances of these estimates are supplied by Land (6).
This article explores the basic principles of radiation physics that impact the exposure a physician experiences in the routine use of intraoperative fluoroscopy for spine cases; the levels of radiation exposure that have been identified with intraoperative procedures, including spine procedures; the biological effects of this radiation; and the best-practice techniques that will reduce a physician’s radiation exposure.
Radiation is a known carcinogen and there is little argument about the existence of random neoplasm due to low-dose radiation, but the frequency of such events is speculative. These stochastic events are thought to have a linear dose‑response relationship without defined thresholds. Current models of stochastic radiation-induced neoplasm are assumed to be unaffected by dose fractionation; therefore, an individual physician’s risk increases as total radiation exposure increases over the course of a career. This implies that for a physician who routinely uses fluoroscopy in surgical cases, small per case savings in radiation exposure realized by altering technique can lead to a significant reduction in personal risk. Many physicians are misled by the claim that one would have to do thousands of fluoroscopic procedures per year in order to reach the defined occupational threshold risk for radiation exposure that is associated with disease. Because these levels of radiation are related to deterministic risk, the claim is accurate but insufficient for understanding physician risk in that it fails to account for statistically identifiable, nondeterministic (stochastic) risks.
X-rays are high-energy photons that penetrate a patient’s body and are received by a contralateral image intensifier. The amount of X-rays that penetrate in a given unit of space is related to the density of the tissues that are encountered between the source beam and the image intensifier. Differential penetration creates contrast between dense substances such as bone and iodine contrast, intermediate-density substances such as soft tissues, and very low-density substances such as air. The X-rays generated by the fluoroscopy machine that do not penetrate to be intercepted by the image intensifier are either absorbed by the patient’s tissues or scattered. Absorption of X-rays is related to potential adverse biological effects that will be discussed later in this article. Scattered X-rays spread throughout the operative suite and represent the majority of radiation to which physicians and their team members are exposed.
The output of a fluoroscope is described in terms of entrance skin exposure, or ESE. The units of this exposure are roentgens per minute, R/min. Individual exposure to radiation is described by the unit rem. One rem is equal to the energy imparted per unit mass of tissue when a patient is radiated. (One rad delivered to a patient results in one rem of exposure to that patient). The FDA limits fluoroscopy units to a maximum ESE of 10 rads per minute (13). Higher radiation rates, often referred to as “boost” modes, can deliver an ESE of up to 20 rads per minute for a short duration.
Modern fluoroscopy machines often have an automatic brightness control, or ABC, that is designed to automatically increase the intensity of X-rays generated per unit volume, depending on whether adequate signal is received by the image intensifier. A unit equipped with an ABC feature can increase the exposure to an operating physician during a procedure without warning. This is often encountered when the patient is large and the X-ray beam is significantly weakened by the mass of the patient.
The key to understanding operating room risk is the insight that the members of the operating team are predominantly affected by scattered radiation rather than by the primary radiation beam (2). It is an infrequent event that physicians or other members of the operating team will expose themselves to the path of a primary beam. Therefore, the largest source of X‑ray exposure is from scattered radiation ( Compton radiation) that results from the beam interfacing with the patient. Scattered radiation is noncoherent, multidirectional radiation that is highest near the patient’s body surface and diminishes based on the distance between the patient’s body surface and the physician (7). In a lateral exposure of the spine, personnel on the side of the beam source are exposed to the highest dose of radiation due to the large amount of radiation that is backscattered by the patient, the positioning frame, and the table (1). During pedicle screw placement, radiation exposure to the thyroid is three to four times greater on the X-ray beam source side of the table than on the image intensifier side of the table (8). The dose to a surgeon’s torso is significantly increased when the surgeon stands on the side of the X-ray source (53 millirem/min) compared to standing on the side of the image intensifier (2.2 millirem/min) (8).
The reason for this differential is that backscattered radiation is greatest at the initial beam-patient interface and thus accrues to personnel ipsilateral to the beam source. Individuals standing on the image intensifier side of the table are not affected by radiation that is attenuated by patient body absorption, the backscattering effect described above, and image intensifier absorption.
It is well established that the amount of radiation one is exposed to during a fluoroscopic procedure is related to the distance between the individual and the source of the radiation (10, 4). In this instance, regardless of the side of the table on which one stands, the amount of radiation exposure can be dramatically reduced by increasing the distance between the physician and the patient. Given the noncoherent, multidirectional nature of scattered radiation, the radiation per unit volume diminishes significantly with increasing distance from the source. This is referred to as the inverse square law which characterizes the reduction in radiation exposure as an exponent of distance from the source. This mathematical relationship predicts that a significant reduction in radiation exposure is achieved by adding even a fraction of a meter to the distance between a Compton radiation source and an operating physician.
The biological effects of radiation are determined by the amount of radiation exposure, the sensitivity of the specific cell line involved and the susceptibility of the individual (11, 5). The relative sensitivity of human cell lines is greatest for lymphocytes, erythrocytes, and epithelial and endothelial cells and is lowest for neurons, bone, and muscle. This correlates with the mitotic activity of the cell line, and vulnerability is driven by the proportion of cells undergoing mitotic activity per unit time; the higher the rate, the higher the sensitivity to radiation exposure. The absorption of energy from a high-energy photon often results in cell destruction but on occasion can result in an injury that produces a biologically modified cell which may initiate a neoplasm.
The role of radiation exposure in spine surgery has been specifically evaluated and found to be 10 to 12 times greater than the radiation exposure during other fluoroscopically assisted nonspinal musculoskeletal procedures (8). This increase in exposure is related to the amount of energy required to penetrate the torso, which is thicker than limbs, and the proximity of the surgeon’s hand to both the primary and backscatter sources of radiation during operative imaging.
Best Practices
Based on the stochastic model of injury related to low-dose radiation and
the physics of radiation exposure in the operating room, a series of best practices
can be identified and in many instances verified for reducing the radiation
exposure to the operating physician and team, as well as for reducing the likelihood
of injury related to low-dose radiation.
A primary issue is the distance between personnel and the patient during an X-ray exposure. This distance should be as great as possible. Typically, the radiation exposure becomes extremely low at a distance of three meters from the patient. The opportunity to move three meters from a patient during placement of a pedicle screw, for example, is sometimes limited, but even making small increases in the distance from the patient will reduce radiation exposure. The operating physician who is most likely to be required to stay closest to the patient during an X-ray exposure should always be positioned on the side of the image intensifier. Those personnel who can move at least three meters from operative field during this portion of the case should be positioned on the source side of the fluoroscopy unit.
All personnel should use lead aprons, which markedly reduce the amount of radiation exposure. The most effective lead shielding is a wraparound two-piece garment, which gives 360-degree protection of both the upper and the lower torso. So-called lightweight lead aprons may sacrifice lead thickness for comfort, resulting in proportionately less protection.
The fluoroscopic technique should be designed to produce an adequate image with the minimal amount of penetrating beam energy for the shortest period of time. The use of short “looks” is preferable to a continuous exposure. The use of boost and magnifying modes, which increase the amount of high-energy photons generated, should be limited as much as possible.
The amount of radiation exposure also will be lessened by keeping the image intensifier as close as possible to the patient’s body surface. This significantly reduces the exposure to a physician on the side of the image intensifier and reduces the likelihood that an ABC system will compensate for a decreased signal due to the scatter between exiting body surface and the image intensifier. Dimming the room lights often can improve the contrast of the image displayed on the screen, reducing the need to boost X-ray beam energy to achieve contrast resolution.
When taking anteroposterior projections of the spine, it is best to place the image intensifier above the table and the source beam below the table. There is a significant amount of scatter from the table with the source below the table, and this relatively predictable scatter constrains much of the exposure profile to the lower torso; an adequate distance from the table and a wraparound lead apron protecting the tissues below the waist adequately compensate for this exposure.
Patrick W. McCormick, MD, FACS, MBA, associate editor of AANS Neurosurgeon, is a partner in Neurosurgical Network Inc., Toledo, Ohio. The author reported no conflicts for disclosure.
What Do You Think?
What potential operating room hazards are of particular concern to you? Tell us in a letter to the editor or send us your ideas for topics to be explored in future articles. A link to instructions for all types of submissions to AANS Neurosurgeon is available at www.aansneurosurgeon.org.
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