Answer to Question #825 Submitted to "Ask the Experts"

Q

How much x-ray dose is on average absorbed during cardiac catheter procedures, and what is the risk to the patient, either short-term risk, or long-term in causing cancer, for example?

A

The dose from a cardiac catheterization procedure is going to depend on the total time of x-ray exposure and the actual output or exposure rate (at the patient's skin) from the x-ray tube. Keep in mind the configuration of the x-ray equipment: an x-ray tube is located below the patient on the table and an image intensifier is located above the patient. The image intensifier intercepts the radiation being transmitted through the patient and is used to produce the images on the monitors observed by the cardiologist standing next to the patient. As the cardiologist angles the x-ray tube/image intensifier assembly (to view the heart and vessels at different angles), the equipment is constantly adjusting the x-ray energy to produce the highest-quality images. As a result, the actual exposure rate to patient changes slightly throughout the procedure. Furthermore, the exposure rate will vary slightly from unit to unit (depending upon the manufacturer and the equipment design). At the hospital where I work, the average exposure time for a cardiac catheterization procedure is under 10 minutes. Some procedures are diagnostic in nature and may require short exposure times. Other procedures, however, are therapeutic (such as angioplasty to reduce blockages in the vessels) and may require longer exposure times. So the actual exposure time difference from patient to patient may be significant (depending upon the procedure, the patient's condition, and whether there are any complications during the procedure). There is a federal limit for the entrance skin exposure (known as "ESE") to the patient (when the equipment is operated in "normal" dose mode): 10 R per minute. (If the equipment includes "high" dose mode, in normal mode the limit is 5 R per minute and the recommended limit during high dose mode is 20 R per minute.) The exposure rate to an average patient is about 2.5 R per minute. (Again, this will vary, depending upon the equipment and the patient size.) Assuming that 1 R of exposure is approximately 1 rad of absorbed dose in soft tissue, for 10 minutes of exposure time, the skin dose (localized to the area on the patient's back) will be about 25 rad. With regard to risks of radiation exposure, there are two types of effects: deterministic (nonstochastic) and stochastic effects. Deterministic effects are those effects for which severity is a function of dose (i.e., the more radiation dose, the more "worse" the effect). A minimum dose (or threshold) exists for these types of effects. Examples include erythema (skin reddening) and epilation (hair loss) at the location of radiation exposure. These effects may be observed a few hours to many weeks after exposure. Stochastic effects are those effects for which the probability is a function of dose (i.e., the higher the dose, the higher the probability of the effect). Examples include cancer and genetic effects. The latent period for diagnosis of these effects may be a few years to several decades. While it is hypothetically possible that any dose of radiation, no matter how small, could induce cancer, the risk of stochastic effects is low. Keep in mind that with cardiac catheterization procedures it is not the entire body that is being exposed to radiation; only a localized area on the patient's back is exposed. So, with regard to radiation exposure, the risk from these procedures is related to skin effects. Transient or temporary erythema (skin reddening) occurs at doses greater than about 200 rad. With the information given above (2.5 R per minute for an average patient), about 80 minutes of x-ray exposure time would be required . . . and this is unlikely to occur. HOWEVER, in high-dose mode, assuming 20 R per minute, only about 10 minutes of exposure time would be required to produce the effect. And since severity is a function of dose, the effects for very long exposure times may be worse than erythema; in the most extreme, worst cases, ulceration and necrosis may occur. (And the onset of symptoms may occur hours after exposure . . . to many weeks after the procedure for the more severe effects.) As with all medical procedures involving exposure to ionizing radiation, the benefits outweigh the risks associated with exposure. I urge you to speak with your physician to address any concerns you have. (Generally, risks from procedures are discussed with the patient during the "patient consent" process.) Carmine M. Plott, Ph.D., CHP