Louis K. Wagner, PhD
You Do Not Know What You Are Doing Unless You Know What You Are Doing1 Why does that pesky 5-minute timer exist on all fluoroscopes? What purpose does it serve? Twenty-five years ago, I was taught that it existed to keep the fluoroscopist mindful and wary about the duration of fluoroscopy, with the assumption that this would encourage discretionary use of fluoroscopic radiation. In my years of experience, I have witnessed wise use and blatant abuse of the timer. Some have considered it a nuisance and others have disabled the timer, a violation of federal and state regulatory requirements. In my experience, the timer has proved useful in managing radiation. A few years ago, complaints arose from technologists that physicians were using too much fluoroscopy time during gastrointestinal procedures, as judged from the number of times the 5-minute timer had been reset. We instituted a policy that all cumulative fluoroscopy times would be recorded in a quality-control log and then reviewed at monthly performance-improvement meetings. In general, gastrointestinal tract fluoroscopy times were on the order of 2–5 minutes, well within rea-
Index terms: Editorials Fluoroscopy, technology Radiations, exposure to patients and personnel Radiations, injurious effects Radiations, measurement Published online before print 10.1148/radiol.2252020805 Radiology 2002; 225:327–328 1
From the University of Texas–Houston Medical School, 6431 Fannin St, Houston, TX 77030. Received July 1, 2002; accepted July 2. Address correspondence to the author (e-mail: firstname.lastname@example.org).
See also the article by Miller et al in this issue. ©
son for the procedures. A few of the lengthy outlier examinations were found to be justified on the basis of the difficulty of the procedure. On rare occasions, an unusually long examination was recorded, which prompted a modification of procedures. This performance-improvement review had several benefits. There were no more complaints from technologists. Perhaps their initial complaints were exaggerated, or perhaps the physicians were more conscientious about their fluoroscopy times because they were being watched. One thing was clear, before the performanceimprovement policy was introduced we had no idea about how reasonable or unreasonable our fluoroscopy times were. After the policy was initiated, we felt confident about the adequacy of our practice, and we were able to inaugurate some helpful changes as a result. This once again proved one of my favorite adages: “You do not know what you are doing unless you know what you are doing.” Today, things are different. Many complex interventional procedures routinely require 20 – 40 minutes of fluoroscopy time, and some have recorded over 200 minutes. That means that the timer must be reset anywhere from four to 40 times during procedures. How are we to use the timer for these procedures? Should we just ignore it? Obviously not, since debilitating radiation-induced skin injuries are a proven risk of extended procedures (1). But just how useful is the timer for these procedures? It records fluoroscopy time only and not fluorography time or frames, which make up a good portion of many procedures. The timer has little direct relationship to skin dose, which is the dose of most immediate concern for the health of the patient. By using advanced dose-saving equipment in some patients, 200 minutes of fluoroscopy time might not even reach the threshold for the dose that induces the delayed phase of skin erythema. For other pa-
tients examined with identical fluoroscopy time and settings but with different equipment, the dose could well exceed that capable of causing deep tissue necrosis (2). With such a wide range of possibilities for the same fluoroscopy time, simply being mindful of the 5-minute reset or simply recording fluoroscopy time for quality-control and performanceimprovement purposes is no longer adequate. In the current issue of Radiology, Miller et al (3) compare several real-time methods that can be used to monitor radiation dose during interventional procedures: fluoroscopy time, dose-area product (DAP), cumulative dose at a reference point, and skin-dose mapping. What these authors teach us is that the 5-minute timer ranks dead last in utility as a skin-dose monitor. Equally important, they teach us that most current and easily available methods to measure radiation are inaccurate and sometimes misleading. For complex interventional fluoroscopy, the literature on skin injury (4) indicates that we need to know the entrance skin dose in order to prevent skin damage— something that the majority of fluoroscopes do not measure or display. On most new fluoroscopy units sold in the United States, DAP meters are available. Miller et al (3) along with other authors (5) also teach us that DAP meter readings correlate poorly with skin dose and may be more misleading than informative in some circumstances. For example, for a well-collimated beam, the DAP is low because the field size is small; contrarily, however, the actual skin dose is greater with the application of collimation. In other words, neither the 5-minute timer nor the DAP meter empower us with information on skin dose, knowledge that we need in order to know what we are doing. This lack of a reliable skin-dose monitor during fluoroscopy needs to change. The accurate measurement of skin dose 327
in real time is a nontrivial task. Skin dose depends substantially on many factors, including source-to-skin distance, collimation, and radiation output that automatically varies with multiple machine settings and with changing patient anatomy. Additionally, the entrance skindose site varies as the table and the C-arm move during the procedure, which makes the tracking of skin dose a convoluted problem. Miller et al (3) report on a technique that accounts for these variables and provides a real-time map of the skin-dose distribution that accrues during a procedure. This map of skin dose shows the user a lot about dose delivered during procedures and empowers users with the knowledge they need in order to know what they are doing. This knowledge is used by the authors to improve dose management, something that is wanting when we do not know what we are doing—an ignorance that has resulted in severely debilitating injuries in some patients (1,4). With hard data on which to act, Miller et al discuss not only the effectiveness of established principles of dose reduction but are also able to vividly reemphasize the effectiveness of skindose reduction in extended interventional procedures by combining collimation and changes in beam orientation. To quote Miller et al: “Before this capability was available, we all believed that we were managing dose effectively. However, all of us have modified our technique based on what we have learned from the use of this tool.” This proves once again the adage, you do not know what you are doing unless. . . The sad truth is that this dose-mapping tool is no longer offered for sale on new equipment manufactured by the vendor. This has come about due to a lack of
demand for the device, in part because of added cost to the unit and low interest from the buyer. This tool teaches us much that we would never know without it. It is unfortunate that we as consumers have not placed a greater demand for acquisition of this tool for our interventional suites. So, unless the community demonstrates a revitalized attitude toward real-time skin-dose mapping, it is likely that this device will be relegated to the dust heap of history as a highly useful but largely ignored instrument. But there is still hope. The International Electrotechnical Commission has specified that fluoroscopy equipment intended for interventional use be equipped with a cumulative-skin-dose monitor (6), and the U.S. Food and Drug Administration has discussed a proposal of a similar nature. These monitors provide the user with information about the cumulative dose at a reference point that approximates the skin-dose site. The reference point is located 15 cm from the isocenter toward the x-ray tube side of the C-arm. Miller et al (3) point out that this particular tool is not as good as the mapping tool because these data may over- or underestimate the actual skin dose; in lieu of a skin-dose mapping monitor, however, this is the next best thing. Cumulative dose does not inform the user about the virtues of collimation, the changes in beam orientation, or the effects of table height and position on the distribution of dose across various skin sites, but it will be useful in helping the physician manage the benefit/risk tradeoff for the patient and prevent the use of dangerous dose levels. For users of established equipment, some vendors offer a device that can be retrofitted onto existing
equipment to perform essentially the same function (7). In the meantime, if you are an interventionalist who operates a fluoroscope that has no skin-dose monitoring equipment and you have not initiated any program to help estimate what the doses might be for your patients, then all you have is a 5-minute timer and perhaps a DAP meter, neither of which let you know what you need to know in order to know what you are doing. Acknowledgment: The author thanks Alan Cohen, MD, for his thoughtful discussions on skin dose monitors. References 1. Koenig TR, Wolff D, Mettler FA, Wagner LK. Skin injuries from fluoroscopically guided procedures. I. Characteristics of radiation injury. AJR Am J Roentgenol 2001; 177:3–11. 2. Wagner LK, Archer BR, Cohen AM. Management of patient skin dose in fluoroscopically guided interventional procedures. J Vasc Interv Radiol 2000; 11:25–33. 3. Miller DL, Balter S, Noonan PT, Georgia JD. Minimizing radiation-induced skin injury in interventional radiology procedures. Radiology 2002; 225:329 –336. 4. Koenig TR, Mettler FA, Wagner LK. Skin injuries from fluoroscopically guided procedures. II. Review of 73 cases and recommendations for minimizing dose delivered to patient. Am J Roentgenol 2001; 177:13–20. 5. Vano E, Gonzalez L, Ten JI, Fernandez JM, Guibelalde E, Macaya C. Skin dose and dose-area product values for interventional cardiology procedures. Br J Radiol 2001; 74:48 –55. 6. International Electrotechnical Commission. Medical electrical equipment: part 2-43—particular requirements for the safety of x-ray equipment for interventional procedures, IEC 60601-2-43. Geneva, Switzerland: International Electrotechnical Commission, 2000. 7. Cusma JT, Bell MR, Wondrow MA, Taubel JP, Holmes DR. Real-time measurement of radiation exposure to patients during diagnostic coronary angiography and percutaneous interventional procedures. J Am Coll Cardiol 1999; 33:427– 435.