Clinical Editor's Corner

Do You Know Your Radiation Dose During Your Cath?

Morton Kern, MD Clinical Editor, Professor of Medicine, Associate Chief Cardiology University of California Irvine, Orange, California mortonkern2007@gmail.com

Morton Kern, MD Clinical Editor, Professor of Medicine, Associate Chief Cardiology University of California Irvine, Orange, California mortonkern2007@gmail.com

I recently learned that I could see my real-time x-ray dose while doing a case. My x-ray system (Philips, Bothell, Wash.) has a display (see Figures) on the left side of the monitors that shows my table height, source-to-image distance (SID), and flat detector (FD) distance, and below it shows the fluoroscopy settings (low, normal), fluoro time, K mGy/min, patient dose area product (DAP) (I think) and air kerma (AK) in mGy. It was also interesting that by raising and lowering the tube, table, or fluoro setting, the operator can change dosage and know, in real time, in which direction. This seems to be a real step forward toward the better awareness of radiation safety in the lab. I inquired of my cath lab colleagues across the country about radiation monitoring and safety in their labs and here’s what I found.  

Q: Is the same information (about radiation dose) displayed on other x-ray units by Siemens, Toshiba, GE, and other manufacturers? 

Most said yes, but the display may be in the control room and not necessarily on the screen in front of the operator. Dr. Charles Chambers of Harrisburg, Pennsylvania State University College of Medicine, MS Hershey Medical Center, Hershey, Pennsylvania, reported to me that since 2006, all newly manufactured fluoroscopy units are required to display dose. The current definition of radiation exposure nomenclature and terms was published in March 2011 in Catheterization and Cardiovascular Interventions,1 and is certainly a must-read for anyone interested in radiation safety.

Q: How do we assess patient radiation dose?

We measure fluoroscopy time (FT, min), total air kerma, air kerma area product, and peak skin dose. Each measurement tells us something about what the patient receives. 

Fluoroscopy time is the time during a procedure that fluoroscopy is used, but does not include cine acquisition imaging. Therefore, FT alone is not a useful indicator of patient radiation dose.

Total air kerma (AK) at the interventional reference point (also called Ka,r, Gy) is the cumulative AK. AK is the x-ray energy delivered to air at the interventional reference point. This is the radiation monitoring value required on interventional x-ray systems since 2006. Ka,r is used to monitor patient dose as it is associated with deterministic skin effects. The deterministic effect is the radiation dose that produces a direct health effect. Experience and research have established threshold values for various effects. AK is, however, not the true peak skin dose.

Air kerma area product (PKA, Gy/cm2) is the sum of the product of instantaneous air kerma and x-ray field area. It is commonly reported by modern systems and is used to monitor patient dose and the possible risk of stochastic effects (radiation-induced chances of producing cancer). The stochastic effect is the biologic effect of radiation that occurs by chance to a population of persons, whose probability is proportional to the dose and severity independent of the dose. A single threshold is not known and not specific for stochastic effects to occur.

Peak skin dose (PSD, Gy) is the maximum radiation dose received by any local patient skin area. Both the probability and severity of skin effects increases as PSD increases. PSD is highly dependent upon instantaneous dose rate and the duration that the x-ray is directed at a specific body area. PKA is the total radiation delivered to a patient and can be utilized to accurately estimate skin dose as a research tool, but not in clinical practice. There is no currently available method to measure PSD. However, it can be estimated when the air kerma and x-ray geometry details are known. Therefore, when a significantly high Ka,r occurs, it is important to estimate early post-procedure PSD, employing a qualified physicist so that accurate, relevant information regarding potential radiation-induced injury can be addressed and treated by health care providers.

Q: What is the best number to use for radiation dose? 

Dr. Chambers tells us that for best assessment of dosage, the Europeans use DAP (dose area product), or KAP (kerma area product), or what the National Council on Radiation Protection & Measurements (NCRP) now classifies as air kerma area product in Gy/cm2. This dose can be correlated to skin injury, but is more often used for stochastic (likelihood of cancer in the future) assessment. The true reflection of skin injury, used by the Joint Commission as a sentinel event, is peak skin dose (PSD), but this requires calculation by a physicist.

Q: What is the maximum radiation dose that triggers an incident report? 

The Joint Commission follows Gray units (Gy) and considers exposure over 15 Gy as a sentinel event. The Quality Assurance committees should be following Gy as the total air kerma at the interventional reference point (IRP). Also, keep in mind that, ideally, you want to stay under 5 Gy for most procedures.

Q: Does your state require special training to use radiation equipment? 

Yes, in California. In many states, regulatory agencies are requiring training in fluoroscopy and the recent NCRP publication for operators of potential high-dose cases (>2 Gy) is to have mandatory training prior to independent use. It can be documented in the cardiology fellowship training years and then requires one hour of CME each year to maintain proficiency and licensure. The best safety is achieved by remembering the acronym ALARA — that is, reduce exposure to ionizing radiation to a level that is As Low As Reasonably Achievable (ALARA) for the medical necessity. 

Q: Is anyone using this dose meter during his or her cases either as teaching tool or in another clinical application? 

Most said no. On this point, Dr. Chambers and others suggested that we need better early warning systems. When the procedure reaches 3 to 5 Gy, the operator should know that they are getting to the deterministic (tissue injury) threshold for adverse skin effects. Many now believe that the annoying beep for fluoroscopy timers should be removed and some mechanism for recognizing total air kerma counts should be instituted.

Q: How do we assess radiation doses to the operators?

Cath lab operators should be aware of the potential for significant cumulative dose when multiple imaging studies are performed. A busy interventional cardiologist using good technique and proper protective equipment receives 2-4 mSv/year, with dose dependent upon time in the lab and case complexity. During the case, the physician should consider the following variables (summarized in Tables 2 and 3). However, among the most practical methods to monitor radiation is to maintain communication between staff and operator, noting monitor displays, especially in high-dose cases. The staff should notify the physician operator during the procedure when Ka,r is in excess of 3 Gy and then every 1 Gy thereafter. 

Keeping an eye on the radiation dose is good for everyone and essential for safe cardiac catheter procedures. Let’s put this practice to work.

References

  1. Chambers CE, Fetterly K, Holzer R, et al. Radiation safety program for the cardiac catheterization laboratory. Cathet Cardiovasc Interv 2011;77:510–514.
  2. Chambers CE. Radiation dose in percutaneous coronary intervention. OUCH…Did that hurt? J Am Coll Cardiol Interv 2011;4:344–346.
  3. Balter S, Hopewell JW, Miller DL, et al. Fluoroscopically guided interventional procedures: a review of radiation effects on patient’s skin and hair. Radiology 2010;254:326–341.
  4. Balter S, Moses J. Managing patient dose in interventional cardiology. Cathet Cardiovasc Interv 2007;70:244–249.
  5. Hirshfeld JW Jr, Balter S, Brinker JA, et al. ACCF/AHA/HRS/SCAI clinical competence statement on physician knowledge to optimize patient safety and image quality in fluoroscopically guided invasive cardiovascular procedures, A report of the American College of Cardiology Foundation/American Heart Association/American College of Physicians Task Force on Clinical Competence and Training. J Am Coll Cardiol 2004;4:2259–2282.
  6. Klein LW, Miller DL, Balter S, et al. Joint Inter-Society Task Force on Occupational Hazards in the Interventional Laboratory. Occupational health hazards in the interventional laboratory: Time for a safer environment. Cathet Cardiovasc Interv 2009;73:432–438. 
  7. Suzuki S, Furui S, Isshiki T, et al. Methods to reduce patients’ maximum skin dose during percutaneous coronary intervention for chronic total occlusion. Cathet Cardiovasc Interv 2008;71:792–798.

Disclosure: Dr. Kern reports that he is a speaker for Volcano Therapeutics and St. Jude Medical, and is a consultant for Merit Medical and InfraReDx, Inc.