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Safety First: Ionizing Radiation in the Cardiac Cath Lab
The annual patient radiation dose from medical imaging has increased sixfold since 1982, with CV dose alone increasing approximately 20%. However, with this increased risk, patients have also seen dramatic gains, including a decrease in CV death risk to 188/100,000, an improvement of threefold since the 1950s. Although these gains are not solely created through ionizing radiation techniques, the exact risk from radiation exposure is similarly unclear. Nonetheless, this dramatic increase in radiation from medical imaging has appropriately generated attention toward radiation safety and a wholly appropriate focus on best practices for radiation safety in the cardiac cath lab.
Dose Assessment
Assessment of radiation dose in the cardiac cath lab is much more than fluoroscopy time (FT [min]). With several limitations, the most obvious of which is its failure to include cine imaging, FT alone is not adequate to assess patient radiation dose. For this reason, since 2006, all fluoroscopic equipment sold in the United States has additional parameters to identify patient dose measured, recorded and displayed during the procedure.
As of 2006, there are two standard parameters reported on interventional fluoroscopic equipment: total air kerma at the interventional reference point (Ka,r, gray [Gy]) and air kerma area product (PKA, Gycm2). Ka,r, also referred to as cumulative air kerma (CAK), is the procedural cumulative air kerma (X-ray energy delivered to air) at the interventional reference point. This point in “space” where kerma, or kinetic energy released in matter, is measured rests 15 cm on the X-ray tube side of isocenter — the point at which the primary X-ray beam intersects with the rotational axis of the C-arm gantry. This identifies associated skin injury called deterministic effects (Figure).
PKA is the product of air kerma and the X-ray field, also referred to as dose area product (DAP) and kerma area product (KAP). PKA is used to monitor the potential for genetic defects or cancer risk over time, called stochastic effects. Using PKA to estimate deterministic skin injury risk requires an estimate of a factor 100 times less. For instance, initial concerns for skin injury arise when the Ka,r >5 Gy would approximate a PKA of 500 Gycm2.
Figure. Adverse radiation skin effects. Evolution beyond the initial erythema and epilation, on the patient’s back, with central necrosis and subsequent eschar formation 4 months following complex chronic total occlusion PCI, with Ka,r of 16.4 Gy.
Image: Charles E. Chambers, MD, FSCAI
Peak skin dose (PSD, Gy) is the maximum dose received by any area of a patient’s skin. This is not a point in space but rather an actual skin dose that requires calculations dependent on gantry angulations and table height. There is no current method to measure PSD during the case. A qualified physicist should be notified early to calculate PSD, if a high dose is to be delivered to a patient.
Safety in the Laboratory Environment
All cardiac cath labs should have a radiation safety program with active participation from the physicians, staff and physicist. All interventional cardiologists should apply two basic principles of radiation protection to their practice: reduce radiation exposure to “as low as reasonably achievable” (ALARA); and ensure procedure justification, such that no patient receives radiation without potential benefit.
Although only certain states mandate fluoroscopy training, it is important that everyone receive radiation dose management and safety training commensurate to their responsibilities. The National Council on Radiation Protection and Measurements recommends both didactic and “hands-on” training. The didactic program should include initial training with periodic updates covering the topics of radiation physics and safety. Hands-on training should be provided for newly hired operators and all operators on newly purchased equipment.
It is the individual’s responsibility to wear a dosimeter. Although a single dosimeter worn outside the collar can be used, two properly worn dosimeters — one at the waist underneath and one at the collar outside the protective garment — provide a better reflection of effective dose. Protective garments stop approximately 95% of scatter radiation. Radiation glasses are effective but must fit properly, have 0.25-mm lead equivalent protection and additional side shielding. Ceiling-mounted and below-table shielding are effective; both should be used routinely.
Current fluoroscopic X-ray systems offer features for dose management, including frame rate adjustment, virtual collimation, last image hold, X-ray store and real-time dose display. Image quality is a function of multiple patient, procedure and equipment variables. As a general rule, image quality and radiation dose are tightly woven. Automatic dose rate controls increase dose for a specific patient size in a specific projection to achieve adequate image quality. Knowing the equipment and working with a qualified physicist are essential for dose optimization.
Procedure-Based Radiation Dose Management
Outlined in the Table is procedure-based dose management. Pre-procedure planning is an essential component to radiation dose management. High-risk patients include those who are obese, have complex disease or have undergone fluoroscopic procedures within the previous 30 to 60 days. Informed consent should include radiation safety information, most especially with high-risk patients.
During each case, the physician should manage dose from the outset. Staff should notify the physician when Ka,r is in excess of 3 Gy and then every 1 Gy thereafter. When high-dose radiation has been administered, the operator must balance risk with benefit to proceed. For the operator and staff, developing good techniques is essential. It is important to use fluoroscopy only when looking at the monitor and limit cine imaging. Steep angles, frame rate, collimation, protective shielding, and table and image receptor height are all important variables during the procedure. Operator and staff must maximize their distance from the X-ray tube (ie, the inverse square law), which is of particular importance for radial access cases. All appendages — operator’s and patient’s — should be out of the imaging field.
Abbreviations: Ka,r, total air kerma at reference point; PKA, air kerma area product; PSD, peak skin dose.
Source: Modified from Chambers CE. J Am Coll Cardiol Intv. 2011;4:344-346; reprinted with permission.
Cardiac catheterization reports should include all available radiation parameters: FT, Ka,r, Gy and PKA (Gycm2). Patient notification, chart documentation and communication with the primary care provider should routinely occur following procedures with high radiation dose. For Ka,r >5 Gy (PKA>500 Gycm2), it is important that patients be educated regarding potential skin changes (eg, red patch on the back) with 30-day phone call follow-up and office visits, as required.
For Ka,r >10 Gy (PKA >1,000 Gycm2), a qualified physicist should promptly calculate PSD with skin examined at 2 to 4 weeks. The Joint Commission identifies peak skin doses >15 Gy as a sentinel event; hospital risk management and regulatory agencies need to be contacted within 24 hours. Suspected tissue injury should be referred to a specialist made aware of potential radiation etiology. A biopsy should be performed only if required, as the biopsy “wound” may potentially be more severe than the radiation effects.
Conclusion
Establishing a radiation-safety-conscious environment in the cardiac cath lab is a collaborative effort involving physicians, staff, physicists, quality assurance personnel and hospital administration. Establishing safe radiation practice improves patient, staff and physician safety. The interventional cardiologist, as the person responsible for all aspects of patient care in the cath lab, must be actively involved in managing radiation dose to maximize patient safety.
Disclosure: Chambers reports no relevant financial disclosures.