CASE STUDY INVOLVING RADIATION EXPOSURE

Case presented by:

BARRY BELANGER, PHD

A 75-year-old patient is undergoing a complex atrial fibrillation (AF) ablation procedure using a biplane x-ray imaging system oriented for anteroposterior (AP) and lateral (LAT) views.

Question No. 1: Cumulative dose readings reported by the x-ray system are as follows:

Given these cumulative dose readings mid-procedure, what decisions should be made concerning the rest of the procedure?

1. Terminate the procedure and reschedule the completion. Dangerous radiation levels have been reached.
   
2. Continue as planned. Radiation risk is minimal.
   
3. Proceed with caution, paying attention to cumulative dose and in making the following adjustments as appropriate or possible (select all that apply):
   
   
   
   1. Change LAT view to oblique.
      
   2. Verify whether AP and LAT detectors are as close to patient as possible.
      
   3. Switch to a smaller field of view.
      
   4. Verify dose settings and frame rates are as low as possible without jeopardizing visibility required for procedure.
      
   5. If adjustment is available, move x-ray tube away from patient.
      
   6. If adjustment is available, move x-ray tube toward patient.
      
   7. Store fluoroscopy (fluoro) sequences in lieu of digital cine recordings.

Discussion

Types and Severities of Radiation Risks

Radiation risks are usually categorized as Stochastic versus Deterministic. Stochastic risk is an assessment of the increase in likelihood of the occurrence of cancers resulting from exposure to radiation. Lickfett et al¹ have reported that the additional lifetime risk for a fatal malignancy associated with the ablation of AF was 0.15% for female patients and 0.21% for male patients in their study. The average age of the patients in the study was 56 ± 11 (39–78). Special considerations may be made for individual patients based on their status and medical condition. For example, should a needed interventional procedure using x-ray be performed on a patient who is pregnant or can it be safely deferred until after delivery? Pediatric interventions are an important case of stochastic risk, since the likelihood of cancer expression increases with lifetime following exposure, and sensitivity to radiation increases with the rate of cell division. And stochastic risk is an important consideration for the physicians and staff working within the procedure room.

The main concern for patients in interventional procedures is usually Deterministic risk, which is the tissue damage that will occur if radiation exposure exceeds certain thresholds. The tissue at greatest risk is usually the skin at the entrance location of the incident x-ray beam, where the intensity is greatest.

Table 68.1 provides a list of the known injuries that can be caused by the type of radiation used in interventional procedures (x-rays in the normal diagnostic imaging energy range), and the thresholds at which they can be expected to occur. In the US, the Joint Commission (TJC), formerly the Joint Commission on Accreditation of Healthcare Organizations (JCAHO), has set a level of 15 Gray exposure to a single area of skin as the threshold of a sentinel event. A sentinel event is a serious medical error, such as operating on the wrong organ. To retain TJC accreditation, a medical facility must respond to the occurrence of a sentinel event with a thorough investigation and implementation of an effective remediation plan, including ongoing monitoring.

National regulatory agencies have also set upper limits on patient entrance dose rates in fluoroscopy, for example, 88 mGy/min in the US. However, with complex procedures often lasting hours, with significant dependence upon and use of fluoroscopy, skin injuries can still occur due to long fluoroscopy times and corresponding high cumulative radiation exposures.

Table 68.1. A List of the Known Injuries that can be Caused by x-Rays Used in Interventional Procedures, and the Thresholds at which They can be Expected to Occur

Sources: US FDA Public Health Advisory: Avoidance of Serious x-ray-induced Skin Injuries to Patients During Fluoroscopically-guided Procedures. May 2, 1996.2 Interventional Fluoroscopy: Reducing Radiation Risks for Patients and Staff. Newsletter from NCI and SIR: 2005.³

Although the occurrence rate of reported deterministic radiation injuries is low, it is not zero,^4,5 and a significant fraction of these have resulted from cardiac radiofrequency (RF) catheter ablation procedures.⁴ A very pertinent, well-conducted study on a small cohort of patients undergoing RF ablation procedures demonstrated that, even with careful attention to radiation exposure, including use of low-dose fluoroscopy techniques, the maximum measured skin exposures for AF ablation procedures were close to the threshold at which deterministic skin injuries occur.¹ The maximum skin doses accurately measured on a cohort of 15 patients was 1.48 ± 0.37 (0.8–2.05) Gray versus a threshold of 2.0 Gray for deterministic skin injuries. The weights and body mass indices (BMIs) of the patients were 82 ± 12 (64–108) kg and 26 ± 2 (23–32), respectively, which does not indicate a bias toward obesity. So it is not difficult to imagine how factors including case difficulty and patient obesity could conspire to drive cumulative skin doses into the range of deterministic injury during AF ablation procedures.

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