Registry Study: AMI Patients at Increased Cancer Risk from Low-Dose Radiation

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Results of a Canadian registry study suggest that patients who undergo cardiac imaging and therapeutic procedures involving low-dose ionizing radiation within 1 year of a myocardial infarction (MI) are at increased risk for developing a new cancer. The study appeared February 7, 2011, online ahead of print in the Canadian Medical Association Journal.

Researchers led by Louise Pilote, MD, PhD, MPH, of the McGill University Health Centre (Quebec, Canada), evaluated registry data on 82,861 patients admitted for a first acute MI between April 1996 and March 2006. Patients with a history of cancer were excluded. Overall, 77% underwent at least 1 diagnostic or therapeutic cardiac procedure involving low-dose ionizing radiation within 1 year of admission. These included (average effective dose estimates based on the literature):

• Myocardial perfusion imaging (15.6 mSv)
• Diagnostic cardiac catheterization (7.0 mSv)
• PCI (15.0 mSv)
• Cardiac resting ventriculography (7.8 mSV)

The researchers then tracked cancer diagnoses beginning 1 year after the index admission. According to the authors, this lag time was meant to minimize the risk of spurious associations between cardiac radiation exposure and cancer diagnosed soon after exposure and to decrease the risk of workup bias (ie, imaging tests ordered in the diagnostic workup of cancer, notwithstanding that cardiac imaging tests are seldom performed as a diagnostic workup for cancer).

Risk Dose-Dependent

The most commonly performed cardiac procedures were:

• PCI: in 40.7% of patients, accounting for 40.3% of total exposure
• Myocardial perfusion imaging: in 33.8% of patients, accounting for 29.9% of exposure
• Diagnostic cardiac catheterization: in 31.1% of patients, accounting for 23.6% of total exposure
• Cardiac resting ventriculography: in10.6% of patients, accounting for 6.3% of total exposure

 

Diagnostic cardiac catheterization and PCI together accounted for 63.9% of radiation exposures from cardiac procedures.

The cumulative exposure to low-dose ionizing radiation from all cardiac procedures was 5.3 mSv per patient-year, of which 84% occurred during the first year after AMI. Patients with the highest levels of exposure (> 30 mSv) tended to be younger males with fewer comorbidities (mean age, 59.5 years). The level of exposure was higher among patients whose treating physician was a cardiologist compared with those whose treating physician was a general practitioner.

In all, 12,020 new cancers were diagnosed during a mean follow-up of 5 years. The researchers found a dose-dependent relationship between exposure to radiation from cardiac procedures and subsequent risk of cancer: For every 10 mSv of radiation exposure, there was a 3% increase in the risk of age- and sex-adjusted cancer during the follow-up period (HR 1.003 per mSv; 95% CI 1.002-1.004).

The most common cancer sites were the abdomen/pelvis (41.9%) and thorax (26.8%), followed by bone/soft tissue/skin (22%), head and neck (9%) and blood (8.2%).

When patients were stratified by sex, the adjusted hazard ratio for incident cancer was 1.005 (95% CI 1.003-1.007) in women and 1.002 (95% CI 1.001-1.003) in men. The interaction for sex and radiation exposure was statistically significant (P < 0.001).

Something Doesn’t Add Up

Stephen Balter, PhD, of Columbia University Medical Center (New York, NY), commented that the types of cancers found in the study are “interesting, but very strange.” In a telephone interview with TCTMD, he also questioned the finding of a higher risk for men vs. women, saying it is contrary to established data.

“That just makes no sense,” he said. “And, if you look at the procedures, none of them involve any substantial radiation to the head, so how they ended up with 9% head and neck cancers is a mystery. It’s out of balance with the American Cancer Society (ACS) 2010 data with regard to what you would expect in a US population.”

According to ACS, the most common new cancers are prostate, lung/bronchus, colon/rectum, and urinary/bladder in men and breast, lung/bronchus, colon/rectum, uterine, and thyroid in women.

Confounding factors may explain some of the disparity. Dr. Balter pointed out that the study contains no information on smoking status. Also, most cancers thought to be associated with low-dose radiation exposure would not be apparent for 3 to 5 years, he said, so the origin of cancers occurring within 1 year of an MI is debatable.

Enthusiasm for Cardiac Procedures Questioned

Still, Dr. Pilote and colleagues maintain that although most patients who have had an MI likely will die of cardiac-related causes, the increased exposure to radiation could increase their risk of cancer-related mortality.

“These results call into question whether our current enthusiasm for imaging and therapeutic procedures after acute myocardial infarction should be tempered,” the authors write. “We should at least consider putting into place a system of prospectively documenting the imaging tests and procedures that each patient undergoes and estimating his or her cumulative exposure to low-dose ionizing radiation.”

On that point, Dr. Balter agreed. “The issue for interventional cardiologists is when you do a procedure, write down the dose,” he said. “The FDA has mandated dosimeters on all fluoroscopes in the United States since 2006, which means there is no reason for cardiologists not to monitor [exposure]. It’s very important to document prospectively so we can make some sense out of the data instead of relying on retrospective studies with dose estimates.”

In an editorial accompanying the study, Mathew Mercuri, MSc, Tej Sheth, MD, and Madhu K. Natarajan, MD, MSc, of McMaster University (Hamilton, Canada), state that the real concern for patients is not an individual medical test but multiple procedures that can result in a cumulative exposure “that approaches or exceeds the level for which there is reasonably good epidemiologic evidence of an increased risk of cancer (ie, cumulative dose > 50 mSv).”

Study Details

The patient median age was 63.2 years and 31.7% were women. Clinical characteristics were similar between exposure groups.

A patient was considered to have cancer if at least one diagnostic code for cancer was recorded during follow-up in any of the following settings: (a) admission to hospital with a primary diagnosis of cancer, (b) a non-cancer-related admission with cancer listed as a secondary diagnosis and (c) any outpatient visit with a cancer diagnosis.

 

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Sources:
1. Eisenberg MJ, Afilalo J, Lawler PR, et al. Cancer risk related to low-dose ionizing radiation from cardiac imaging in patients after acute myocardial infarction. CMAJ. 2011;Epub ahead of print.

2. Mercuri M, Sheth T, Natarajan MK. Radiation exposure from medical imaging: A silent harm? CMAJ. 2011;Epub ahead of print.

 

 

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