Low-Level Ionizing Radiation From Noninvasive Cardiac Imaging: Can We Extrapolate Estimated Risks From Epidemiologic Data to the Clinical Setting?

Abstract

Clinical decision-making regarding the use of low-level ionizing radiation for diagnostic and/or therapeutic purposes in patients with cardiovascular disease must, as in all other clinical scenarios, encompass the broad range of the risk-benefit ratio. Concerns regarding the late carcinogenic effects of exposure to low levels, i.e., <100 mSv, of ionizing radiation stem from extrapolation of exposure-outcome data in survivors of World War II atomic bomb explosions. However, ongoing debate regarding the true incremental risk to subjects exposed to doses currently administered in cardiovascular procedures fails to take into account the uncertainty of the dose-response relationship in this lower range, as well as tissue-specific reparative responses, also manifest at lower levels of exposure. The present discussion draws attention to both of these aspects as they relate to clinical decision-making.

Stress only myocardial perfusion imaging: Is it time for a change?

“…16,17 This level of exposure can be achieved with a \ 33 mCi dose of Tc-99m sestamibi and a \ 38 mCi dose of Tc-99m tetrofosmin. 6,7 Based on these assumptions, and if administered Tc-99m sestamibi, 76% (or 6,125) of our 8,034 stressonly patients would have received a \ 10 mSv exposure as compared to only 2.6% (or 231) of those undergoing stress/rest imaging (P \ .001). If administered tetrofosmin, the percentages increase to 87% (n = 7,009) of stress-only patients vs 3.0% (n = 266) of stress/rest patients (P \ .001).…”

“…Since there is a requirement for two separate radiotracer injections, the radiation exposure from this protocol is relatively high at *11-18 mSv (for Tc-99m tracers) and *27-30 mSv (for dual isotope rest Tl-201/ stress Tc-99m procedures). 6,7 An alternative approach is a same day stress/rest imaging protocol where a relatively low dose of radiotracer is injected at peak stress (i.e., 8-15 mCi) with avoidance of the higher 25-35 mCi rest dose if the stress acquisition is normal. This affords a significant reduction in radiation exposure to patients who have a normal stress study and conserves Tc-99m which has become increasingly more difficult to obtain due to ongoing worldwide shortages.…”

“…Conservation of Tc-99m translates into less reliance on Tl-201 with its associated higher radiation exposure (22 mSv for a 3.5 mCi dose). 6,7 Furthermore, a stress-only imaging protocol should improve laboratory efficiency by eliminating rest imaging in a sizeable percentage of patients, increase patient satisfaction and convenience by markedly decreasing their time spent at the laboratory and reduce cost by eliminating the higher second dose of radiotracer and the second imaging procedure.…”

Integration of Quantitative Positron Emission Tomography Absolute Myocardial Blood Flow Measurements in the Clinical Management of Coronary Artery Disease

“…Accordingly, although tracer dose must be reduced to prevent overwhelming the response capabilities of the camera, patient radiation exposure also is reduced, an increasingly important clinical consideration. 9 A typical rest/stress PET examination (3D mode) with 13 N-ammonia or 82 Rb imparts a nominal radiation dose of ≈2 mSv compared with ≈14 mSv for SPECT rest/ stress 99m Tc-MIBI. 10,11 Thus, even operating in 3D mode, which sacrifices some spatial resolution compared with the 2-dimensional mode, PET technology provides substantially better image resolution, counting statistics, and, most important, true absolute quantization of tracer concentration (nCi/mL) in blood and myocardium compared with SPECT.…”

Low-dose 3D 82Rb PET

“…Although the true effect of radiation exposures lower than 100 mSv on cancer risk is unknown as the risk estimates are extrapolated linearly from higher doses, 5,6 radiation exposure and discussion of cardiac imaging's contribution to hypothetical (but possible) cancer have been recently highlighted. Therefore, from the patient's perspective, radiation exposure ''as low as reasonably achievable'' is very desirable.…”