Development of Simple Methods to Reduce the Exposure of the Public to Radiation from Patients Who Have Undergone ¹⁸F-FDG PET/CT

DISCUSSION

The ever-expanding use of diagnostic imaging continues to raise concerns about the increased cancer risk from ionizing radiation exposure. This issue has been especially critical for children and young adults. Multiple media reports have surfaced describing the increased cancer risk from medical imaging examinations. During this same period, the use of diagnostic imaging has been increasing at a steady pace. Although the natural background radiation has not significantly changed, the radiation exposure from medical imaging has increased more than 6-fold. According to a report issued in March 2009 by the National Council on Radiation Protection and Measurements, medical imaging contributed about 15% of the overall radiation dose in the United States in the 1980s, compared with about 50% in 2006 (5). The use of PET/CT in the clinical setting has also been expanding with the advancement of medicine and technology. The number of PET/CT systems increased over 10-fold since 2001, and the number of clinical scans performed in the United States alone is estimated at nearly 2 million, which is an increase of 13% over 2015 (6,7).

The concerns about exposure of the public to ionizing radiation from medical imaging have brought about a movement in both the pediatric population with the Image Gently campaign and the adult population with the Image Wisely campaign. To maximize the risk-to-benefit ratio, the medical community attempts to optimize diagnostic image quality while at the same time following the as-low-as-reasonably-achievable principle. The Image Gently and Image Wisely campaigns were developed to educate both providers and consumers in an attempt to curtail unnecessary imaging and reduce radiation exposure (8–10). However, there continues to be debate on whether low-dose radiation exposure can cause cancer (11). In a recent publication, Siegel et al. made the case for terminating these campaigns because of the erroneous extrapolation of the linear no-threshold model from high- to low-dose radiation as well as propagation of radiophobia. The linear no-threshold model for radiation-induced cancer has guided radiation protection policies since the 1950s and is supported by national and international advisory bodies. It is based on the concept that low levels of radiation increase mutations, which lead to increased cancers. However, Siegel et al. claim that low-dose radiation may even help prevent cancer by inducing repair of preexisting and ongoing DNA damage while repairing radiogenic damage (12–15). Nevertheless, from the regulatory standpoint, the linear no-threshold model is accepted and continues to be supported by national advisory bodies (National Research Council and National Council on Radiation Protection and Measurements) (16,17). Furthermore, those questioning the linear no-threshold model are basing the risk versus benefit on only the patient. This point of view should not be the same for the general public or potentially vulnerable patient populations in hospital or clinic waiting areas. It has been shown that the cancer risk increases substantially in patients with a suppressed immune system such as in young AIDS patients, for whom the cancer incidence increases by a factor of 40, or in young organ-transplant patients, for whom the cancer mortality rate increases by a factor of 60 (18,19). Nonetheless, a balance needs to be made between radiation risk and radiation aversion.

Various techniques have been developed to decrease the level of radiation exposure by using weight-based protocols for both the radiopharmaceutical delivered and the CT scan. Furthermore, PET/CT systems with higher sensitivity and improved performance can take advantage of enhanced detector technology (better time-of-flight performance, continuous bed motion, or an extended axial field of view) to reduce the ¹⁸F-FDG dose (20,21). According to the Society of Nuclear Medicine and Molecular Imaging guidelines, the typical administered ¹⁸F-FDG dose is 370–740 MBq (10–20 mCi) (22). However, patients who weigh more than 75 kg should receive a slightly higher dose to compensate for degraded image quality due to a lower signal-to-noise ratio from excessive attenuation (20).

Although these advancements and techniques address the radiation exposure of patients and radiation workers, they do not address the exposure of the public to patients or the repeated exposure of unmonitored nonradiologic medical staff to patients. Many patients schedule multiple tests and clinical appointments on a single day, exposing not only other patients in waiting areas but also general medical personnel. Multiple studies have been performed to assess the amount of radiation received by medical personnel in various roles (23). Regardless of the validity of the linear no-threshold model, there is justification for the radiation exposure of both patients and radiation workers; however, the same cannot be said for the general public.

Our study evaluated simple noninvasive interventions to reduce the overall radiation exposure of the public with no additional cost and only an additional 30-min wait for the patient before being released after imaging. At a time when the Nuclear Regulatory Commission has strict guidelines on the release of a patient treated with a radioisotope, the recommendations about releasing a patient undergoing diagnostic PET/CT are not as clear. In addition, national and international guidelines (Society of Nuclear Medicine and Molecular Imaging, American College of Radiology, and European Association of Nuclear Medicine) neither give recommendations for radiation exposure from patients nor recommend voiding or waiting after a scan before release (20,22,24). Although our data on the reduced radiation exposure after voiding were similar to prior studies, we found that nearly 75% of patients undergoing an ¹⁸F-FDG PET/CT scan leave the imaging facility exceeding 20 μSv/h at 1 m (25). We also attempted to address the concern about reduced renal function in these patients. Although renal failure has been shown not to have a significant impact on ¹⁸F-FDG biodistribution in PET/CT, our initial data showed a possible correlation between impaired renal function and elevated radiation exposure (26). However, because of our small sample size, this correlation was not statistically significant.

Moreover, the initial data from patients undergoing a ¹⁸F-fluciclovine study showed that they exposed the public to a higher level of radiation than did patients undergoing an ¹⁸F-FDG study. The higher exposure is most likely due to the elimination of the 60-min uptake phase, given the different pharmacodynamics. Sörensen et al. demonstrated that tumor uptake of ¹⁸F-fluciclovine peaks at around 3 min, with a plateau from 3 to 12 min and gradual washout. Lymph node uptake was rapid, with washout even faster than from tumor, and the urinary bladder had gradual uptake (27). Therefore, standard imaging with ¹⁸F-fluciclovine is performed 3–5 min after radiotracer administration, compared with 60 min with ¹⁸F-FDG (28). Although the ¹⁸F isotope is the same in both the ¹⁸F-fluciclovine and the ¹⁸F-FDG studies, as well as there being a similar injected dose—370 versus 421.8 MBq (10 vs. 11.4 mCi)—the difference in the protocol with ¹⁸F-fluciclovine requires a significantly shorter uptake phase. In our study, this difference caused 75% of the ¹⁸F-fluciclovine patients to remain above the 20 μSv/h threshold after waiting 30 min plus voiding, versus only 3% for the ¹⁸F-FDG patients.

This study may be the first to evaluate the radiation exposure of the public to a patient undergoing an ¹⁸F-FDG PET/CT scan and to assess the feasibility of implementing simple interventions to significantly reduce that exposure. Other studies have addressed radiation exposure but in the context of exposure to a patient or technologist (25,29). In addition, the effects of voiding after imaging to reduce exposure have also been studied, but not the combination of waiting plus voiding. In theory, these principles can also be used for other diagnostic PET and nuclear medicine studies before a patient leaves an imaging facility.

Our study is not without limitations. The relatively small sample size limits the statistical significance of the findings, especially when evaluating the effect of renal impairment. A larger sample size could allow for evaluation of various stages and severities of renal disease. Additionally, only 12 patients undergoing an ¹⁸F-fluciclovine study were evaluated. Other commonly used Food and Drug Administration–approved PET tracers such as ¹⁸F-sodium fluoride and ⁶⁸Ga-DOTATATE should also be evaluated. We found that even the same isotope can expose the public to vastly different levels of radiation, depending on the protocol. Lastly, this study was limited to the experience of a single institution with a fixed scanner. Because of financial concerns, a fixed PET/CT scanner is not always cost-effective and a mobile PET/CT scanner is often used. However, given the space and time constraints of a mobile scanner, these simple interventions may not be feasible. A prospective multiinstitute study would be better able to assess different patient populations, radiotracers, and protocols and to compare both fixed and mobile facilities.