Optimization of Pediatric PET/CT
Introduction
Positron emission tomography/computed tomography (PET/CT) is the most common form of hybrid imaging, one that combines 2 different imaging modalities. PET uses a radiopharmaceutical to provide functional data whereas CT, with its higher spatial resolution and tissue characterization, provides anatomical information that increases the diagnostic yield of the functional study by improving localization and providing attenuation correction (AC) maps. Although PET/CT imaging combines the best aspects of both imaging modalities, that gain is not without penalty. Both techniques are sources of ionizing radiation, contributing to increased patient radiation exposure. Additionally, the combination of 2 serial acquisitions, at slightly different times and with different temporal resolutions, leads to additional challenges.1
The clinical use of PET/CT is well established in the pediatric population for both oncologic and nononcologic indications. In oncology, 18fluorine-2-fluoro-2-deoxy-d-glucose (18F-FDG)-PET/CT is used for tumor staging, risk stratification, selection of biopsy sites, radiotherapy planning, monitoring of therapeutic response, and restaging of potentially curable FDG-avid tumors.2 Its use has been particularly proven in lymphoma and sarcoma.3, 4, 5 It is also helpful in cases of metastases of unknown primary malignancy, differentiating benign from malignant lesions, and monitoring for malignant transformation of known benign lesions, such as neurofibromatosis.6, 7 18F-FDG-PET/CT is particularly helpful in the evaluation of the distribution and monitoring of disease activity in entities such as Langerhans histiocytosis.8 In nononcologic applications, 18F-FDG-PET/CT has been shown useful in certain infectious and inflammatory conditions such as tuberculosis, chronic osteomyelitis, chronic granulomatous disease, sarcoidosis, Takayasu arteritis, Crohn’s disease, and in localizing the underlying source of disease in children with fever of unknown origin.9, 10, 11, 12, 13, 14, 15 18F-FDG-PET/CT of the brain can be performed not only for tumor imaging but also to localize epileptogenic foci.16, 17, 18 PET radiopharmaceuticals other than FDG are also available for imaging of brain tumors in both the clinical and research domains, mostly radiolabeled amino acids, the most extensively researched of which is 11C-methionine.19, 20 Cardiac PET/CT is an uncommonly performed procedure in pediatrics that is helpful in cases of myocardial inflammation, in the preoperative and postoperative evaluation of certain cardiac structural, and coronary anomalies, as well as in evaluating myocardial ischemia in patients with Kawasaki disease.21, 22, 23, 24
Under appropriate clinical conditions, the benefit of PET/CT significantly outweighs the potential risks related to radiation.25 Nievelstein et al26 evaluated the benefits and risks of 18F-FDG-PET/CT in malignant lymphoma, one of the best-established and most common oncologic indications for PET/CT, and concluded that “The modest radiation risk that results from imaging with CT and 18F-FDG-PET can be considered as justified, but imaging should be performed with care, especially in children.”
Optimizing PET/CT is a complex multifactorial process that can be generally divided into 2 competing interests. The first is the desire to maximize the benefits of the study by improving its diagnostic performance. In much of PET/CT practice, improved performance often comes as increased lesion detectability or more accurate and precise lesion localization and quantification. Additional benefits for an examination could include improving the patient experience and efficiency of departmental workflow. The other interest is to minimize any risks of the study. Some risks include those related to anesthesia, iodinated contrast, and most importantly in children, radiation exposure.
In this article, we will review a variety of strategies for radiation dose optimization in 18F-FDG-PET/CT as well as measures to improve quality and minimize the risks of these transformative hybrid imaging studies.
Section snippets
Minimizing Radiation
It is widely accepted that children are more sensitive to the effects of radiation than adults. Not only are children’s organs and soft tissues more radiosensitive compared with adults owing to a rapid rate of cell division, but after radiation exposure, children have a longer postexposure life expectancy in which to exhibit adverse radiation effects.25, 27, 28, 29 It is not surprising that estimated risk of inducing fatal cancer per exposure unit at the age of 10 years is approximately 5-15
Strategies for Dose Optimization in Pediatric 18F-FDG-PET/CT
The strategies for decreasing radiation exposure in 18F-FDG-PET/CT are individually discussed in the following sections and are summarized in Table 1.
CT Methodology
There are several methods of performing the CT component of an 18F-FDG-PET/CT study. These include (1) AC CT with intravenous (IV) contrast using either diagnostic or low-dose scan parameters; (2) AC non-IV contrast CT using either diagnostic or, more typically, low-dose scan parameters; (3) AC CT with limited noncontrast CT; and (4) PET with AC CT only in which no diagnostic CT information is imparted.
The effective radiation dose to the patient varies substantially based on the CT protocol
Optimizing 18F-FDG-PET/CT: Pediatric Patient Preparation; Use of Pediatric-Specific Scanning Protocols
Although many groups have proposed guidelines for the performance of 18F-FDG-PET/CT, with the exception of those proposed by the EANM80 as well as the Royal College of Radiologists22, 81 and a statement about sedation in the SNMMI guidelines,82 few are focused on the unique needs of the child where age-appropriate patient preparation and acquisition protocols, along with knowledge of the common developmental and physiologic patterns of 18F-FDG uptake, facilitate correct image interpretation.
Conclusion
In conclusion, the radiation dose from pediatric 18F-FDG-PET/CT varies substantially depending on the protocol used. Both PET and CT components contribute significantly to total-patient radiation dose. Optimization of both components, through careful patient preparation and appropriate patient immobilization, use of recommended pediatric 18F-FDG administered activities, thoughtful selection of pediatric-specific CT imaging parameters designed for appropriate diagnostic, localization, or AC-only
Acknowledgment
The authors wish to acknowledge Jason Qi, PhD, who generated the CT effective dose data used in Table 2.
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