Expanding Applications for PET-CT Fusion Imaging of Children
Hossein Jadvar, MD, PhD, MPH, MBA
The incorporation of positron emission tomography (PET) and PET-computed tomography (CT) in the imaging evaluation of children is rapidly expanding; new indications for imaging children were prominent at the 54th annual meeting of the Society of Nuclear Medicine in Washington, DC, by scientific sessions dedicated to this important topic.
Investigators from the St. Jude Children's Research Hospital in Memphis, Tennessee, characterized the relationship between tumor histology and tumor glucose metabolism in 26 children with brain tumors.[1] Fluorodeoxyglucose (FDG) PET scans and tumor biopsy were performed to distinguish residual neoplasm from post-radiotherapy changes on magnetic resonance imaging (MRI). In patients with glial tumors, the standardized uptake value (SUV) ranged from 3.3 to 24.8 (mean 8.5) while the non-glial tumors SUV ranged from 3.2 to 14.5 (mean 8.8). Due to this considerable overlap, the authors suggested that unlike the findings in adult brain tumors, metabolic activity may not accurately indicated tumor grade in children and that low metabolic activity does not exclude residual neoplasm.
The group from Germany compared retrospectively the potential additional diagnostic value of hybrid PET-CT to stand-alone PET in 53 patients with Ewing sarcoma.[2] On a lesion-based analysis, sensitivity, specificity, accuracy, positive predictive value and negative predictive value of PET were 71%, 95%, 88%, 87%, and 88%, respectively, compared to 87%, 97%, 94%, 92%, and 95%, for hybrid PET-CT. The areas under the curve of the receiver-operating characteristic analysis were 0.79 for PET alone and 0.92 for PET-CT. The investigators concluded that PET-CT is significantly more accurate than stand-alone PET in staging patients with Ewing sarcoma. Other investigators reported on the role of FDG PET-CT in the follow up of 23 pediatric patients (age range 8 months to 18 years) with sarcoma (14 Ewing sarcoma, 5 rhabdomyosarcoma, 2 synovial sarcoma, 2 osteosarcoma) after neoadjuvant chemoradiotherapy.[3] Histology and radiologic follow-up for at least 6 months were employed as the standard of reference for imaging findings. The overall negative and positive predictive values in this setting were 95% and 67%, respectively. The authors concluded that PET-CT is useful in excluding residual disease post chemoradiotherapy and assessing for distant metastases in children with sarcoma.
A comparative study of PET-CT and diagnostic CT was performed in pediatric patients with lymphoma.[4] In view of a high rate of correlation between the 2 modalities and lack of false negative results, the authors concluded that PET-CT may suffice for routine surveillance of pediatric patients with lymphoma sparing them the additional radiation exposure of diagnostic CT. On this latter important desire for reducing radiation exposure in children, the Canadian investigators from the British Columbia determined whether low-dose whole body CT (80 kVp, 40-60 mAs) can be adequate for attenuation correction of PET data in a hybrid PET-CT system.[5] The absorbed radiation dose using this low-dose CT technique in a 55-kg, 15-year-old patient was 9 mSv compared to a nearly double dose of 21 mSv in a similar child using standard PET-CT technique, 7 mSv for PET alone, and in 40-mSv range for Ga-67 citrate and Tl-201 chloride scintigraphy. However, in another investigation, it was determined that low-dose end tidal expiration CT scan may be inadequate for the detection of pulmonary nodules.[6] In this study, 2 experienced readers reviewed 72 paired low-dose end tidal expiration chest CT with standard pediatric dose full inspiration chest CT. Significantly more nodules were detected on the diagnostic CT than on the low-dose CT. Despite this observation, the authors acknowledged that more data should be acquired to determine if diagnostic quality full inspiration CT would be needed, especially in specific conditions such as in lymphoma where malignant pulmonary nodules are relatively uncommon. Additionally, the radiation cost-higher detectability benefit analysis for standard CT will need to be performed and the impact on the clinical management and patient outcome will need to be established.
The Saudi experience with FDG PET-CT over 1 year at a tertiary care setting was also presented.[7] A total of 146 children (age range 2 months to 18 years) with various types of cancer were scanned with 8-slice PET-CT imaging system with using 80 kVp, 75 mAs, pitch of 1.675:1 and slice thickness of 3.75 mm. general anesthesia was performed in all patients younger than 7 years. Children were referred for initial staging in 25%, for assessing recurrence in 33%, and for evaluating treatment response in 42% of cases. Brown fat uptake was seen in 62% and thymic uptake in 82% of patients. The most common pathology was lymphoma (57%) followed by sarcoma (25%). In a similar report from Japan, the researchers concluded that FDG PET-CT is safe and useful in pediatric oncology despite the observation that some malignant tumors may not sufficiently accumulate FDG such as some neuroblastomas and clear cell sarcoma of kidney.[8]
Non-FDG based PET was also discussed in a few presentations. The Taiwanese investigators evaluated prospectively the use of F-18 DOPA in the characterization of pediatric neuroendocrine tumors in comparison to conventional imaging and with surgical histopathologic findings as the standard of reference.[9] Ganglioneuroma and ganglioneuroblastoma accumulated FDOPA while all cases of poorly differentiated neuroblastoma did not demonstrate significant tracer uptake. Based on this limited study of 7 patients, the authors concluded that FDOPA uptake in the tumor indicates the ability to decarboxylase FDOPA by the aromatic amino acid decarboxylase in the well-differentiated tumor component. A similar German study assessed the diagnostic utility of FDOPA PET-CT in patients with pheochromocytoma.[10] The maximum SUV for surgically-proven tumors was 16.2 ± 8 (mean ± s.d.). The authors concluded that FDOPA PET-CT is diagnostically valuable in the workup of patients with pheochromocytoma and multiple endocrine neoplasia (MEN) syndrome.