JNM: PET protocols differ widely, stymie sound evidence & standards

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4D PET/CT images, showing a tumor after treatment.
Image source: University of Pittsburgh Medical Center

The protocol and technique for oncologic PET/CT varies widely, intensifying the need for general standards but underscoring variability’s impedance to comparable, sound evidence for establishing such standards, concluded a study published in the February issue of The Journal of Nuclear Medicine.

“PET with 18F-FDG has been established as a broadly useful technique in cancer imaging, especially in cancer diagnosis, staging and treatment response assessment,” explained Michael M. Graham, MD, PhD, of the department of radiology at the University of Iowa in Iowa City, and colleagues. “However, comparing PET studies from institution to institution can be challenging, in part because of methodologic variabilities.”

As part of the National Cancer Institute (NCI) Imaging Response Assessment Team’s aim to improve the appropriate use of quantitative imaging in clinical trials, the PET/CT subcommittee surveyed 15 academic member institutions. The surveys examined oncologic PET/CT processes among the 15 Imaging Response Assessment Team institutions, looking at protocols for patient preparation; methods for performance of scans; and display, analysis, review and archiving of images.

According to the authors, the survey “showed considerable variability in patient preparation, 18F-FDG dose, CT technique, tracer uptake, imaging time, reconstruction methods and suitability of PACS for PET/CT display.”

The average administered 18F-FDG dose for adults varied from 259 to 740 MBq, while at least two institutions reported giving as much as 925 MBq of the ligand. The authors argued that 370 to 555 MBq is “reasonable,” while noting that European standards are considerably lower than the higher ends of the range observed among these institutions.

“[W]ith newer tomographs and with the increased general concern about patient radiation exposure, recommendation of a lower range may be appropriate,” the authors considered. “In general, it would be more appropriate to specify an administered dose per kilogram of body weight, because patient weight ranges widely.”

Despite the wide range in allowance for uptake deviations (30 to 90 minutes), the authors held back on confirming general recommendations that imaging take place within 60 +/- 10 minutes of administering 18F-FDG, citing a lack of evidence and saying that “the value [of this time frame] is also probably not optimal.”

Large variations were also observed in institutions' dietary recommendations to patients, including diabetic individuals, prior to scans. While the National Cancer Institute has recommended low-carbohydrate diets the day before imaging, followed by varying lengths of fasting, the authors noted of specific time frames, “It is not really known how well patients adhere to the recommendation or how effective it is in improving the quality or reproducibility of 18F-FDG PET/CT studies. In the absence of any compelling evidence, this recommendation is difficult to support, and patient compliance is difficult to verify.”

Further contributing to uncertain standards, the institutions surveyed demonstrated “broad agreement” that 18F-FDG PET/CT should not be done if patients’ blood glucose levels are below 200 mg/dL, yet “both the NCI and the European group suggest that the limit should be much lower,” the authors pointed out.

“Because of increasing concern about radiation exposure it may be appropriate to recommend 30 to 50 mAs or less for attenuation imaging for an average-size patient and to adjust for body size at a reasonably high pitch,” the authors considered. The average dose was 50 mAs among respondents, but doses did range from 8 to 120 mAs among institutions using fixed amperages. In response to two institutions not adjusting dose for pediatric studies, the authors averred: “Adjustment for body size, collimation and pitch is particularly important in children.”

“The wide variation certainly means that it is quite difficult to use the retrospective data of a site in any meaningful multicenter quantitative analysis of efficacy, especially if absolute quantitation is required across centers,” Graham and co-authors argued. “This wide variation also impedes the ability of the various sites to participate in prospective clinical trials, because baseline studies are often done before a patient is recruited into a trial.”

Graham and colleagues also said it is “virtually