18F cocktail shows diagnostic promise

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The combined administration of sodium 18F and 18F-FDG in a single PET/CT study has the potential to improve cancer diagnosis, staging and possibly therapy monitoring, according to the results of a prospective study conducted at Stanford University Medical Center in Stanford, Calif.

“Bone scintigraphy with sodium 18F was performed before the introduction of 99mTc-based agents, achieving excellent quality studies,” the authors wrote in the April issue of the Journal of Nuclear Medicine. “18F is a positron emitter, allowing for PET. Thus, imaging skeletal lesions with 18F PET/CT appears a logical approach for acquisition of highly sensitive and specific images.”

A 75-year-old man with prostate cancer. (A) MIP image of 18F-FDG PET shows lymph node metastases (arrowheads) and faint uptake in osseous lesions, such as a right rib (arrow). (B) MIP image of 18F PET shows intense radiotracer uptake in multiple bone lesions, including right rib lesion (arrow) seen on 18F-FDG PET. (C) MIP image of combined 18F /18F-FDG PET shows both lesions noted on 18F-FDG PET (arrowheads) and skeletal lesions noted on 18F PET (reference right rib lesion marked with arrow). Image and caption courtesy of SNM.

A preclinical evaluation of the technique was carried out on a murine model using a Concorde Microsystems microPET R4 scanner and GE Healthcare eXplore RS MicroCT system.

“The image processing involved in this preclinical study required obtaining a bone mask from CT data, combining 18F/18F-FDG PET data with micro-CT data for coregistration (using fiducial markers), and displaying the 18F /18F-FDG uptake in the osseous structures on the PET scan,” the researchers noted.

A total of 14 consecutive patients with proven malignancy (six sarcoma, three prostate cancer, two breast cancer, one colon cancer, one lung cancer and one malignant paraganglioma) were recruited for the pilot study. Each patient underwent separate 18F and 18F-FDG PET/CT scans as well as a combine 18F/18F-FDG PET/CT for evaluation of the malignancy. Whole-body PET/CT images were obtained in 2D mode using a GE Discovery LT PET/CT system.

“With the availability of PET/CT, a cocktail approach allows for a new strategy for patient management not previously possible,” the authors noted.

The 18F PET/CT, 18F-FDG PET/CT and combined 18F/18F-FDG PET/CT scans were interpreted by two board-certified nuclear medicine readers unaware of the diagnosis and results of the other imaging studies. In addition to the separate interpretation of the three scans for each patient, the CT data from the combined 18F/18F-FDG scan were used to create a bone mask that allowed the display of 18F/18F-FDG in the osseous structures on the PET scan, the scientists reported.

“Through image processing, the combined 18F/18F-FDG scan yielded results for bone radiotracer uptake comparable to those of the 18F PET/CT scan performed separately,” the authors wrote. “Thus, the combined 18F/18F-FDG cocktail tracer administration followed by single PET/CT appears to be feasible in this patient population referred for pre-therapy evaluation of the extent of a known malignancy.”
The successful approach performed by the Stanford University team is based on the eventual biodistribution of the 18F nearly exclusively to the skeletal structures. In addition, they noted that in terms of radiation exposure for the patients that a 99mTc-MDP bone scan delivers approximately 4.2mSv while a standard 18F-FDG PET/CT workup can deliver approximately 26.5 mSv from 18F-FDG and 10 mSv from the low-dose CT.

The Stanford cocktail protocol allows for a low-dose of 18F/18F-FDG, resulting in a total radiation exposure of 26 mSv.

“Thus, instead of patients having to get separate SPECT and PET/CT studies, usually on different days, this strategy allows for one combined PET/CT study with potentially more utility, lower costs, lower radiation dose, and much greater patient convenience,” the authors noted.

As promising as these initial results appear, the scientists are cognizant that mores studies, with a larger patient cohort, will need to be conducted to validate their results.

“Larger prospective trials are needed for a better understanding of the precise indications for the combined 18F /18F-FDG PET/CT method and for refinement of the image-processing algorithm,” they wrote.