[18F]Fluorodeoxyglucose ([18F]FDG) is a positron emission radiotracer whose biodistribution is similar to glucose. The similar biodistribution of [18F]FDG and glucose in the human body requires a fasting condition for at least six hours prior to performing a [18F]FDG positron emission tomography ([18F]FDG PET) study.

In human studies, FDG PET images, in either the fasting state or the glucose-loaded state, have demonstrated that [18F]FDG uptake is decreased in the tumour, and thus the PET image quality is impaired, when plasma glucose levels are increased. All these results suggest that patients should fast before FDG PET studies, and their plasma glucose concentration needs to be considered when assessing tumour glucose metabolism. However, for lymphomatous disease, the data are contradictory and there are reports that insulin does not induce major changes in glucose uptake of lymphomatous tissue.

Here, we report two cases of lymphoma in which [18F]FDG PET/computed tomography ([18F]FDG PET/CT) was used for chemotherapy response evaluation. In both cases, initial [18F]FDG PET/CT scans were negative for neoplastic lesions but showed increased and diffuse FDG uptake in muscles. This led us to investigate better the importance of a fasting condition. A second [18F]FDG PET/CT performed 3–4 days later revealed pathological uptake in the lymphomatous lesions in both cases.

We demonstrate the importance of a euglycemic state before [18F]FDG administration, and that a fasting period of at least six hours is required prior to administration.