Effects on the FDG distribution by a high uptake of brown adipose tissue at PET examination

At fluorodeoxyglucose/positron emission tomography (FDG/PET) examinations, a generally increased uptake of the skeletal muscles is sometimes encountered. As the tracer distribution constitutes a 'zero-sum-game', the uptake of lesions as well as of normal tissues is reduced in these patients. This has to be considered at calculation of standardised uptake values (SUVs), especially at longitudinal examinations in the same patient. In the current study, a possible similar influence on the FDG distribution by a high uptake of brown adipose tissue (BAT) was studied.

Twelve patients with strongly increased BAT uptake were examined twice with a mean of 5 days (study group). In six of these patients, there was at least one pathological lesion with increased uptake. The BAT uptake was normalised at the second examination after pretreatment with propranolol. SUVs of the pathological lesions and of the liver, spleen, lung, blood, skeletal muscles, bone marrow, gluteal fat, abdomen and heart were assessed. In order to control the effects of propranolol on normal organs/tissues, which could interfere with the findings, 25 age and gender matched normal controls were also studied (control subjects).

In the study group, there was only a lower bone marrow uptake after propranolol administration. Comparing the study group with the control subjects, the bone marrow activity was higher at examination before propranolol treatment compared to the control subjects. There was also a higher uptake of the spleen in the study group before propranolol treatment compared to the control subjects. There were no differences between the study group after propranolol administration and the control subjects.

The differences found are small and cannot be explained, why they could be random phenomena. Together with, there were no differences between the study group after propranolol administration and the control subjects; it is concluded that an effect on the FDG distribution in patients with a strong BAT uptake by can be disregarded in clinical praxis. This is important at longitudinal examinations of patients undergoing tailored tumour therapy and in contrast to examinations in patients with a generally increased uptake of the skeletal muscles which significantly affects the distribution of the radiopharmaceutical.