Clearance of unusual muscular uptake of F-18 FDG in the lower lumbar region on a repeat FDG-PET scan after oral chlorzoxazone intervention.

A 29-year-old woman, a diagnosed case of primitive neuroectodermal tumor (PNET) of the right gluteal region, had an F-18 FDG PET scan before treatment. Whole-body PET scan that was performed 60 minutes after the intravenous injection of 370 MBq (10 mCi) of tracer showed avid uptake with necrotic areas at the site of the primary tumor in the right gluteal region. In addition, a linear band of intense FDG uptake was noted in the soft tissue posterior to the lower lumbar spine in an area where there was no evident abnormality on clinical examination except for mild stiffness in the erector spinae muscle on the left. A repeat regional PET scan was done the next day after oral administration of 1000 mg chlorzoxazone in 2 divided doses at 12-hour intervals. The repeat PET scan did not show the linear abnormal uptake confirming it to be in the tense musculature. The use of long-acting muscle relaxants such as chlorzoxazone before a repeat regional study may be helpful to avoid false-positive interpretation of FDG PET scans.