Flip-flop fungus sign - FDG PET-CT

Last revised by Kevin Banks on 3 Oct 2023

The flip-flop fungus sign suggests benign pulmonary disease on FDG-PET 1-4.

Lung cancer nodules demonstrate increasing FDG-avidity as the tumor grows. Lymph node spread occurs later and is therefore at an earlier stage of development and usually exhibits lower FDG-avidity 4.

Acutely, FDG-avidity in fungal lung nodules exceeds avidity in draining lymph nodes and can mimic cancer. As inflammation subsides, the lung nodule avidity diminishes but lymph node avidity can persist for months. The flip-flop fungus sign refers to this situation of nodal activity exceeding lung nodule avidity, the reverse of the usual findings in lung cancer 5-7.

In areas of endemic histoplasmosis or coccidioidomycosis, serology makes the diagnosis and directs treatment. Fungal disease will regress on treatment.

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