A patient is described with non-Hodgkin lymphoma and erythematous skin nodules suspected to be erythema nodosum. The patient underwent serial fluorodeoxyglucose (FDG) positron emission tomography (PET), which demonstrated normalization of FDG uptake by the lymphoma after 2 cycles of chemotherapy, but there was new abnormal uptake involving the subcutaneous tissues of the lower extremities. A typical skin lesion was sampled and showed the appearance of erythema nodosum with no evidence of lymphoma. The FDG uptake gradually diminished on serial PET imaging after treatment with nonsteroidal antiinflammatory drugs. In view of the recognized association of erythema nodosum with malignancy and the differential rate of response to chemotherapy, the lesions of erythema nodosum may be a source of a false-positive PET interpretation, and histologic assessment should be considered.
ositron emission tomography (PET) using fluorine-18 fluorodeoxyglucose (FDG) for the evaluation of malignant diseases has limitations in the setting of coexisting granulomatous diseases such as tuberculosis, histoplasmosis, and sarcoidosis. Granulomatous lesions have increased FDG uptake and may result in a false-positive PET interpretation. Erythema nodosum (EN) is a panniculitis consisting of an infiltrate of lymphocytes and multinucleate giant cells within subcutaneous fat. Malignancy can be associated with EN and has been documented previously with both Hodgkin disease and non-Hodgkin lymphoma. EN tends to parallel the course of the underlying malignancy and can predate the diagnosis. We describe a patient with non-Hodgkin lymphoma who developed EN proved on histology and underwent serial FDG PET. The lesions of EN show increased FDG uptake and are another potential cause for a false-positive FDG PET interpretation.