Aggressive fibromatosis: is PET-CT useful in lesion characterization?

Deep extra-abdominal fibromatoses (desmoids) are rare benign fibrous mesenchymal tumours occurring in adults, which may mimic primary malignancy on imaging. We present a case of a 64-year-old man with a hard painless lump in his left calf. The solid and partially hypervascular appearance on ultrasound, the infiltrative appearance, lesion heterogeneity and heterogeneous enhancement pattern on Magnetic Resonance Imaging (MRI) were suspicious for a soft tissue sarcoma. Moreover, PET-CT demonstrated FDG-avidity. Despite these aggressive imaging features, histopathology revealed a benign but locally aggressive desmoid tumour. The radiologist should be aware that PET-CT is not always helpful as an additional tool for differentiation between malignant and benign soft tissue lesions. Intralesional bandlike areas of low signal intensity on all pulse MR sequences and intimate relationship with the muscle fascia are more useful clues to the diagnosis of this soft tissue lesion.