Desmoplastic neurotropic melanoma in a patient with trigeminal neuralgia: FDG PET/CT and MRI.

A 51-year-old man presented with a hard subcutaneous nodule adhering to the underlying bone structures of the left eyelid in 2004. Histopathology showed a desmoplastic neurotropic melanoma (DNM) with perineural invasion. Patient presented with a first recurrence in October 2005, which was treated by surgery. In August 2006 he presented with trigeminal neuralgia of the left face in the area innervated by the first and second branches of the trigeminal nerve. A PET scan clearly shows the tumoral hypermetabolism of the subcutaneous recurrence of the neurotropic melanoma with invasion of the second or maxillary branch that follows the nerve up to the trigeminal ganglion, which was detected despite the physiological high uptake in the temporal lobe. MRI confirms the invasion and a second PET and MRI 6 months later done to evaluate resectability showed progressive disease. DNM is a rare subtype of spindle cell melanoma. It corresponds to dermal proliferation of desmoplastic cells of neural differentiation. Unlike other melanomas, however, survival for DNM may be better compared with other forms of melanoma. This rare case report presents PET imaging involving cranial nerve invasion by this uncommon melanoma subtype.