Low-fat, plexiform spindle cell lipoma of the lip expressing S100 protein: a neural tumor simulator

The plexiform variant of spindle cell lipoma is very uncommon. In fact, as far as we are aware only seven cases have been previously reported. We describe herein the case of a 49-year-old man with a smooth nodule of the mucosa of the lower lip that was gradually increasing. Surgical excision of the lesion was done and the study revealed the histological and immunohistochemical features of a plexiform spindle cell lipoma (PSCL). Peculiar to this case was the location in the lip, the presence of abundant S100-positive dendritic cells, and scarce mature lipogenic cells. S100 protein reactivity has rarely been observed in classical and plexiform spindle cell lipoma. To our knowledge, no case of PSCL displaying abundant S100-positive dendritic cells has been described. This feature may lead to a diagnostic pitfall. The main differential diagnosis includes the neuroma group, plexiform intraneural neurofibroma (PIN), plexiform schwannoma and plexiform hybrid tumor of perineurioma and cellular neurothekeoma. It is imperative correctly diagnose and differentiate PSCL from neural tumors because they may show syndromic associations, have different prognosis, including malignant transformation in PIN, and the management of all these lesions differs.