Adult rhabdomyoma with oncocytic changes affecting the floor of the mouth: optical, immunohistochemical, and ultrastructural study.

Adult rhabdomyoma (AR) is an extremely uncommon benign neoplasm with mature skeletal muscle differentiation comprising approximately 2% of muscle tumors, usually affecting the soft tissue of the head and neck. Although histology of AR is characteristic, several differential diagnoses (granular cell tumor, hibernoma, oncocytoma) should be considered, and one needs to be familiar with this rare entity to exclude other neoplastic diseases. We present a case of AR, in a 54-year-old man, affecting the floor of the mouth, and call attention to the oncocytic appearance (including antimitochondrial and peroxiredoxin I immunoreactivity) of this case and its differential diagnosis analyzed at the optical, immunohistochemical, and ultrastructural level, showing the morphological and immunohistochemical features that can be confused with a salivary oncocytoma.