Unusual benign myxoid nerve sheath lesion: myxoid palisaded encapsulated neuroma (PEN) or nerve sheath myxoma with PEN/PEN-like features?

A 38-year-old woman had a polypoid nodule on her scalp. The lesion histopathologically demonstrated an extensive myxoid lobular lesion associated with a nonmyxoid and cellular area in the peripheral area of the lesion. The features of the peripheral cellular area resembled those of palisaded encapsulated neuroma (PEN). Most of the lesion was myxoid, with a large lobule in which several thin fibrous septa were present, dividing it into smaller lobules. The myxomatous area was composed of spindle cells or stellate cells with elongated cytoplasmic processes. There was a loose cellular network with a reticular and netlike network pattern or a lamellar pattern. From the immunohistochemical findings, as well as the histopathologic features, the whole lesion was thus considered to be a PEN, whereas the main, myxoid, lobular lesion was thought to have been caused by myxoid changes within a PEN, namely, myxoid PEN. It cannot be completely ruled out, however, that this case could be that of a nerve sheath myxoma with a PEN/PEN-like lesion. Thus, the present case may suggest that PEN can show extensive myxoid change or may demonstrate a relationship between PEN and nerve sheath myxoma.