Phyllodes tumor of the breast containing the intracytoplasmic inclusion bodies identical with infantile digital fibromatosis.

This report illustrates a case of the phyllodes tumor of the breast with intracytoplasmic inclusion bodies that are diffusely distributed over the tumor. The inclusion bodies were identified as aggregated actin filaments by modified immunohistochemical and ultrastructural analyses. These structures are thought to be identical to those of the characteristic intracytoplasmic inclusion bodies observed in the case of infantile digital fibromatosis. To date, tumors with such intracytoplasmic inclusion bodies have been recognized exclusively as infantile digital fibromatosis. Although a few rare cases in which the infantile digital fibromatosis-like tumor occurred outside the digit or in adulthood, all these cases were in the category of fibromatosis or fibroma. This case is the first instance of the tumor containing intracytoplasmic inclusion bodies identical with the infantile digital fibromatosis other than fibromatosis or fibroma.