label: Inclusion body fibromatosis
label_lc: inclusion body fibromatosis
description: A benign tumor made up of mostly myofibroblasts that appears almost exclusively on the digits of the hands and feet, rarely involving the thumb or big toe. The lesion displays a proliferation of bland intradermal spindle cells arranged in whorls, fascicles, or a storiform pattern in a collagenous background of varying degrees. Also usually present are perpendicular tumor cell fascicles that extend to the epidermis. The small intracytoplasmic inclusions are said to appear similar to red blood cells. The inclusion bodies have been shown to be made up of densely packed vimentin and actin filaments. The tumor often causes a dome-shaped elevation of the overlying structures, forming a protuberant or polypoid nodule. The overlying epidermis can display a host of changes, including acanthosis, hyperkeratosis, parakeratosis, rete ridge flattening, entrapment of adnexal structures, and, rarely, ulceration.