Infantile digital fibromatosis (IDF), also termed inclusion body fibromatosis, Reye tumor, or Reyes tumor, usually occurs as a single, small, asymptomatic, nodule in the dermis on a finger or toe of infants and young children. IMF is a rare disorder with approximately 200 cases reported in the medical literature as of 2021. The World Health Organization, 2020, classified these nodules as a specific benign tumor type in the category of fibroblastic and myofibroblastic tumors. IDF was first described by the Australian pathologist, Douglas Reye, in 1965.IDF consists of an overgrowth of spindle-shaped cells in a collagen fiber-rich background located in the dermis (i.e. the layer of skin between the  epidermis and subcutaneous tissue) but may extend into the subcutaneous tissue. These spindle-shaped cells contain distinctive inclusion bodies within their cytoplasm that greatly help in distinguishing IDF from other fibrous skin diseases. These inclusions are composed of densely packed vimentin and actin filamentsIDF lesions are usually painless and  have a tendency to regress spontaneously. Consequently, the current recommended treatment for IDF nodules is conservative observation. If the lesions cause local deformities and/or functional impairments or continue to increase in size over long observation periods, surgical removal is recommended. However, surgically removed IDF lesions have had high rates of recurrence and this surgery may cause post-operative digital deformities.
Presentation
IDF typically presents as a solitary painless, smooth, flesh-colored to red, dome-shaped nodule located on the dorsal or lateral aspect of a finger or toe but sparing the thumb or great toe; they occur principally in the first two years of life with about 1/3 of cases having a lesion since birth. However: 1) lesions with appearances similar to, and diagnosed as, IPF have been described in extra digital sites such as the arm, breast, tongue, thigh, and chest; 2) unusual cases have presented with multiple lesions, i.e. 2–3, uncommonly 7–15, and one case with 74 lesions in a 2018 review study; 3) cases have been reported in children up to the age 10 and one case was reported in a 52-year-old adult; 4) the lesions are typically smaller than 2 cm in maximum diameter but rarely have been tumor-sized, e.g. 4.5 cm; 5) the lesions, particularly when larger-sized, may be painful and/or compromise the functions of nearby joints and digits; and 6) the lesions occasionally invade the periosteum and erode a nearby bone. Pathology
Microscopic histopathological analyses of appropriately dye-stained IDF tissues typically show a non-encapsulated small tumor composed of bundles of uniform spindle-shaped cells that combine physical features of fibroblasts (the most common cell type in connective tissue) with those of myofibroblasts  (contractile, spindle-shaped cells that are identifiable by their expression of α-smooth muscle actin) in a background of collagen fibers. In hematoxylin and eosin-stained tissues, the spindle-shaped cells have pale eosinophilic (i.e.