Inclusion body fibromatosis (Infantile digital fibromatosis) is a rare, benign fibrous growth of childhood. It was identified as a distinct entity by Reye in 1965.

This lesion has also been referred to as Reye tumor, infantile digital fibroma and recurrent digital fibrous tumour of childhood.

Age:  These lesions are usually present in young infants of either sex. Most of the cases are diagnosed within the first year of life and up to one third of cases are congenital. Some cases have been reported in adults (non-digital sites).

Site:   These lesions usually present as single or multiple nodules involving the extensor surfaces, most commonly the dorsal and lateral aspect of the distal or middle phalanx. Thumbs and great toes are usually spared.                               Cases have been reported in extradigital locations such as tongue, breast, arm and leg.

Gross features:  Pale, hard , poorly circumscribed, nontender nodule with a firm broad-base. The lesion is covered by intact skin and is often less than 1cm in diameter.

Microscopic features: 

The lesion replaces most of the dermis.
It is a non-encapsulated tumour composed of moderately cellular whorls, sheets and interlacing bundles of eosinophilic myofibroblasts set in a collagenous backround.

The most characteristic feature is the presence of 'cytoplasmic round eosinophilic inclusions'  in a paranuclear position. The inclusions are sometimes surrounded by a clear halo. These are slightly larger than red blood cells. With maturation of the lesion there may be decrease in number of inclusions.  These inclusions are composed of actin which has been confirmed by immuno-electron microscopy.

Adnexal structures and fat may be entrapped in the fibrous proliferation. The lesion may extend to the periosteum and erode it. There is no invasion of bone.

[The following features may be indicative of tumour recurrence:    Incomplete surgical or histological resection; Patient's age more than 5 years ; Mitotic index of 5 or more/ 10HPF; Areas of necrosis and inflammation within the tumour].

Special stains: Eosinophilic inclusions are negative for PAS, but are positive for Masson’s trichrome (bright red ),PTAH (deep purple),elastin van Gieson (yellow).

Immunocytochemistry reveals that the inclusions are reactive with smooth muscle actin and vimentin. The tumour cells are keratin, vimentin, desmin and actin positive. S-100 protein is negative.

Digital fibromas may be associated with facial pigmentary dysplasia, focal dermal hypoplasia, metacarpal and metatarsal disorganization, and limb malformations. A syndrome of digital fibromas, facial pigmentary dysplasia, and metacarpal and metatarsal disorganization. Am J Med Genet. 1998;80(1):1-5.

This rapidly growing indolent nodule may lead to interphalangeal deformity and may also erode the underlying bone.

The most common treatment is surgical excision. Spontaneous regression has been reported in a few cases. There is a high recurrence rate of about 60%.        The tumour in extradigital locations of adults does not seem to recur after excision. This tumour does not metastasize.

Differential diagnosis:  Age, location and presence of distinctive eosinophilic bright cytoplasmic inclusions helps in establishing the diagnosis.

The following lesions may be included in the differential diagnosis: Other fibrous tumours of infancy ; Dermatofibroma ; Smooth Muscle tumours ; Myofibroblastic tumour;  Peripheral nerve sheath tumour