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Infant with nodule on 3rd digit distal interphalangeal joint (DIP) for 3 months.
Inclusion body fibromatosis is a rare benign fibroproliferative tumour of infancy and early childhood that preferentially affects the fingers and toes. Sections show acral skin that is mildly acanthotic. Present within the dermis is a spindle cell neoplasm with abundant amphophilic cytoplasm arranged in short and long fascicles intersecting the dermal collagen bundles. The nuclei are bland, round to oval, and with open chromatin. Mitotic activity is not easily identified. Necrosis is absent. Occasional eosinophilic intracytoplasmic globules are noted.
Immunohistochemical studies demonstrate the neoplastic cells to express smooth muscle actin (SMA), calponin and desmin. Factor XIIIa, CD68, and CD34 are negative. PAS and elastic trichrome stains highlight the eosinophilic cytoplasmic inclusions.