Inclusion body fibromatosis is a superficial benign myofibroblastic tumor characterized by eosinophilic intracytoplasmatic inclusions. These tumors are also known under the terms infantile digital fibroma or fibromatosis, recurring digital fibroma of childhood, recurring digital fibrous tumor of infancy or infantile dermal fibrosis and are typically seen in infancy and feature a propensity for local recurrence 1,2.
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Epidemiology
Inclusion body fibromatosis is a rare condition and accounts for about 2% of fibroblastic tumors in children and approximately 0.1% of soft tissue tumors 1. Almost all cases have been described in children younger than 5 years old. About 30% are detected at birth and the majority of cases are seen within the first year of life. There is no gender predilection 1,2.
Clinical presentation
Inclusion body fibromatosis typically manifests as polypoid cutaneous nodules that are usually otherwise asymptomatic 1.
Diagnosis
The diagnosis of inclusion body fibromatosis is histological 1.
Diagnostic criteria
The diagnostic criterion according to the WHO classification of soft tissue and bone tumors (5th edition) 1:
- cytologically bland, fibroblastic or myofibroblastic, dermal proliferation
- cytoplasmic or paranuclear eosinophilic inclusions
Pathology
Inclusion body fibromatosis is a proliferative skin-based myofibroblastic tumor with eosinophilic paranuclear cell inclusions 1.
Etiology
The etiology of inclusion body fibromatosis is unknown 1.
Location
The classical location of inclusion body fibromatosis involves the dorsal aspects of the distal and middle digital phalanges of hands or feet, with fingers being more frequently involved than toes. The fifth digits are less likely affected. The thumb and great toe are typically spared. Other extradigital locations are rare and include other regions of the extremities, the oral cavity and the breast 1-4.
Macroscopic appearance
Macroscopically inclusion body fibromatosis appears as an ill-defined, dome-shaped, cutaneous nodule of white-tannish color and fibrous firm consistency 1.
Microscopic appearance
The histological spectrum of inclusion body fibromatosis includes the following 1-4:
- spindle cells with eosinophilic cytoplasm and bland elongated nuclei
- intracytoplasmic inclusions which can be stained with trichome
- short fascicular, whorling or storiform growth pattern typically perpendicular to epidermis extending into dermis or subcutis
- low mitotic activity
- no cell atypia
Immunophenotype
Immunohistochemistry stains can be positive for actin, calponin and occasionally caldesmon 1-3.
Radiographic features
Imaging of inclusion body fibromatosis usually reveals a non-specific soft tissue mass. Appearance on cross-sectional imaging has been described as poorly demarcated, heterogenous round to oval, subcutaneous soft tissue mass with similarities to other superficial fibromatoses. Osseous involvement has been described in rare events 2,4.
Ultrasound
Lesions have been described as isoechoic to hypoechoic 4.
Radiology report
The radiological report should include a description of the following:
- location and size of nodules
- relation to adjacent bones
- relation to adjacent tendons
Treatment and prognosis
Some lesions might regress spontaneously but local recurrence after excision is very common. Thus management varies and depends on symptoms. Treatment options include function preserving excision and/or intralesional steroid injections for symptomatic tumors but can also include watchful waiting for asymptomatic lesions 1.
History and etymology
The entity was initially described by the Australian pathologist Ralph Douglas Kenneth Reye probably in 1965 3,5,6.
Differential diagnosis
Conditions that can mimic the appearance of inclusion body fibromatosis include 3:
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