Surgical Pathology Criteria

Infantile Fibromatosis

Differential Diagnosis

Applies to immature / diffuse / mesenchymal type
(for differential diagnosis of adult type see extra-abdominal desmoid fibromatosis)

Infantile Fibromatosis Infantile Fibrosarcoma
Mild to moderate cellularity Usually more cellular
Fine chromatin Usually clumped chromatin
Rarely > 5 mitotic figures per HPF Usually > 5 mitotic figures per HPF
Atypical mitotic figures rare Atypical mitotic figures common
Variably collagenous stroma Little collagen in stroma
No herringbone pattern Frequent herringbone pattern
Hemorrhage and necrosis rare Frequent hemorrhage and necrosis
Muscle fibers preserved Muscle fibers destroyed
Lacks t(12;15) t(12;15) in most cases
Distinction between grade I fibrosarcoma and desmoid fibromatosis may not always be clear


Fibrous Hamartoma of Infancy Infantile Fibromatosis
Triphasic lesion with spindle cell fibrous areas, adipose tissue and clusters of immature mesenchymal cells Not triphasic
Primarily superficial Usually centered on muscle


Inflammatory Myofibroblastic Tumor Infantile Fibromatosis
Frequent in abdomen Rare in abdomen
Usually age 0-14 years Age <5 years
Prominent inflammation Variable, peripheral inflammation
Variable locations Centered in muscle


Lipoblastomatosis Infantile Fibromatosis
At least focal plexiform capillaries No plexiform capillaries
Contains lipoblasts Mature fat cells only
Lobular pattern Infiltrative pattern


Myxoid Liposarcoma Infantile Fibromatosis
Chicken-wire vascular pattern No chicken-wire pattern
Contains lipoblasts Mature fat cells only
Essentially does not occur <5 years of age <5 years of age


Embryonal Rhabdomyosarcoma Infantile Fibromatosis
Rarely involves skeletal muscle Usually centered on skeletal muscle
Cytologically atypical Cytologically bland
No fat component Fat frequently part of lesion
MyoD1 positive MyoD1 negative
Desmin positive Desmin variable


Infantile Fibromatosis Inclusion Body Fibromatosis
Rare in hand Vast majority in hands or feet
Usually >2 cm Rarely >2 cm
Involves skeletal muscle Muscle not involved
No inclusions Inclusions usually present

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