Infantile Fibrosarcoma

Differential Diagnosis

Myofibroma	Infantile Fibrosarcoma
Random spindle cell pattern	Fascicular pattern
Biphasic mature and immature areas	No biphasic pattern
Atypical mitotic figures rare	Atypical mitotic figures common

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

Infantile Fibrosarcoma	Monophasic Synovial Sarcoma
May have extensive herringbone pattern	Herringbone pattern usually only focal
Keratin, EMA negative or at most focal	Keratin, EMA usually positive
No ropy collagen	Ropy collagen frequent
Calcification rare	Calcification common
Elongate nuclei	Plump nuclei
Chromatin not stippled	Stippled chromatin
t(12;15) usually present	SYT-SST gene fusion present

Both may have hemangiopericytomatous vessels

Embryonal Rhabdomyosarcoma	Infantile Fibrosarcoma
Desmin and myogenin positive	Desmin and myogenin negative
No areas of conventional fibrosarcoma	Areas of conventional fascicular pattern present

Infantile fibrosarcoma may have round cell areas reminiscent of rhabdomyosarcoma

Leiomyosarcoma of Deep Soft Tissue or Retroperitoneum	Adult or Infantile Fibrosarcoma
Frequently desmin positive	Desmin negative
Frequently pleomorphic	Uniform
May occur in retroperitoneum or pelvis	Rare in retroperitoneum and pelvis
Lacks t(12;15)	t(12;15) usually present in infantile fibrosarcoma

Distinction may be arbitrary in some cases but desmin positive neoplastic cells exclude fibrosarcoma

Dermatofibrosarcoma Protuberans	Adult or Infantile Fibrosarcoma
CD34 positive	CD34 negative
Storiform pattern	Fascicular, herringbone pattern
Involves dermis and subcutaneous tissue	Virtually always in deep soft tissue

DFSP may dedifferentiate with a fibrosarcoma pattern
If fibrosarcoma occurs in the dermis or subcutaneous tissue, it is usually dedifferentiated DFSP

Primitive Peripheral Neuroectodermal Tumor	Infantile Fibrosarcoma
CD99 positive	CD99 negative
No areas of conventional fibrosarcoma	Areas of conventional fascicular pattern present
Various translocations involving 22q12	t(12;15) (p13;q26) present in most cases

Infantile fibrosarcoma may have round cell areas reminiscent of PNET

Malignant Peripheral Nerve Sheath Tumor	Adult or Infantile Fibrosarcoma
May be S100 positive (50%)	S100 negative
May be pleomorphic	Uniform
May arise from nerve or neurofibroma	No relation to nerve or neurofibroma
May arise in von Recklinghausen disease	Not related to von Recklinghausen disease
Lacks t(12;15)	t(12;15) usually present in infantile fibrosarcoma

Cellular Fibroma of Tendon Sheath	Adult or Infantile Fibrosarcoma
Common in hands	Rare in hands in adults but may involve hands in infantile version
Rare >2 cm	Frequently >4 cm
Cytologically bland	Cytologically atypical
No atypical mitotic figures	Atypical mitotic figures may be seen
Smooth muscle actin strongly positive	Actin variable
Lacks t(12;15)	t(12;15) present in most infantile cases