Adult Fibrosarcoma

Differential Diagnosis

Nodular Fasciitis	Adult Fibrosarcoma
Rare over 5 cm	Usually over 4 cm
Undulating pattern	Herringbone pattern
Atypical mitotic figures rare	Atypical mitotic figures common
Fine, pale, even chromatin	Coarse, granular, irregular chromatin
Torn appearance of stroma	Lacks torn appearance

Extra-abdominal Desmoid Fibromatosis	Adult 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 (except in low grade fibrosarcoma)
Atypical mitotic figures rare	Atypical mitotic figures common
Cells separated by collagen	Cells usually touch each other

Distinction may not always be clear and may be indistinguishable in low grade areas

Adult Fibrosarcoma	Monophasic Synovial Sarcoma
Usually extensive herringbone pattern	Herringbone pattern usually only focal
Keratin, EMA negative	Keratin, EMA often positive
No ropy collagen	Ropy collagen frequent
Calcification rare	Calcification may be present
No hemangiopericytomatous vessels	Hemangiopericytomatous vessels frequent
Thin elongate nuclei	Plump nuclei
Chromatin not stippled	Stippled chromatin
No SYT-SSX gene fusion	SYT-SST gene fusion present
TLE1 0% (0/3)	TLE1 97%

Malignant Fibrous Histiocytoma	Adult Fibrosarcoma
Pleomorphic	Uniform
May show histiocytic differentiation	No histiocytic differentiation

Distinction is definitional as pleomorphism is not permitted in fibrosarcoma

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

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