Proximal Type Epithelioid Sarcoma

Differential Diagnosis

Keratin positivity is typically seen in both

Squamous Carcinoma	Proximal Type Epithelioid Sarcoma
CD34 negative	CD34 50% positive
May have history of epithelial primary	No primary epithelial lesion
Uncommon in young people	Most patients under 40 years
May have in situ lesion or keratinization	No in situ lesion or keratinization
CK5/6 commonly extensive	CK5/6 rare, focal
CA125 negative in cutaneous SCC, frequently positive in SCC of other sites	CA125 90%

CD34 reactivity is very rare in carcinoma of any type

Proximal Type Epithelioid Sarcoma	Monophasic Synovial Sarcoma
Necrosis frequently extensive	Necrosis usually focal
CD34 50% positive	CD34 negative
No ropy collagen	Ropy collagen frequent
Calcification infrequent	Calcification frequent
No hemangiopericytomatous vessels	Hemangiopericytomatous vessels frequent
No SYT-SSX gene fusion	SYT-SST gene fusion present

Both may present as keratin positive spindle cell neoplasms

Epithelioid or rhabdoid cytoplasm may raise the following possibilities

Epithelioid Angiosarcoma	Proximal Type Epithelioid Sarcoma
CD31 usually positive	CD31 negative
May have adjacent atypical small channels	No small atypical channels

CD34 and keratin may be positive in both.

Angiomatoid variant epithelioid sarcoma may be more difficult to distinguish as it may have large blood filled spaces.

Keratin positivity as well as cytoplasmic features may cause confusion between these two.

Extrarenal Rhabdoid Tumor	Proximal Type Epithelioid Sarcoma
CD34 negative	CD34 50% positive
No intensely eosinophilic collagen	Intensely eosinophilic collagen
Median age 8-9 years	Median age 35-40 years
Wide range of sites	Predilection for genitalia

It is probable that malignant rhabdoid tumors do not occur in adults - most if not all are carcinoma, melanoma or proximal epithelioid sarcoma