This pathology case study examines a biopsy taken from a slow-growing, diffuse mass on the distal extremity (hand) of a middle-aged adult. Histologically, the tumor presents as sheets of large, pleomorphic cells with abundant pale pink cytoplasm and highly atypical, vesicular nuclei.
Immunohistochemical analysis provides the diagnostic confirmation: the tumor cells exhibit strong, diffuse positivity for both Pan-Cytokeratin and EMA (Epithelial Membrane Antigen). Crucially, the tumor shows a complete loss of nuclear expression for INI1 (SMARCB1), a key molecular signature. The combination of morphology and immunohistochemistry confirms the diagnosis of Epithelioid Sarcoma (conventional type).
Epithelioid Sarcoma is a rare but highly aggressive soft tissue sarcoma known for its propensity for local recurrence, metastasis (including to lymph nodes), and challenging surgical eradication due to its infiltration of fascial and tendinous structures. The video emphasizes key diagnostic pitfalls, particularly its low-power mimicry of benign granulomatous processes like rheumatoid nodules.
Epithelioid Sarcoma: Distal Extremity Mass
0:13 Case Presentation: The specimen is a mass biopsied from the hand of a middle-aged adult, characterized by slow growth and diffuse swelling.
0:28 Histologic Features: The neoplasm consists of sheets of large, atypical cells with abundant pale pink/dense pink cytoplasm, large pleomorphic nuclei, and a distinct pale/washed-out, vesicular chromatin structure.
1:24 Positive Immunohistochemistry (IHC): The tumor shows strong, diffuse expression of Pan-Cytokeratin (pancK) and EMA (Epithelial Membrane Antigen) in the cytoplasm, confirming the epithelioid/carcinomatous differentiation often seen in this sarcoma.
1:49 Critical Diagnostic Loss: The tumor exhibits a complete loss of expression of INI1 (also known as SMARCB1) in the tumor cell nuclei, a feature considered definitive for Epithelioid Sarcoma (ES) in the correct clinical and morphological context.
2:04 IHC Caveat (Negative Stains): The speaker stresses the importance of adequate internal controls when interpreting negative stains like INI1/SMARCB1 to avoid misdiagnosis, noting that failure of the background staining can lead to an inappropriate interpretation of loss.
3:02 Diagnosis and Types: The diagnosis is Epithelioid Sarcoma (conventional type), which typically affects the distal extremities of young or middle-aged adults. A more aggressive "proximal type" exists, which is often rhabdoid-looking and found closer to the trunk (e.g., groin).
3:47 Diagnostic Pitfall: ES can sometimes mimic benign conditions like a rheumatoid nodule or deep granuloma annulare at low power, especially when it features central necrosis surrounded by tumor cells.
4:32 Secondary IHC Considerations: Approximately half of ES cases can stain positive for the vascular marker ERG. Pathologists must be careful not to mistake ES for a malignant vascular tumor (like epithelioid angiosarcoma) just based on positive ERG staining, especially since angiosarcoma can also be keratin-positive.
5:16 Prognosis and Treatment: ES is a very serious, locally aggressive tumor with a tendency to metastasize (including to lymph nodes, which is unusual for most sarcomas). Due to its infiltrative nature along tendon sheaths and fascia, surgical eradication is difficult, often leading to recurrence and sometimes requiring eventual amputation.
5:43 Gross Appearance: A gross photograph of the resection shows a fleshy tumor infiltrating the subcutaneous adipose tissue and mingling with the fascia and tendon sheath, underscoring the challenge of obtaining clear surgical margins.