This video by Jared Gardner details the diagnostic approach to pleomorphic spindle cell neoplasms of the skin, focusing on differentiating atypical fibro xanthoma (AFX) from undifferentiated pleomorphic sarcoma (UPS), also known as pleomorphic dermal sarcoma (PDS) when confined to the skin and subcutis. These lesions commonly present on the sun-exposed head and neck of elderly patients, often clinically resembling squamous cell carcinoma. Histologically, they are characterized by highly atypical, pleomorphic spindle cells with prominent nucleoli and atypical mitotic figures, necessitating a comprehensive immunohistochemical panel to exclude other aggressive entities such as spindle cell melanoma, spindle cell squamous cell carcinoma, leiomyosarcoma, and angiosarcoma. The critical distinction between AFX and PDS/UPS lies in the depth of invasion: AFX is confined to the dermis and has negligible metastatic potential, while PDS/UPS invades the subcutis and carries a risk of metastasis. The video emphasizes the challenge of making this distinction on often transected biopsy specimens and advises pathologists to report these as pleomorphic spindle cell neoplasms with a differential, recommending complete excision to determine the full extent and, thus, the prognostic implications.
Summarizing Atypical Fibroxanthoma (AFX) vs. Undifferentiated Pleomorphic Sarcoma (UPS)
0:00:05 Understanding Terminology: The video introduces the challenge of differentiating Atypical Fibroxanthoma (AFX) from Undifferentiated Pleomorphic Sarcoma (UPS), formerly Malignant Fibrous Histiocytoma (MFH), and Pleomorphic Dermal Sarcoma (PDS) when confined to the skin/subcutis. These are common but confusing entities.
Important Details: Lesions typically occur on the head and neck of elderly, sun-exposed patients (similar to SCC, BCC, melanoma). They clinically present as ulcerated, erythematous, scaly nodules, often indistinguishable from squamous cell carcinoma without biopsy. Biopsies are frequently transected, making complete assessment difficult.
Key Takeaway: Clinically, these lesions are non-specific and require biopsy for diagnosis.
Important Details: The dermis is broadly filled with atypical spindle cells. The cells show huge, pleomorphic nuclei and can be arranged in fascicles or haphazard sheets. No in situ malignancy in the overlying epidermis.
Key Takeaway: Malignant spindle cell neoplasm filling the dermis, requiring further differentiation.
Important Details: Three primary entities to consider: spindle cell melanoma, spindle cell squamous cell carcinoma (SCC), and atypical fibro xanthoma (AFX). All can appear histologically identical on H&E stains.
Key Takeaway: A broad differential is necessary when encountering ugly, atypical spindle cells in the skin.
Important Details: Cells exhibit significant pleomorphism (largest nucleus 3x smallest), are hyperchromatic, have huge central nucleoli, and can be multinucleated. Atypical mitotic figures (e.g., Y-shaped, tri-polar) are present. These features indicate chromosomal instability or aneuploidy.
Key Takeaway: Presence of marked atypia and atypical mitoses strongly suggests malignancy and genetic instability.
0:05:54 The Necessity of Immunohistochemistry (IHC):
Important Details: IHC is crucial for distinguishing these entities, as H&E alone is insufficient, especially in the absence of obvious in situ components. It's also important to remember patients can have multiple tumors in the same area.
Key Takeaway: IHC is essential for accurate diagnosis and differentiation in pleomorphic spindle cell lesions of the skin.
Important Details: Use S100 and/or SOX10. SOX10 is preferred as a clean nuclear stain that doesn't stain Langerhans/dendritic cells, reducing background confusion in inflamed/ulcerated lesions, unlike S100. Diffuse positivity in the tumor rules in melanoma; negativity makes melanoma highly unlikely. Internal controls are vital.
Key Takeaway: SOX10 is the preferred marker to rule out spindle cell melanoma.
0:09:25 IHC for Squamous Cell Carcinoma Exclusion:
Important Details: Pan-cytokeratin can be used but may be negative in poorly differentiated spindle cell SCC. P63 or P40 (nuclear markers) are highly preferred as they are rarely negative in spindle cell SCC. Strong negativity argues against SCC.
Key Takeaway: P63/P40 are strong markers to rule out spindle cell SCC.
Leiomyosarcoma: Consider if fascicular growth is present. Use Desmin (preferred, specific) over Actin (non-specific, requires strong diffuse staining). Dermal leiomyosarcomas have a very low risk of metastasis.
Angiosarcoma: Consider if background hemorrhage is present. Use ERG (highly sensitive, nuclear) or CD31 (excellent, but can stain histiocytes). CD34 is less reliable.
Rhabdomyosarcoma: Very rare in skin, but Desmin can help exclude.
Key Takeaway: A broader IHC panel (Desmin, ERG/CD31) may be necessary to exclude other rare but aggressive spindle cell sarcomas.
0:15:48 AFX vs. PDS/UPS: The Critical Distinction & Prognosis:
Important Details: If melanoma, SCC, leiomyosarcoma, and angiosarcoma are excluded, the diagnosis is a pleomorphic spindle cell neoplasm. The depth of invasion determines the prognosis and specific name:
AFX: Confined entirely to the dermis, almost never metastasizes despite its atypical appearance.
PDS/UPS: Extends into the subcutis, considered a sarcoma with metastatic potential. Some studies show a significant metastasis rate (e.g., ~30%).
Key Takeaway: Depth of invasion is the most crucial factor for differentiating AFX from PDS/UPS and predicting metastatic behavior.
Important Details: Due to common broad transection of biopsies, pathologists often cannot definitively confirm AFX. The recommended practice is to report "pleomorphic spindle cell neoplasm" with a differential including AFX and PDS/UPS, explicitly stating the need for re-excision to assess the full depth.
Key Takeaway: Communicate clearly with clinicians that complete excision is needed to determine the tumor's depth, which dictates treatment and prognosis.
Important Details: Dermatofibrosarcoma Protuberans (DFSP) is a common misdiagnosis. However, DFSP is almost always cytologically bland, lacking the pleomorphism and atypical mitoses seen in AFX/PDS/UPS (unless previously radiated).
Key Takeaway: Marked pleomorphism and atypical mitoses effectively rule out DFSP.