This pathology video analyzes a shave biopsy of a scalp papule from an 80-year-old man, presenting a diagnostic challenge due to a dermal proliferation of highly pleomorphic, malignant-appearing spindle cells. A comprehensive immunohistochemistry (IHC) panel (including markers for melanoma, squamous cell carcinoma, rhabdomyosarcoma, and angiosarcoma) was uniformly negative, successfully ruling out aggressive mimics.
The remaining differential diagnosis is between Atypical Fibroxanthoma (AFX) and its more aggressive counterpart, Pleomorphic Dermal Sarcoma (PDS). Since AFX and PDS are morphologically and immunophenotypically identical, the distinction relies on aggressive features, primarily invasion into the subcutis, which cannot be assessed on the transected shave biopsy. The pathologist recommends reporting the diagnosis as a pleomorphic spindle cell neoplasm (AFX vs. PDS) and emphasizes that complete excision is mandatory to determine depth, establish the final diagnosis/prognosis, and ensure negative margins.
Pathology Case Review: Pleomorphic Spindle Cell Neoplasm (AFX vs. PDS)
0:00 Case Introduction: The case involves an 80-year-old male with a papule on the sun-damaged scalp, biopsied via a shave specimen.
1:23 Morphologic Findings: The dermis is replaced by a proliferation of ugly, highly pleomorphic spindle cells with bizarre nuclei. Mitotic activity, including atypical forms, is present. The overlying epidermis is atrophic with no evidence of dysplasia or carcinoma in situ.
3:30 Differential Diagnosis (DDx): The primary differential includes spindle cell squamous cell carcinoma, spindle cell melanoma, Atypical Fibroxanthoma (AFX), and Pleomorphic Dermal Sarcoma (PDS).
5:51 IHC Strategy Takeaway: The pathologist advocates for SOX10 over S100 for melanoma workup in the skin because S100 frequently stains benign background dendritic/Langerhans cells, potentially leading to an overcall, especially in AFX.
8:00 Vascular Mimics: ERG is the preferred screening marker for angiosarcoma due to high sensitivity. A strong, diffuse staining pattern equal to background blood vessels is required to call a lesion ERG positive; patchy staining is non-specific (8:43).
13:31 AFX vs. PDS Distinction: AFX (small, confined to dermis, excellent prognosis) and PDS (deeper, invades subcutis, increased risk of metastasis/death) are morphologically identical. The distinction hinges entirely on the presence of aggressive features, particularly depth of invasion.
14:45 Diagnostic Limitation: Because the shave biopsy is transected at the base, the depth (a critical factor for distinguishing AFX from PDS) cannot be assessed. This is the "tip of the iceberg" scenario.
14:47 Recommended Reporting: The diagnosis should be hedged as "Pleomorphic Spindle Cell Neoplasm, Atypical Fibroxanthoma versus Pleomorphic Dermal Sarcoma (See Comment)," ensuring the clinician understands the malignant potential.
15:14 Management Takeaway: Complete excision is required to assess depth, rule out PDS, and achieve negative margins. Mohs surgery debulk specimens must be saved and examined for this tumor type.
17:54 Avoidance of Non-Specific Stains: Non-specific markers like CD10, while often positive in AFX, are deemed unhelpful because they also stain many mimics (SCC, melanoma, angiosarcoma) and do not influence the critical differentiation between AFX and PDS.