This transcript details the diagnostic approach to a challenging case of a dense, dermal spindle cell tumor, emphasizing that morphology alone is insufficient for diagnosis. The pathologist introduces the broad differential known by the acronym SLAM (Squamous cell carcinoma, Leiomyosarcoma, Angiosarcoma, Melanoma, and Atypical Fibroxanthoma/Pleomorphic Dermal Sarcoma). The video provides specific criteria for separating the locally aggressive Atypical Fibroxanthoma (AFX) from the potentially metastatic Pleomorphic Dermal Sarcoma (PDS) based on size, depth of invasion, and necrosis. A comprehensive immunohistochemistry (IHC) panel is outlined, targeting key lineage markers (keratins, S100/SOX10, Desmin, ERG/CD31) necessary to exclude major mimics and guide appropriate management, noting that the final classification often hinges on complete excision and deep assessment.
Diagnosing Dermal Spindle Cell Tumors: Differential Diagnosis and IHC Panel
0:12 Dermal Tumor Presentation: The input case involves a dense dermal tumor composed of spindle cells, abutting and slightly separating the epidermis, highlighting the challenge of morphological diagnosis.
0:26 The "SLAM" Differential: The primary differential diagnosis for spindle cell tumors includes: Squame (Spindle AFX), Leiomyosarcoma (LMS), Angiosarcoma, and Melanoma.
0:55 Differentiating AFX vs. PDS: The distinction between Atypical Fibroxanthoma (AFX) and Pleomorphic Dermal Sarcoma (PDS) is based on risk features.
PDS Criteria: Larger than 2 cm, presence of necrosis, perineural invasion, or invasion into the subcutaneous tissue, indicating a more aggressive tendency.
AFX Criteria: Small tumors strictly limited to the dermis.
1:24 Essential IHC Stain Panel: Due to indeterminate cell morphology, stains are required:
Epithelial: P63/P40 and high molecular weight keratins (e.g., CK5/6) to rule out spindle cell squamous carcinoma.
Melanocytic: S100 or SOX10 to rule out melanoma.
Muscle/Rhabdo: Desmin to cover leiomyosarcoma and rhabdomyosarcoma.
Vascular: ERG or CD31 to rule out angiosarcoma.
2:07 ERG Staining Caution: The pathologist notes that ERG may show patchy, moderate staining in AFX, which should be ignored, reserving a positive result for angiosarcoma when staining is strong and diffuse, matching background vessel intensity.
2:38 Key Takeaway on Depth: If the tumor is shaved or incompletely excised, the pathologist cannot definitively distinguish between AFX and PDS, necessitating a comment that complete excision is required to assess depth and risk factors.
2:52 Clinical Management Note: When a lesion goes to Mohs surgery, the debulk specimen should be saved so the full depth of the tumor can be examined.