This presentation appears to be a review or teaching session focused on the histopathological diagnosis and differential diagnosis of a specific type of cutaneous spindle cell neoplasm, identified microscopically as an ulcerated, pleomorphic lesion slamming against the epidermis with high mitotic activity. The primary focus is on distinguishing between Atypical Fibroxanthoma (AFX) and other high-grade spindle cell tumors like Spindle Cell Squamous Cell Carcinoma (SCC) and various sarcomas (e.g., Leiomyosarcoma, Rhabdomyosarcoma, Angiosarcoma). The presenter outlines a systematic immunohistochemical approach, emphasizing markers like S100/SOX10 (to rule out melanoma), P40/CK5 (for SCC), and ERG/CD31 (for Angiosarcoma). While AFX and Pleomorphic Dermal Sarcoma (PDS) are considered the same entity at different stages, AFX is reserved for smaller, dermally confined lesions with a good prognosis, whereas PDS applies when the tumor invades the subcutis or shows aggressive features. A specific question regarding the occurrence of Rhabdomyosarcoma in the skin is addressed, explaining its presumed origin from a primitive precursor stem cell, despite the lack of obvious normal skeletal muscle counterpart in that location.
Pathology Case Review: Differential Diagnosis of Pleomorphic Spindle Cell Lesions in the Skin
0:00 Visual Findings: The scanned slide shows a dermal proliferation composed of very pleomorphic, spindle-shaped cells with high mitotic activity, including atypical tripolar forms, pressing against the epidermis.
0:02:12 Immunohistochemical Strategy (Differential Diagnosis - SLAM): A systematic approach is required to rule out mimics of Atypical Fibroxanthoma (AFX):
S - Spindle Cell SCC (Rule out with P40, CK5).
L - Leiomyosarcoma (Usually look less ugly, have fascicles, ruled out with Desmin).
A - Angiosarcoma (Rule out with ERG or CD31; CD34 is cautioned against as it can be lost).
M - Spindle Cell Melanoma (Rule out with S100 or SOX10).
0:03:09 AFX Presentation: Typically presents as an erythematous, ulcerated nodule on sun-damaged head/neck of elderly individuals, characterized by ugly, pleomorphic cells with many mitoses, often lacking the foam cells originally associated with the name.
0:04:30 AFX vs. PDS: AFX is the term used when the pleomorphic spindle cell neoplasm is small, confined to the dermis, and has a good prognosis. If it extends into the subcutis, is large ($\geq 2$ cm), or shows necrosis/perineural invasion, it is termed Pleomorphic Dermal Sarcoma (PDS); the speaker believes they are the same tumor with different features.
0:04:15 Ruling out Rhabdomyosarcoma (RMS): Desmin staining is used to exclude RMS, which rarely occurs cutaneously in older adults and is highly aggressive even when small.
0:07:03 Utility of CD10: CD10 stains most entities in the differential (including AFX, SCC, Melanoma, Angiosarc) and is therefore considered non-specific and not personally relied upon for primary diagnosis.
0:08:01 Origin of Cutaneous RMS: Rhabdomyosarcomas in the skin are likely derived from a primitive precursor stem cell present in the tissue, as there is no obvious normal skeletal muscle counterpart in the dermis.
0:09:32 Next Case: The presentation concludes the discussion on this case, prompting transition to the next one.