This video features a pathology case discussion focusing on the differential diagnosis of a highly atypical, pleomorphic spindle cell neoplasm observed in the dermis, typically arising on the sun-damaged head and neck of an elderly patient.
Microscopic examination reveals pleomorphic, geomorphic spindle cells with numerous atypical mitoses, growing directly beneath the epidermis (lacking a grenz zone). The main diagnostic challenge involves excluding other aggressive spindle cell tumors, summarized by the acronym SLAM: Spindle cell Squamous Cell Carcinoma (SCC), Leiomyosarcoma (LMS), Angiosarcoma (AS), and Melanoma.
The preferred diagnosis, achieved through exclusion via immunohistochemistry (IHC), is Atypical Fibroxanthoma (AFX). AFX is defined as a small, dermis-confined pleomorphic sarcoma with a good prognosis. The same tumor, if larger, extending into the subcutis, or exhibiting necrosis/perineural invasion, is classified as Pleomorphic Dermal Sarcoma (PDS), indicating a potentially more aggressive behavior. The discussion outlines the specific IHC markers required (S100, Sox10, P40, CK5, Desmin, ERG, CD31) to confidently rule out melanoma, SCC, rhabdomyosarcoma, and angiosarcoma.
Summary of Spindle Cell Neoplasm Diagnosis
0:33 Microscopic Findings: The tumor consists of geomorphic, pleomorphic spindle-shaped cells in the dermis with high mitotic activity, including tripolar and atypical forms.
1:28 Location: The tumor is "slammed up against the epidermis," meaning there is no clear grenz zone or space between the tumor and the basal layer.
2:38 Clinical Context: The typical presentation is an erythematous, ulcerated, or crusted papule/nodule on the head and neck of an elderly, sun-damaged patient.
3:34 Main Differential Diagnosis (SLAM): The differential is summarized by the acronym SLAM: Spindle cell SCC, Leiomyosarcoma, Angiosarcoma/AFX, and Melanoma. Leiomyosarcoma (LMS) is often discounted as cutaneous LMS usually appears less ugly and more fascicular than this pleomorphic presentation.
4:32 Defining AFX (Atypical Fibroxanthoma): AFX is an ugly, spindled lesion confined to the dermis and is essentially a diagnosis of exclusion. It has a surprisingly good prognosis despite its high-grade microscopic appearance.
4:52 Required IHC for Exclusion: To confidently diagnose AFX, one must exclude other entities using IHC: S100/Sox10 (Melanoma), P40/CK5 (SCC), Desmin (Rhabdomyosarcoma), and ERG/CD31 (Angiosarcoma).
5:55 AFX vs. PDS: When the tumor is small and confined to the dermis, it is called AFX. When it invades the subcutis, is larger (β₯2cm), or shows necrosis/perineural invasion, it is termed Pleomorphic Dermal Sarcoma (PDS). They are considered the same tumor, with PDS reflecting features associated with higher aggressive potential.
7:07 CD10 Utility: While AFX often stains positive for CD10, the marker is considered non-specific and generally unhelpful because it stains a wide variety of spindle cell neoplasms (SCC, melanoma, angiosarcoma, etc.).
8:04 Takeaway on Sarcomas: Rhabdomyosarcoma is extremely rare but can occur in the skin, even in older adults, arising from primitive stem cells rather than existing skeletal muscle. Desmin must be used to rule this out due to its highly aggressive nature.