This pathology case study focuses on the diagnosis of a pleomorphic spindle cell neoplasm identified in a shave biopsy from the scalp of an 80-year-old patient with sun-damaged skin. The tumor displays marked cellular pleomorphism and mitotic activity, but the overlying epidermis lacks dysplasia. An extensive immunohistochemistry (IHC) panel (SOX10, P40, CK5/6, Desmin, ERG) was uniformly negative, ruling out the major mimics: spindle cell melanoma, squamous cell carcinoma (SCC), rhabdomyosarcoma, and angiosarcoma. The final diagnosis rests between Atypical Fibroxanthoma (AFX) and Pleomorphic Dermal Sarcoma (PDS). Since AFX and PDS are indistinguishable histologically and immunophenotypically—differing only by aggressive features, primarily invasion into the subcutis—the pathologist emphasizes that a definitive diagnosis and prognosis require a complete excision specimen to assess the tumor's depth, necessitating a cautious initial diagnosis of "AFX versus PDS."
Summarizing the Pleomorphic Spindle Cell Neoplasm
0:00 Case Presentation: An 80-year-old man presents with a papule on the scalp. Initial shave biopsy utilized an IHC panel (SOX10, P40/P63, CK5/6, Desmin, ERG), all of which were negative.
1:13 Morphological Findings: The lesion is a purely dermal proliferation of ugly, highly pleomorphic spindle cells, some foamy, completely replacing the dermis of a sun-damaged specimen. Mitotic figures, including atypical forms, were present. The epidermis is atrophic with no in situ component (no SCC or melanoma in situ).
3:30 Differential Diagnosis (DDx): The primary differential includes Spindle cell SCC, Spindle cell melanoma, Atypical Fibroxanthoma (AFX), and Pleomorphic Dermal Sarcoma (PDS). Rare considerations include spindled Angiosarcoma and Rhabdomyosarcoma.
Melanoma: SOX10 is preferred over S100 because S100 can non-specifically stain background dendritic cells (Langerhans cells), which can lead to overcalling AFX as spindle cell melanoma.
SCC: P40/P63 is used alongside CK5/6 because some spindle SCCs lose keratin expression but retain P40/P63 expression.
Vascular/Muscle: Desmin screens for rare but aggressive cutaneous Rhabdomyosarcoma (4:37). ERG is a preferred, highly sensitive vascular screen for Angiosarcoma (8:01), though patchy ERG staining in AFX must be disregarded.
13:31 AFX vs. PDS: AFX and PDS are pathologically identical; the distinction is based on prognosis/behavior. AFX is small and confined to the dermis (excellent prognosis), while PDS invades the subcutis or shows other aggressive features (necrosis, size >2cm), behaving like a high-grade sarcoma.
14:45 Key Takeaway (Reporting Strategy): Due to the transected nature of the shave biopsy, the pathologist cannot assess depth or rule out PDS. The recommended top-line diagnosis is: "Pleomorphic spindle cell neoplasm, Atypical Fibroxanthoma versus Pleomorphic Dermal Sarcoma. See comment."
15:14 Management Recommendation: The comment must explain that complete surgical excision is mandatory to evaluate the depth of invasion (specifically checking for involvement of the subcutis) and secure negative margins. The entire debulk specimen (if Mohs surgery is performed) must be examined.
17:28 Staining Philosophy: The pathologist dismisses the utility of non-specific positive stains like CD10, arguing that a complete panel of negative stains ruling out all aggressive mimics (Melanoma, SCC, Angiosarc, Rhabdo) is sufficient to confirm the diagnosis of AFX/PDS based on classic morphology, as CD10 positivity does not change the required differential or management plan.