Abstract
Pleomorphic dermal sarcoma (PDS) and atypical fibroxanthoma (AFX) are rare mesenchymal tumors that share similar clinical, histological, and immunohistochemical characteristics. Careful histopathological examination of a biopsy specimen that includes subcutaneous fat remains the preferred way to differentiate between these tumors. AFX is limited to dermal invasion, whereas PDS demonstrates deeper invasion. Moreover, PDS may present with tumor necrosis and high-grade histological findings, such as lymphovascular and perineural invasion, features absent in AFX. However, like PDS, AFX is a diagnosis of exclusion, and an exhaustive immunohistochemistry panel is recommended to distinguish these tumors from other spindled cell tumors in the differential diagnosis. The authors present the case of an 86-year-old man with biopsy-suspected AFX who was referred for Mohs micrographic surgery for tumor excision. During Mohs, the tumor was observed to have invaded deeply into the subcutaneous tissue and galeal aponeurosis, aligning more closely with a PDS. The diagnosis of PDS was confirmed using en face processing during Mohs surgery, which captured the intravascular involvement of a solitary vessel. Differentiating between PDS and AFX is important because PDS is a more aggressive tumor, with a higher rate of local recurrence and metastasis, and requires closer monitoring.