This pathology case presentation details the microscopic analysis of a hypercellular, multinodular spindle cell proliferation on the foot of an elderly man. Initially, the diagnosis was challenging as the lesion lacked distinguishing features like significant pleomorphism or expression of common markers (S100, Keratin), leading to a broad differential diagnosis including sarcoma and melanoma.
The key learning point is highlighted by comparing the case with a second, similar lesion that displayed the classic slit-like spaces filled with extravasated erythrocytes, confirming the entity as Kaposi Sarcoma (KS). The first case was unusual because it presented with minimal hemorrhage, underscoring that not all vascular tumors exhibit abundant blood. The video emphasizes the necessity of using vascular markers (ERG, CD31, CD34) in difficult spindle cell tumors, and definitively confirms the diagnosis using the highly specific HHV8 immunostain.
Pathology Case Study: Nodular Kaposi Sarcoma Lacking Hemorrhage
0:08 Initial Case Presentation: An elderly adult man presented with a nodule on the foot. Histologically, it showed a multinodular, hypercellular proliferation of spindled to plump spindle/epithelioid cells filling the dermis, arranged in vague streaming fascicles.
0:52 Morphology and Differential Diagnosis: The cells were atypical, hyperchromatic, and enlarged, with numerous mitotic figures. Standard immunohistochemical stains (S100, Keratin, Desmin, Actin) were negative, ruling out common entities like melanoma or carcinoma. This left an unusual differential diagnosis, including atypical fibrous xanthoma or pleomorphic dermal sarcoma in this location.
2:12 Identifying the Key Feature (Case Comparison): A second, similar case of an acral nodule was introduced. This tumor exhibited a crucial feature missing in the first case: extravasated red blood cells (erythrocytes) filling slit-like spaces between the spindled tumor cells.
2:55 Final Diagnosis: The presence of the slit-like spaces with erythrocytes confirmed the entity as Kaposi Sarcoma (KS), specifically the nodular or tumoral stage. The initial challenging case (Case 1) was also determined to be Kaposi Sarcoma, despite the extreme paucity of blood/hemorrhage.
3:35 Key Lesson: Not all vascular tumors, including Kaposi Sarcoma, present with abundant extravasated red blood cells. The absence of blood should not automatically exclude vascular tumors from the differential.
4:02 Diagnostic Strategy for Spindle Cell Tumors: When a spindle cell neoplasm is difficult to classify and the standard panel is negative, dermatopathologists should always perform a vascular marker (CD31, CD34, or ERG).
4:30 Preferred Markers and Caveats:ERG is preferred due to its clean and strong signal, though it is not specific (can be positive in prostate carcinoma, epithelioid sarcoma, etc.). Caution must be used with CD34 (often lost in solid angiosarcoma) and CD31 (can be downregulated by the HHV8 virus in Kaposi Sarcoma).
5:05 Definitive KS Diagnosis: The most specific and reliable immunostain for Kaposi Sarcoma is HHV8 (Human Herpes Virus 8), the virus responsible for driving KS.
5:30 Clinical Context: This case was a "classic type" Kaposi Sarcoma (not associated with HIV/AIDS), typically seen in elderly adults. It is thought to be linked to reduced immune function in old age and often presents on the feet or lower legs, sometimes alongside lymphedema changes.