This pathology case presentation details the diagnosis of an Angiofibroma of Soft Tissue, a benign subcutaneous mass found in the thigh of a middle-aged woman. Microscopically, the tumor is characterized by bland fibroblastic spindle cells set within a prominent mixoid (edematous) stroma, most strikingly defined by a "crazy amount" of prominent, branching, plexiform blood vessels. The presentation emphasizes the crucial distinction between neoplastic vascular tumors and tumors exhibiting dense background vascularity. Key differential diagnoses discussed include Myxoid Liposarcoma (ruled out by vessel thickness and location), Dermatofibrosarcoma Protuberans, and Solitary Fibrous Tumor (which can be ruled out by STAT6 or CD34 immunohistochemistry, depending on the overlap). The tumor is confirmed to be a distinct entity, despite having a confusing nomenclature that overlaps with several unrelated "angiofibromas," and is associated with an AHR::NCOA2 translocation.
Angiofibroma of Soft Tissue
0:17 Case Presentation: The slide shows a subcutaneous mass on the thigh of a middle-aged adult woman, appearing as a nodule in the dermis pushing into the subcutis.
0:55 Striking Vascularity: The most notable feature is an exceptionally high density of thin-to-medium sized, branching, plexiform blood vessels, giving the lesion a hemangiopericytoma-like pattern.
1:13 Key Distinction: The speaker stresses that despite the prominent vessels, this is not a vascular tumor. Pathologists must distinguish between neoplastic vessels (endothelial tumors) and vessels forming the background vascular network of the neoplastic spindle cells.
1:51 Cytology and Stroma: The tumor cells are bland, non-atypical, fibroblastic-appearing spindle cells. The background is mixoid or edematous, with variable amounts of collagen fibers. Mitotic activity is essentially none (2:07).
3:11 Diagnosis and Nomenclature: The tumor is diagnosed as Angiofibroma of Soft Tissue. The speaker acknowledges the frustrating and confusing nomenclature, as this name is shared by several totally unrelated entities (e.g., cutaneous, nasopharyngeal, and cellular angiofibromas).
3:56 Differential Diagnosis (DDx) - Myxoid Liposarcoma (MLS): MLS is the primary pitfall due to its mixoid stroma and branching vessels. Arguments against MLS include:
Location: MLS is typically a deep soft tissue tumor (deep musculature of the thigh).
Vessel Morphology (4:55): MLS vessels are far thinner and more delicate (like one endothelial cell thick), whereas the vessels here are thicker.
Vessel Density: MLS is generally less vascular than Angiofibroma of Soft Tissue.
6:42 Other DDx: Other considerations include Dermatofibrosarcoma Protuberans (DFSP) and Solitary Fibrous Tumor (SFT).
7:42 DDx - Solitary Fibrous Tumor (SFT): SFT can have overlapping features (variable cellularity, mixoid change, small branching vessels). If doubt exists, STAT6 can rule out SFT.
8:15 Immunohistochemistry (IHC): CD34 staining highlights the extensive background vascular network. In this case, the spindle cells were mostly CD34 negative, which argues against SFT.
8:48 Molecular Correlate: Angiofibroma of Soft Tissue is associated with a translocation involving AHR::NCOA2.
9:45 Related Entities (Cellular Angiofibroma): The speaker notes that Cellular Angiofibroma of the genitals (unrelated to this specific tumor) is important to know because it shares molecular findings (RB1 loss) with Spindle Cell/Pleomorphic Lipoma, Mammary Myofibroblastoma, and Superficial Acral Fibromyxoma.