This video presents a detailed microscopic analysis of an exceptionally large, deep, and cellular dermatofibroma (DF), likely representing the aneurysmal/hemosiderotic variant. The case highlights key histopathologic features necessary to confidently diagnose DF and distinguish it from the clinically concerning dermatofibrosarcoma protuberans (DFSP), particularly when the DF exhibits atypical features such as high cellularity, pleomorphism, mitotic activity, and deep infiltration into the subcutis. The diagnostic criteria focus on classic DF features (epidermal hyperplasia, collagen trapping) and ancillary findings (abundant hemosiderin, hemorrhage, foamy histiocytes, and giant cells—all rare in DFSP). Immunohistochemical stains are reviewed, confirming hemosiderin (Prussian Blue) and showing the tumor cells are negative for CD34 (unlike DFSP), while cautioning against pitfalls in interpreting CD34 and S100 stains due to prominent background dendritic cell staining.
Histopathology of a Large Aneurysmal Dermatofibroma
0:04 Case Introduction: The specimen is an unusually large dermatofibroma (DF) on the leg of a young woman, excised due to clinical concern over the mass size. The lesion extends deep into the subcutis.
0:35 Classic DF Features (Superficial): Exhibits typical epidermal hyperplasia (induction) and elongation of the rete ridges over the tumor, particularly at the periphery.
1:32 Collagen Trapping: A highly useful diagnostic feature showing spindled and stellate tumor cells intercalating between and wrapping around large bundles of reticular dermal collagen. This pattern helps differentiate it from DFSP.
2:48 Prominent Vascularity: The tumor displays dilated, branching "staghorn" vessels. Takeaway: This vascular pattern is not specific to solitary fibrous tumors (formerly hemangiopericytoma) and is commonly seen in large, deep DFs and certain DFSP variants.
3:41 Cytology and Mitosis: The tumor is highly cellular with plump, oval-to-round cells displaying hyperchromatic and somewhat pleomorphic nuclei. Takeaway: This pleomorphism and the presence of mitotic figures (not specifically shown but noted elsewhere in the slide) are acceptable in large, deep DFs (cellular or aneurysmal types) and should not automatically suggest malignancy.
5:04 Recurrence Risk: Deep and large DFs, if only superficially biopsied (e.g., shave biopsy), often appear to "recur," but this usually represents continued growth of the large remaining deep tumor, rather than true local aggressiveness.
5:46 Hemosiderin and Histiocytes (Aneurysmal Features): Abundant hemosiderin deposition, hemorrhage, foamy histiocytes, and Touton-type histiocytic giant cells are present. Takeaway: These findings are strong clues for DF and are extremely rare in DFSP.
9:28 Deep Subcutis Infiltration: The tumor margin shows "trickling" growth, sending small tendrils or "little feet" down into the subcutaneous fat. Takeaway: While fat infiltration is a buzzword for DFSP, the DF pattern is usually trickling/splaying, contrasting with the deeply infiltrative, "honeycomb" pattern typical of DFSP.
12:16 Special Stain - Prussian Blue: The stain beautifully highlights the extensive hemosiderin in the tumor as a bright blue color, confirming the diagnosis of a hemosiderotic DF.
12:48 Special Stain - CD34: The tumor cells are completely negative for CD34. Takeaway: This is a critical finding, as DFSP is typically CD34 positive. Caution is needed, as CD34 strongly stains the normal background dermis and vessels, potentially forming a confusing "halo" effect around the tumor edge due to clustered dendritic cells.
16:13 Special Stain - S100: S100 shows negative staining in tumor cells but highlights abundant clusters of S100-positive dendritic cells, particularly common in inflamed/hemosiderotic DFs. Takeaway: S100 is generally not recommended for DF vs. DFSP differentiation due to this background staining pitfall, especially if trying to rule out a melanocytic neoplasm; Sox10 might be a superior alternative.
19:35 Conclusion: The vast majority of DFs and DFSPs can be reliably separated using H&E morphology alone, focusing on the peripheral and deep features, and utilizing IHC sparingly.