This pathology video analyzes a challenging skin nodule case from a 15-year-old male, ultimately diagnosed as a Diffuse Neurofibroma. The lesion presents a diagnostic pitfall, exhibiting histological features highly suggestive of Dermatofibrosarcoma Protuberans (DFSP), including bland spindle cell proliferation and extensive "honeycomb" fat entrapment in the deep dermis and subcutis. The crucial differentiator is the immunohistochemical staining profile: while both lesions can express CD34, the presence of S100 positivity confirms the lesion’s Schwann cell component, ruling out DFSP. Furthermore, the case demonstrates features consistent with Neurofibromatosis Type 1 (NF1), including adjacent expanded nerve trunks (focal plexiform neurofibroma). The video also explores two key variants of diffuse neurofibroma: the presence of characteristic Wagner-Meissner bodies (resembling Meissner’s corpuscles) and an uncommon round-cell morphology.
Diffuse Neurofibroma: Differentiating from DFSP in a Challenging Case
0:00 Case Introduction: Presentation of Case 10: a skin nodule on the leg of a 15-year-old male.
0:23 Histological Appearance: The proliferation is composed of bland, pale spindle cells extending deeply into the subcutaneous fat, entrapping eccrine coils and creating a distinctive "honeycomb" pattern in the fat.
1:23 Diagnostic Challenge (DFSP Mimicry): The bland spindle cells and honeycomb fat entrapment strongly mimic Dermatofibrosarcoma Protuberans (DFSP). Furthermore, neurofibromas often express CD34 (due to a background dendritic cell component), a marker typically associated with DFSP.
1:37 Key Differentiator (S100): The crucial finding is that the lesion is S100 positive (or SOX10 positive), confirming a Schwann cell component and establishing the diagnosis as Diffuse Neurofibroma. DFSP is almost never S100 positive.
2:01 Plexiform Features & NF1: The specimen contained elongated, expanded nerve trunks filled with neurofibroma, suggesting a tiny bit of plexiform neurofibroma mingled with the diffuse type. The patient was confirmed to have Neurofibromatosis Type 1 (NF1).
3:44 Takeaway: Diffuse neurofibromas can be extensively infiltrative, involving fat, eccrine coils, vessels, and even deep structures like muscle. They are benign but can be disfiguring.
4:20 Wagner-Meissner Bodies: The video introduces a separate case to illustrate Wagner-Meissner bodies—structures (often present in diffuse NF) composed of swirling, oval/spindle cells wrapped around pink, stacked layers of collagen. These bodies closely recapitulate Meissner’s corpuscles (fine touch receptors).
6:26 Round Cell Component: Diffuse neurofibromas can sometimes exhibit a prominent, uncommon round-cell component, potentially leading to confusion with round blue cell tumors, though the cells remain bland with rare mitotic figures.
7:15 Background Features: Neurofibromas often contain mast cells, which are attracted by the myxoid background substance (hyaluronic acid) found between the Schwann cells and collagen.
8:28 Conclusion: When considering DFSP due to the honeycomb fat pattern and CD34 positivity, always add an S100 stain, as its positivity essentially excludes DFSP and diagnoses neurofibroma.