This pathology video presents Case 6, a dermal nodule from the shoulder of a 25-year-old female. Histologically, the lesion displays a highly nested or micro-nodular pattern separated by thick, fibrous septations, contained entirely within the dermis and subcutis. Cellular analysis reveals oval to round nuclei with a distinctive pale, fine, "washed-out" chromatin. Due to its melanocytoid/histiocytoid appearance, the differential includes dermal Spitz nevus; however, the tumor is confirmed as Cellular Neurothecioma (CN) based on its characteristic architecture and necessary exclusion of melanocytic differentiation (negative for Sox10, S100, and Melan-A). The video emphasizes that CN is a benign, indolent tumor, even when displaying significant nuclear atypia or mitotic figures. Despite its name, Cellular Neurothecioma is considered a fibrohistiocytic tumor, not a true neural sheath tumor.
Differentiating Cellular Neurothecioma from Melanocytic Lesions
0:08 Case Introduction: A dermal nodule on the shoulder of a 25-year-old female, excised after a prior biopsy.
0:24 Nested/Micro-Nodular Pattern: The lesion exhibits a striking pattern of aggregates or micro-nodules separated by thick, fibrous septations. It is highly circumscribed and confined to the dermal and subcutaneous tissue, sparing the epidermis.
1:22 Distinctive Cellular Morphology: The cells have oval to round nuclei with very pale, fine, delicate, or "washed-out" chromatin. This morphology can resemble histiocytes or melanocytic lesions, such as dermal Spitz nevus.
2:04 Final Diagnosis: Cellular Neurothecioma: The case is identified as a definitive example of Cellular Neurothecioma.
2:50 Key Diagnostic Exclusion: Differentiation is achieved via immunohistochemistry (IHC): Cellular Neurothecioma is negative for standard melanocytic markers (Sox10, S100, Melan-A).
3:00 Benign Behavior: A crucial takeaway is that CN is an indolent and benign lesion. It may exhibit features like prominent nuclear atypia or mitosis, which would be concerning for melanoma in a melanocytic tumor, but these findings do not indicate malignancy in CN.
3:35 Ancillary Histological Features: Other features often associated with CN include myxoid change (prominent in some cases, though not this one) and multi-nucleated giant cells.
4:43 IHC Buzzwords (Testing Purposes): Although non-specific for routine diagnosis, key markers mentioned for testing knowledge of CN include S100A6, PGP 9.5, and NKIC3.
5:04 Takeaway on Lineage: Despite the historical name "neurothecioma" (which led to confusion with a true nerve sheath tumor called nerve sheath myxoma), CN is currently believed to be a non-neural, potentially histiocytic or fibrohistiocytic tumor.