This video presents a detailed pathology case study of a pure desmoplastic melanoma (DM) in an 80-year-old man, highlighting the tumor's highly deceptive and treacherous nature. DM often mimics benign entities like scar tissue or neurofibroma, particularly in older, sun-damaged patients on the head and neck. Microscopically, DM is characterized by spindle cells, scattered hyperchromatic pleomorphism, and prominent desmoplastic stroma, with the tumor often exhibiting deep infiltration down to the fascia/periosteum, sometimes creating an "hourglass" shape. Key diagnostic clues include the presence of lymphocyte aggregates and the necessary use of immunohistochemistry (IHC). DM stains reliably only with S100 and SOX10, typically being negative for standard melanoma markers like Mart-1 and HMB45. The video concludes by discussing the prognosis of pure DM, which tends to be more locally aggressive but is associated with a better overall survival rate and lower risk of lymph node metastasis compared to other thick melanomas.
Desmoplastic Melanoma: A Treacherous Subtype
0:13 Case Presentation: The case involves an 80-year-old man with a slowly growing, firm, white, scar-like plaque on the scalp.
1:07 Diagnosis and Mimicry: The diagnosis is Desmoplastic Melanoma (DM). DM is treacherous because it does not resemble typical melanoma and often mimics benign spindle cell tumors, scars, or neurofibromas.
1:31 Context Clues: A large excision specimen (down to the fascia/periosteum) suggests the lesion was previously recognized as a serious cancer, providing a critical context clue often useful for diagnosis.
2:48 Invasion Pattern: DM exhibits a characteristic "hourglass" appearance, arising in the dermis, obliterating the subcutis, and pushing aggressively down to the fascia or periosteum.
3:28 Key Histologic Clues: DM usually occurs on the head and neck of older, sun-damaged people. The presence of lymphocyte aggregates ("smart bombs") alongside a scar-like or neural-looking spindle cell lesion is a major indicator of DM.
4:01 Cellular Morphology: The tumor consists of spindled cells (fibroblast-like or Schwann cell-like) with scattered hyperchromatic pleomorphism and mitosis, separated by large amounts of collagen and myxoid stroma, giving a pale pinkish-blue background.
5:12 Neurotropism: DM has a strong propensity for perineural invasion, sometimes to the extent of being termed neurotropic DM.
6:15 Immunohistochemistry (IHC): DM is reliably stained by S100 and SOX10. It is almost always negative for standard melanocytic markers (HMB45, Mart-1), which distinguishes it from most other melanomas.
7:37 Melanoma in Situ (MIS): Only about half of DM cases have an MIS component overlying the dermal tumor; the absence of MIS makes microscopic diagnosis significantly more challenging.
8:31 Clinical Significance: DM often looks clinically benign (scar-like) and is microscopically bland, resulting in delayed diagnosis. DM lesions are frequently very thick (10-15 mm) at the time of initial diagnosis.
9:20 Prognosis of Pure DM: Pure DM (β₯90% desmoplastic component) is associated with a better overall survival for the same tumor depth compared to other melanomas. It tends to be more aggressive locally (e.g., invading skull/dura) but has a much lower chance of metastasizing to lymph nodes or distant sites.
12:00 IHC Confirmation: Example staining shows S100 positivity throughout the lesion (both in situ and deep components) and Mart-1 negativity in the deep desmoplastic tumor, confirming the characteristic IHC pattern.