Background: Desmoplastic melanoma (DMM) is an uncommon melanoma variant with a distinct morphology, including a prominent spindle cell component with fibrosis, as well as a distinct immunohistochemical profile. Histologically, the spindle cell component of DMM can be confused with sclerotic/desmoplastic nevi, nonpigmented blue nevi, scar, and neural tumors. The histological distinction between sclerotic/desmoplastic/blue nevi and DMM using standard light microscopic techniques can be exceedingly subtle. Therefore, we investigated whether immunohistochemical staining for Melan-A and Ki-67 expression can be used to discriminate these lesions, distinguishing between epithelioid and spindle cell compartments of the lesions.
Design: Fifty cases of DMM and 13 cases of sclerotic/desmoplastic/blue nevi were identified. Standard immunohistochemical techniques were used with antibodies towards HMB-45, Melan-A (A103), and Ki-67; 43 of 50 DMM cases were available for staining with Melan-A, 42 of 50 for HMB-45, and 31 of 50 cases were stained with Ki-67. All 13 nevi were stained for Melan-A and 8 for Ki-67. Immunoreactivity to Ki-67 antibody was scored as 0 to 5%, 6 to 10%, 11 to 30%, or greater than 30% positive tumor cells.
Results: Only 3 of 43 and 3 of 42 of spindle cell compartments of DMMs were positive for Melan-A and HMB-45, respectively. Focal staining of epithelioid cells in the junctional component or superficial dermis was observed in 33% (14/43). In contrast, 100% of the 13 nevi were strongly positive for Melan-A (P < 0.001). Seventeen melanomas (55%) were 0 to 5% positive for Ki-67, five (16%) fell into the 6 to 10% category, three (10%) were between 11 and 30%, and six (19%) were at least focally greater than 30% positive. All 8 nevi (100%) had less than 5% positive cells for Ki-67 (P = 0.02), with only 2 cases having more than 2% positive cells.
Conclusion: The sclerotic/desmoplastic and hypopigmented blue nevi were uniformly positive for Melan-A, while the vast majority of DMM were negative in their spindle cell compartments. Melan-A is very useful in distinguishing between DMM and sclerotic nevi. Ki-67 appears to be an inconsistent marker for DMM. However, a high labeling index (over 5%) may be used as a clue in diagnosing DMM.