Aberrant expression of HMB-45 in traumatized melanocytic nevi

Background: Assessment of histologic and immunohistochemical maturation (with HMB-45 and anti-Ki-67) may be helpful in differentiating benign melanocytic nevi (BMN) from malignant melanoma. Recently, we reported loss of maturation and aberrant immunohistochemical findings in melanocytic nevi after liquid nitrogen cryotherapy (Adeniran et al, J Am Acad Dermatol 2009;61:341-5). Herein we report a similar phenomenon identified in traumatized melanocytic nevi (TMN). Objective: We sought to evaluate the histologic and immunohistochemical findings in early and late stages of traumatized nevi. Methods: Twenty-four cases of TMN were retrieved from the pathology archives. These were then assessed by two pathologists (T.L. and A.H.D.) using HMB-45 and MIB-1 (for Ki-67) antibodies. Results: TMN showed some of the following findings: epidermal changes (parakeratosis, ulceration, serum crust, flattening of the epidermis) and dermal changes including fibrosis and the presence of melanophages. In some cases, there was architectural disorder of the overlying melanocytes, with crowding in the basal layer, but without significant pagetoid spread. Occasionally, the dermal scar contained larger, more epithelioid-appearing melanocytes than those beneath the scar. Fifty-four percent of TMN lacked obvious immunohistochemical maturation with HMB-45, since nevus cells within the scar or directly beneath it were strongly labeled. None of the TMN showed appreciable labeling for Ki-67. Limitations: The exact clinical duration between trauma and biopsy could not be determined. Conclusion: Loss of maturation with HMB-45 in TMN can be a diagnostic pitfall in challenging cases. Concurrent evaluation of MIB-1 expression, along with the characteristic histologic features of trauma, should allow the correct diagnosis to be reached.