A 67-year-old man of Chinese descent presented with a painless nodular lesion that had been present on his right forearm for the previous 3 months. A single, well-defined, dome-shaped, firm nodule with a central keratin plug surrounded by erythema was noted. Keratoacanthoma with secondary bacterial infection was suspected and the patient underwent an excision biopsy. Biopsy of the nodule and immunohistochemical staining supported a diagnosis of nodular malignant melanoma. It should be noted both that nodular malignant melanoma may present with a wide variety of clinical appearances, and that the lack of melanin pigment in nodular malignant melanoma may hinder the diagnosis of this aggressive tumour.

A 67-year-old man of Chinese descent presented with a painless nodular lesion on the extensor surface of the right forearm. The lesion had grown from a pinpoint size to a lesion of 7 mm × 3 mm × 5 mm over the preceding 3 months. There was no history of trauma, discharge, pruritus, or bleeding from the lesion. Examination revealed a single dome-shaped, pinkish nodule with a central keratin plug surrounded by erythema [Figure 1]. An initial diagnosis of keratoacanthoma secondary to bacterial infection was made. The patient was prescribed a course of antibiotics prior to an excision biopsy. However, a week later, during the biopsy, the surrounding erythema was noted to be persistent. Macroscopic examination of the biopsy specimen consisted of a piece of skin with a raised nodular lesion. The skin measured 20 × 10 × 5 mm and the nodule measured 10 × 10 × 10 mm. Histopathology of the lesion showed a polypoid intradermal tumour with ulceration of the overlying epidermis composed of nests of epithelioid tumour cells which extended down to the mid-dermis [Figures 2a and b]. The tumour cells had round to oval nuclei, prominent nucleoli, and abundant eosinophilic cytoplasm with some clearing and a distinct cell border. Some cells were large with pleomorphic nuclei and conspicuous nucleoli. Both typical and atypical mitotic figures were observed. Dense lymphoplasmacytic infiltrates were seen in the deep dermis; however, no lymphovascular invasion was noted. Immunohistochemical (IHC) staining for tumour cells showed a strong and diffuse positivity for S-100 [Figure 2c] and a few cells were positive for melan-A [Figure 2d]. The melan-A test, repeated along with HMB45, revealed similar findings. The final diagnosis, based on the histopathology testing and the IHC stains was malignant melanoma, nodular type. The patient was subjected to a second excision biopsy to achieve a clear margin. Histopathological examination of the margin representing the erythematous ring surrounding the lesion showed the presence of lymphocytes and plasma cells, suggesting an inflammatory reaction. A positron emission tomography (PET) scan was negative for any metastasis.

Nodular melanoma may present with a multitude of clinical presentations. Some may be atypical and totally unexpected. Physicians should be aware of these various presentations, especially when attending to patients with skin lesions over sun-exposed areas. They should maintain a high index of suspicion for this condition, especially in managing hypo- or non-pigmented nodular lesions. Nodular lesions with surrounding erythema should be treated with more caution as this may provide a clue to an underlying malignancy.