Vincenzo De Giorgi1, Marta Grazzini1, Imma Savarese1, Susanna Rossari1, Alessia Gori1, Daniela Massi2 and Torello Lotti1

Departments of 1Dermatology and 2Human Pathology and Oncology, University of Florence, Via della Pergola 60, IT-50121 Firenze, Italy. E-mail: vincenzo.degiorgi@unifi.it

Accepted July 7, 2010.

A 70-year-old Caucasian woman attended our pigmented lesions clinic for routine treatment. On examination, numerous seborrhoeic keratoses were found on her trunk and limbs. She reported that a lesion in the centre of her abdomen had increased in size over the previous year. Clinical examination revealed a symmetric lesion comprising two well distinct areas of pigmentation, one light brown and the other dark brown, and exhibiting a typical exophytic verrucous appearance (Fig. 1). Dermoscopic evaluation of the whole lesion revealed the presence of pseudo-horn cysts, comedo-like openings and fissures. Moreover, a pigmented network, as well as globules and pseudopods, were found in one part of the lesion (Fig. 2).

Fig. 1. Macroscopic image of a malignant melanoma arising from a seborrhoeic keratosis.

Fig. 2. Dermoscopic image of the same lesion shown in Fig. 1 demonstrating pseudo-horn cysts, comedo-like openings and fissures. One part of the lesion also displayed a pigmented network, globules and pseudopods.

A tentative diagnosis of collision tumour involving a seborrhoeic keratosis and atypical melanocytic lesion was made, and we decided to surgically remove the entire lesion. Histopathological examination of the excised tissue revealed a hyperkeratotic (papillomatous) seborrhoeic keratosis converging with an exophytic, verrucous lesion (displaying melanocytic activity) at the dermoepidermal junction and superficial dermis. The hyperkeratotic seborrhoeic keratosis displayed papillomatosis, acanthosis and the proliferation of basaloid cells with a varying admixture of squamoid cells, keratin-filled invaginations and horn cysts. Similar to conventional seborrhoeic keratosis, there were, at the dermoepidermal junction in the area of skin affected by seborrhoeic keratosis-like changes, nests of atypical melanocytes, some of which formed pagetoid spread in the epidermal fronds. The dermis, meanwhile, contained tightly-packed aggregates of small darkly-staining cells resembling naevus cells. These cells displayed hyperchromatic, angulated nuclei, scant cytoplasm and occasionally prominent nucleoli. The final diagnosis was hyperkeratotic seborrhoeic keratosis in collision with verrucous and keratotic nevoid melanoma (Fig. 3). The melanoma was a Clark level II tumour in the vertical growth phase. It had a Breslow thickness of 0.8 mm.

Fig. 3. Hyperkeratotic seborrhoeic in collision with nevoid melanoma. Melanoma cells are associated with scattered meanophages and signs of fibrosis in the superficial dermis (inset).

The development of malignant melanoma contiguous with or adjacent to seborrhoeic keratoses is uncommon but has been documented previously (1–5).

A retrospective analysis of 618 cases of malignant melanoma (6) identified 10 lesions with marked verrucous and keratotic patterns that were initially clinically diagnosed as benign lesions.

In our case, the melanocytic lesion had an architecture characterized by verrucous/exophytic features and the presence of marked hyperkeratosis.

At low magnification, the melanocytic proliferation suggested a naevus, but careful scrutiny revealed impaired maturation, expansile nests and occasional pleomorphic nuclei, more suggestive of verrucous and keratotic nevoid melanoma. We were not able to find any healthy skin between the conventional hyperkeratotic seborrhoeic keratosis and the verrucous and keratotic nevoid melanoma, although a sampling bias cannot be excluded. The two lesions both displayed prominent hyperkeratosis, papillomatosis and a general exophytic verrucous appearance, and it is thus conceivable that the melanocytic proliferation developed in the context of a pre-existing seborrhoeic keratosis.

Many physicians remove typical-looking seborrhoeic keratoses without the use of dermoscopy and without confirming the diagnosis through histopathological analysis. This habit should be abandoned. At the very least, every seborrhoeic keratosis a doctor intends to excise should be subjected to dermoscopic examination in order to avoid missing other colocalized malignant lesions, such as melanomas (1–5).

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