Collision of Pigmented Benign Tumours: A Possible Simulator of Melanoma

Abstract

The coexistence of more than one neoplasm in a single cutaneous specimen is relatively uncommon and has been defined as a collision or compound tumour (1, 2). Although some of these collision tumours may arise from the involvement of related cell types, most occur by chance (1). Melanocytic naevus has been reported as occurring associated with several different tumour types (1, 3). The association of a melanocytic naevus arising contiguously with or adjacent to seborrhoeic keratosis is, however, uncommon (1, 4–7). We report here a patient who presented with a pigmented macule that she had had for a long time, and which had recently increased in size and darkened. Histopathological examination showed a pigmented seborrhoeic keratosis that developed contigu-ously with a melanocytic naevus.CASE REPORTA 39-year-old woman had had a brown, asymptomatic cutaneous lesion on her back since childhood. She re-ported that the lesion had increased in size and became darker in the last year. Clinical examination disclosed an asymmetric, pigmented tumour measuring 13 mm in diameter on the right scapular region. The lesion exhibited 2 well-differentiated areas of pigmentation: one part was light brown in colour, 4 mm in diameter and the other was black, 9 mm in diameter, with fol-licular prominence (Fig. 1a). Dermoscopic evaluation of the whole lesion revealed a sharp demarcation, with pseudo-horn cysts, comedo-like openings and fissures in the bigger area. The other part of the tumour showed the presence of a pigmented network with small hy-popigmented areas. Clinical and dermoscopic findings suggested a collision tumour: seborrhoeic keratosis and melanocytic naevus. The lesion was surgically excised. Histopathological examination showed a symmetrical intradermal melanocytic proliferation composed of nests of melanocytes without atypia, consistent with a melanocytic intradermal naevus. Adjacent to the mela-nocytic naevus we observed a proliferation of pigmen-ted small basaloid cells with uniform appearance with hyperkeratosis, acanthosis and pseudohorn cysts (Fig. 1b). The latter features were consistent with pigmented seborrhoeic keratosis.DISCUSSIONThe combination of a melanocytic naevus with other tumours of epidermal or adnexal origin has often been described (1, 3). Common epidermoid cyst and melano-cytic nevus is the association reported most frequently (8). Melanocytic naevus has also been associated with a trichilemmal cyst(9), steatocystoma, hidrocystoma and dermoid cyst(8), syringoma(10), trichoepithelioma (3), trichoadenoma (11) and basal cell carcinoma (1). Despite the fact that melanocytic lesions and seborr-hoeic keratosis are among the skin lesions from which biopsies are more commonly taken, the association of a melanocytic naevus with a seborrhoeic keratosis is uncommon. Boyd & Rapini (1), in a retrospective study of 40,000 cutaneous biopsies, found 14 cases of melanocytic naevus juxtaposed with a seborrhoeic keratosis. Only another 4 cases of melanocytic naevus