Collision of pigmented benign tumours: a possible simulator of melanoma.

Sir, 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 contiguously with a melanocytic naevus.