Cutaneous collision tumour (melanocytic naevus, basal cell carcinoma, seborrhoeic keratosis): a clinical, dermoscopic and pathological case report.

The association of contiguous or 'collision' tumours in the same biopsy specimen is not uncommon and is often reported in the literature. The most common association, basal cell carcinoma (BCC) and naevus, is very difficult to diagnose clinically. We describe a 38-year-old woman with a previous history of melanoma, who presented with a modified pigmented lesion of the hip that had begun to change 6 months earlier. Histologically, the lesion was a melanocytic compound naevus and a BCC with a seborrhoeic keratosis. The case was investigated clinically and by focusing on the dermoscopic features and their pathological correlates. Cutaneous collision tumours are extremely difficult to diagnose preoperatively, even with the help of dermoscopy, in particular when one of the lesions is melanocytic.