Seborrhoeic keratosis and malignancy: collision tumour or malignant transformation?

A retrospective study of 813 histological specimens reported as seborrhoeic keratoses included 43 (5.3%) associated with non-melanoma skin cancer. Intraepidermal carcinoma (squamous cell carcinoma in situ) was the most common of these (36). There were five basal cell carcinomas (one with intraepidermal carcinoma also) and two invasive squamous cell carcinomas. No melanomas were reported. Twenty-seven of the intraepidermal carcinomas appeared to arise within the seborrhoeic keratosis as did one of the invasive squamous cell carcinomas. Of these 28 lesions, the head was the most common site. Fourteen were clinically diagnosed as a non-melanoma skin cancer with only nine clinically felt to be a seborrhoeic keratosis. These lesions may represent malignant transformation within the seborrhoeic keratosis. Twelve specimens reported adjacent dual pathologies, with the trunk and limbs the most common sites. Seven were diagnosed clinically as a skin malignancy, whereas three were thought to be solar keratoses. Clinically, the remaining two were seborrhoeic keratoses. The origin of the malignancy in these cases is less obvious and may represent collision tumours. Three curette specimens could not be assessed for architecture.