Eccrine porocarcinoma arising in a seborrheic keratosis evaluated with dermoscopy and treated with Mohs' technique

A 78‐year‐old white woman returned for a routine 6‐month skin cancer examination. She had a history of actinic keratosis and multiple basal cell carcinomas. She had no personal or family history of dysplastic nevi or melanoma. The patient was asymptomatic and unaware of any new or changing skin lesions. The patient had multiple lentigines, hemangiomas, and actinic and seborrheic keratoses on all sun‐exposed areas. There were no less than 10 seborrheic keratoses on the right mid‐back, and one was found to have a 1‐cm, reddish nodule asymmetrically located within it (Figs 1 and 2). A clear papule on the left preauricular area was found on biopsy to be a basal cell carcinoma. The nodule on the back was still present 1 month later and it was felt that further evaluation was indicated. As melanoma has been reported to develop in seborrheic keratoses, we decided to examine the lesion using digital dermoscopy. 1 The right mid‐back with multiple seborrheic keratoses one of which contains a reddish nodule within it 2 Close‐up gross image of the eccrine porocarcinoma within a seborrheic keratosis With digital dermoscopy, a well‐demarcated reddish nodule was asymmetrically located within a brown lesion. It blanched significantly with pressure. Within the nodule, there were dotted and irregular linear vessels (atypical vascular pattern; also known as polymorphous vascular pattern) and regular‐appearing brown dots. Surrounding the reddish nodule, there were pale and pigmented, comedo‐like openings, fissures, and ridges (brain‐like appearance). Some of the follicular openings appeared to be within the wall of the nodule (Figs 3 and 4). Comedo‐like openings, fissures, and ridges are primary dermoscopic criteria for the diagnosis of a seborrheic keratosis; however, the vascular pattern seen has not been reported in seborrheic keratosis. Due to the patient's age and the rarity of significant pathology arising in a seborrheic keratosis, a shave biopsy was performed. 3 A digital dermoscopic image demonstrating follicular openings, ridges, and furrows, primary criteria diagnostic of a seborrheic keratosis. Within the wall of the reddish nodule, there are dotted and linear irregular small blood vessels (atypical vascular pattern) 4 Firm pressure blanches the nodule making it easier to see the atypical vascular pattern To our surprise, the specimen was interpreted by an experienced dermatopathologist as a well‐differentiated eccrine porocarcinoma. Due to the high local recurren