A woman aged 51 years presented a macular lesion on her abdomen; she was uncertain how long the lesion had been present. Dermoscopy revealed a network pattern. In addition, there were several small round areas with comedo-like openings on dermoscopy (Fig. 1A).

Figure 1.

A Dermoscopy image. The black squares outline areas with comedo-like openings. B, Confocal microscopy image of 3.5×3.5mm. Ring pattern at the dermoepidermal junction (red arrow), polycyclic cords (yellow arrow), and bright annular structures with an onion-skin appearance (green arrow). C, A junctional melanocytic nevus in transition with a seborrheic keratosis, with its characteristic network pattern.

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Confocal microscopy showed a cobblestone pattern of the epidermis and a ring pattern at the dermoepidermal junction. Several areas with bright (hyperreflective) annular structures (with an onion skin appearance) and polycyclic cords were also visible (Fig.1B). Histology revealed a collision tumor between a junctional melanocytic nevus and a seborrheic keratosis (Fig. 1C).

This patient was a 49-year-old man with a history of superficial spreading melanoma excised in 2011. Follow-up using digital dermoscopy revealed growth of a pigmented lesion on his back. The lesion was a pigmented macule measuring 0.6cm in diameter. Dermoscopy revealed a homogeneous globular network pattern with asymmetric globules peripherally. An area of the lesion presented milia-like cysts and comedo-like openings (Fig. 2A).

Figure 2.

A Dermoscopy image. Homogeneous globular network with asymmetric globules peripherally (red arrow). Milia-like cysts and comedo-like openings (in square). B, Confocal microscopy image of 5.5×5mm. The yellow square outlines the area with the bright annular structures and the hyperreflective round structures with a smooth outline (green arrow); on the right, at higher power, the ring pattern is visible at the dermoepidermal junction, with an area of crest fusion (arrow). C, The histologic image shows the presence in the same area of a compound melanocytic nevus and a seborrheic keratosis.

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A honeycomb pattern was observed in the epidermis and ring-like structures at the dermoepidermal junction, with occasional areas of crest fusion. In addition, a ring of bright (hyperreflective) structures (comedo-like openings on dermoscopy) and hyperreflective round intraepithelial structures with a smooth outline (milia-like cysts on dermoscopy) were observed (Fig. 2B).

Histology revealed collision between a compound melanocytic nevus with distorted architecture but no atypia, and a seborrheic keratosis (Fig. 2C).

The term collision tumor is used to refer to the presence of 2 or more different tumors in a single lesion.1 Clinical diagnosis can be difficult (particularly when the collision is between a malignant and a benign tumor), and dermoscopy and confocal microscopy are very useful diagnostic tools. When the collision is between 2 benign tumors, very good concordance is achieved between dermoscopy, confocal microscopy, and histology.2 However, in the cases presented, although both lesions appeared benign on confocal microscopy, they were excised for histologic confirmation of the suspected diagnosis.

The association of seborrheic keratosis with melanocytic nevus is not uncommon. In a retrospective study published by Boyd and Rapini,1 40000 skin biopsies were analyzed and 69 collision tumors were found, 14 of which involved a nevus and a seborrheic keratosis.

In case of doubt, confocal microscopy, a noninvasive imaging technique, helps to improve the evaluation of melanocytic lesions, as it permits in vivo evaluation of microscopic structures.3 Moscarella et al.2 described 24 cases of collision tumors and reported an excellent correlation between the histologic images and those obtained on confocal microscopy.

The authors declare that they have no conflicts of interest.