A pink and purple plaque.

Clinical presentation

A 79-year-old man presented for evaluation of a slowly growing pink scaly plaque on his right knee. An eccentric violaceous papule was noted on its medial edge (Fig 1).

Fig 1

Clinical image of the lesion on the right knee.

Dermoscopic appearance

Dermoscopy showed a purple papule containing milky red areas, shiny white streaks, and irregular linear vessels beside a pink eroded plaque with occasional pin-point vessels, partially obscured by scales (Fig 2).

Fig 2

Dermoscopic image of the lesion on the right knee.

Histologic diagnosis

Histologic examination found a proliferation of atypical keratinocytes with markedly altered maturation throughout the full thickness of the epidermis (Fig 3). A separate dermal aggregate of atypical cells with nuclear pleomorphism, prominent nucleoli, and frequent mitotic activity was also present. Immunostaining with a pankeratin stain highlighted the dysplastic epidermis, whereas the dermal cells labelled with MART-1, supporting a diagnosis of a squamo-melanocytic collision tumor (amelanotic malignant melanoma, Breslow thickness 0.7 mm, with a contiguous plaque of Bowen disease).

Fig 3

A, Biopsy specimen shows atypical keratinocytes throughout the full thickness of the epidermis. B, A distinct dermal aggregate of atypical, mitotically active cells is also present. C, Immunostaining with pankeratin stains (AE1/AE3) highlights the epidermis. D, MART-1 stain highlights the dermal aggregate. (B, Hematoxylin-eosin stain; C, pankeratin stains; D, MART-1 stain; original magnifications: B, ×4; C, ×10; D, ×10.)

Collision tumor refers to the association of 2 or more different neoplasms within the same lesion. By virtue of their composition, they are diagnostically challenging. Dermoscopy can be a valuable tool to aid in diagnosis, allowing identification of different structures within the same lesion. Histopathologically, collision tumors show distinct neoplasms located adjacent to each other while maintaining separate boundaries. Simultaneous development of 2 tumors at the same site may be coincidental; however, it is postulated that development of 1 malignancy could promote development of another via a paracrine effect or that field cancerization with or without local immunosuppression plays a role. Management of collision tumors depends on the histologic features of each individual component. This patient died of metastatic melanoma.