Clinical presentation {#sec1}
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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](#fig1){ref-type="fig"}).Fig 1Clinical image of the lesion on the right knee.

Dermoscopic appearance {#sec2}
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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](#fig2){ref-type="fig"}).Fig 2Dermoscopic image of the lesion on the right knee.

Histologic diagnosis {#sec3}
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Histologic examination found a proliferation of atypical keratinocytes with markedly altered maturation throughout the full thickness of the epidermis ([Fig 3](#fig3){ref-type="fig"}). 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).Key message*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.[@bib1] 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.[@bib2] Management of collision tumors depends on the histologic features of each individual component. This patient died of metastatic melanoma.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.)

Funding sources: None.

Conflicts of interest: None declared.
