A 63-year-old male was referred for evaluation and treatment of a lesion that arose on his left temple. The growth was first noted several months ago. It was asymptomatic until recently; the edge occasionally bleeds. He has experienced ample past sun exposure and smokes cigarettes. He denies prior history of skin cancer. Examination reveals a 1.4-cm hyperkeratotic plaque with an erythematous, slightly friable medial margin.
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A biopsy of the lesion revealed a squamous cell carcinoma in situ arising within a seborrheic keratosis. This so-called collision tumor represents a combination of 2 different skin tumors occurring adjacent to one another or within a single lesion. Some lesions appear to arise fortuitously, whereas others may have been derived from similar cell lineages.1 The most commonly reported neoplastic collision tumors involve basal cell carcinomas, which may arise in conjunction with a seborrheic keratosis, nevus, or neurofibroma.2 In one study, seborrheic keratoses were the most common benign neoplasms found in collision tumors, followed by nevi.3 Dermoscopy and confocal microscopy are noninvasive procedures that may aid in the diagnosis of these lesions.4,5
Stephen Schleicher, MD, is an associate professor of medicine at the Commonwealth Medical College in Scranton, Pennsylvania, and an adjunct assistant professor of dermatology at the Perelman School of Medicine at the University of Pennsylvania in Philadelphia. He practices dermatology in Hazleton, Pennsylvania.
- 1. Burkhalter A, White WL. Malignant melanoma in situ colonizing basal cell carcinoma. A simulator of invasive melanoma. Am J Dermatopathol. 1997;19(3):303-307.
- 2. Boyd AS, Rapini RP. Cutaneous collision tumors. An analysis of 69 cases and review of the literature. Am J Dermatopathol. 1994;16(3):253-257.
- 3. Moscarella E, Rabinovitz H, Oliviero MC, et al. The role of reflectance confocal microscopy as an aid in the diagnosis of collision tumors. Dermatology. 2013;227(2):109-117.
- 4. Menezes N, Rita G, Inês L, Paulo V, Armando B. Letter: Collision tumor: importance of the new auxiliary tools for diagnosis (an illustrative case report). Dermatol Online J. 2011;17(7):12.
- 5. Zaballos P, Llambrich A, Puig S, Malvehy J. Dermoscopy is useful for the recognition of benign-malignant compound tumours. 6. Br J Dermatol. 2005;153(3):653-656.