Abstract This is a case report of malignant melanoma presenting with both clinical and histopathologic features of a seborrheic keratosis. Not a rare phenomenon, this report emphasizes the importance of biopsy to evaluate apparent seborrheic keratoses. We believe that this phenomenon is best considered a presentation of melanoma. Diminished routine histopathologic evaluation of apparent seborrheic keratoses may well increase the number of mistaken diagnoses in such cases.

Introduction Seborrheic keratosis is the most common benign skin lesion in the geriatric population and presents with a variety of clinical and histopathologic appearances (1,2). The resulting differential diagnosis is broadened further by the simultaneous occurrence of seborrheic keratosis and a second overlapping benign, in situ or malignant lesion (3-6). We present a case in which a melanoma occurred with clinical and histopathologic features resembling seborrheic keratosis and which was submitted as such by an experienced dermatologist. The possibility of melanoma should be considered in the differential diagnosis for such lesions. Case Report A seventy-five-year-old Caucasian woman was seen at a nearby community clinic for a raised lesion on her right thigh. She had noticed that the pale tan to light gray color of the lesion was changing. Examination by the referring experienced dermatologist showed a rough and coarsely granular, oval lesion with a slightly irregular border, measuring 1.7 x 0.9 cm. Clinically this lesion was thought to resemble an irritated seborrheic keratosis and was biopsied due to its size, a slight variability in color and a clinical history of color change. The biopsy requisition diagnosis read "Irritated Seborrheic Keratosis"." Histopathologic examination of a deep shave biopsy showed the low-power silhouette of a seborrheic keratosis (Figs. 1,2) including squamous papilloma-like features and delicate, laminated pseudocysts of horn (Fig. 3). The final diagnosis was malignant melanoma, superficial spreading type, Clark''s level III, Breslow 1.44 mm, probable vertical growth phase, simulating a seborrheic keratosis (Figs. 4, 5, 6, 7).

The histopathologic examination of a deep shave biopsy of the right thigh lesion showed a silhouette of a seborrheic keratosis (Fig. 1) including squamous papilloma-like features and delicate, laminated pseudocysts of horn(Fig. 2).

	FIGURE 5	FIGURE 6

The final diagnosis was malignant melanoma, superficial spreading type, Clark's level III, Breslow 1.44 mm, probable vertical growth phase, simulating a seborrheic keratosis (Figs. 3, 4, 5, 6). Clinical Course One peripheral margin of the specimen was frankly involved with melanoma and routine surgical re-excision was performed. Histopathologic examination of this re-excision specimen showed no residual melanoma. The patient is without metastasis or incomplete excision at follow up examination after more than one year. Discussion A seborrheic keratosis-like clinical presentation of melanoma is not a rare event although it is apparently uncommon (7, 8). Blessings, et. al. (9) reported on 20 such cases where in a benign clinical diagnoses was made in over 50% of them. Eight of these cases had metastases, seven of whom died of their disease. Among other conclusions was that these lesions show a poor prognosis. Initially, 100f these cases were mistakenly diagnosed as benign at histopathologic examination thus showing the treacherous possibility of error both clinically and by histopathology. They suggest using the term Verrucous Melanoma. In a letter to the editor, at least one dermatologist noted a different "feel" when performing curettings of clinical seborrheic keratosis which proved to be malignant melanoma by histopathology (10). Of the three different cases recalled, all were noted to have occurred on the back, were more difficult to curette, had a more friable surface and more bleeding when compared to "routine" seborrheic keratosis. In the previously reported cases and the current case, clinical images of the lesion are not available, presumably due, at least in part, to the lesions having the clinical appearance of an irritated seborrheic keratosis when biopsied. This argues for biopsy of unusual seborrheic keratosis-like lesions and the histopathologic examination of them. The classical clinical features suggestive of melanoma have been previously illustrated. A review of 10,000 seborrheic keratosis lesions for the simultaneous occurrence of a second pathologic diagnosis showed no melanomas and 14 cases of basal cell carcinoma, 11 of which having been determined to have developed within the epithelium of the seborrheic keratosis rather than being a collision tumor (11). While intradermal and compounded nevi have been described to have hyperkeratosis, papillomatosis, horn cysts and lace-like downward growth of epidermal strands, specific causative factors linking nevi and seborrheic keratosis-like epidermal changes have been lacking (12). Just as benign melanocytic nevi may induce seborrheic keratosis-like changes in the overlying epidermis, so too may melanoma. We believe it important for clinicians and pathologists to be reminded of this phenomenon. References 1. Cashmore RW, Perry HO. Differentiating seborrheic keratosis from skin neoplasm. Geriatrics, 1985, 40:69-75. 2. Fitzpatrick, Thomas B Arthur Z. Eisen, Klaus Wolff, Irwin M. Freedberg, K. Frank Austen. Dermatology in General Medicine: McGraw-Hill, 4th ed., 1993, pp 855-58. 3. Cascajo CD, Reichel M, Sanchez JL. Malignant neoplasms associated with seborrheic keratoses: an analysis of 54 cases. Am J Dermatopathol, 1996, 18:278-82. 4. Burgess MC, Smith WB, Keeling JH. Seborrheic keratosis with trichilemmomas masquerading as melanoma. Cutis, 1994, 54:351-53. 5. Yakar JB, Sagi A, Mahler D, Zirkin H. Malignant melanoma appearing in seborrheic keratosis. J Dermatol Surg and Oncol, 1984, 10:382-83. 6. Rapini RP. Seborrheic keratosis? J Dermatol Surg Oncol, 1985, 11:74. 7. Kuehnl-Petzoldt C, Berger H, Wiebelt H. Verrucous-keratotic variations of malignant melanoma. A clinicopathological study. Am J Dermatopathol, 1982, 4:403-410. 8. Suster S, Ronnen M, Bubis JJ. Verrucous pseudonaevoid melanoma. J Surg Oncol, 1987, 36:134-137. 9 Blessing K, Evans AT, Al-Nafussi A. Verrucous naevoid and keratotic malignant melanoma: a clinico-pathological study of 20 cases. Histopathology, 1993, 23:453-458. 10. Field LM. Clinical misdiagnosis of melanoma as well as squamous cell carcinoma masquerading as seborrheic keratosis [letter; comment]. J Dermatol Surg and Oncol, 1994, 20:222. 11. Mikhail GR, Mehregan AH. Basal cell carcinoma in Seborrheic keratosis. J Am Acad Dermatol, 1982, 6:500-06. 12. Requena L, Sanchez M, Requena C. Simultaneous occurrence of junctional nevus and seborrheic keratosis. Cutis, 1989, 44:465-66.

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