Expression of p16 alone does not differentiate between Spitz nevi and Spitzoid melanoma.

Background: Spitz nevi and Spitzoid melanomas show overlapping histopathologic features, often making the diagnosis challenging. The p16 protein functions as a tumor suppressor and loss of its expression may be seen in some melanomas.

Methods: We evaluated 18 Spitz nevi and 19 Spitzoid melanomas from the Yale Spitzoid Neoplasm Repository for p16 expression. A staining intensity score (SIS) was calculated by multiplying a score for the percentage of stained cells (0-3) by a score for staining intensity (0-3).

Results: Staining with p16 was positive in 15/18 (83%) Spitz nevi and 15/19 (79%) Spitzoid melanomas (p = 0.73). Both Spitz nevi and Spitzoid melanomas had a similar SISs, 4.9 and 3.8, respectively (p = 0.057). All 19 patients with Spitzoid melanomas had poor outcome with either death (6 patients) or metastases (13 patients) at a median (3 years) and mean (5.4 years) follow up. In contrast, all 18 patients with Spitz nevi had a benign course with no adverse events at a median (4 years) and mean (4 years) follow up.

Conclusions: We found no significant difference in p16 staining in Spitz nevi and Spitzoid melanomas. We conclude that p16 does not appear to be a useful immunohistochemical marker in distinguishing between Spitz nevi and Spitzoid melanomas.