The regressing thin malignant melanoma: a distinctive lesion with metastatic potential.

To validate the supposition that thin malignant melanomas (less than 0.76 mm thick) of ordinarily low risk but with areas of regression may paradoxically metastasize, we observed 121 thin malignant melanomas over a six year period. Of these, 23 displayed readily apparent areas of regression, of which five (21.7%) metastasized. The incidence of metastases in their 98 counterparts without regression was 2.0% (2/98). The difference between the two is statistically significant (p = less than .01). Of the entire group of the two is statistically significant (p = less than .01). Of the entire group of thin melanomas, those with regression represented 19.0% (23/121) yet accounted for a disproportionate 71.4% (5/7) of all metastases. We conclude that regression is a relatively poor prognostic sign, whose occurrence within an otherwise thin melanoma represents a significant caveat to the current histologic staging system that equates thinness with low risk. We thus submit that patients whose malignant melanomas display regression be followed rigorously for evidence of metastases irrespective of the tumor's actual measured thickness or level of invasion.