Prognostic factors in thin cutaneous malignant melanoma

Background. Thin melanomas can metastasize and be lethal. The predictive importance of tumor thickness in thin melanomas and the specific features identifying the patients at risk have not been investigated fully. Methods. Prognostic factors were analyzed in 585 patients with clinical Stage I invasive cutaneous malignant melanoma with a thickness of less than or equal to 0.8 mm. The patients were included in a population-based cancer registry in Stockholm county during 1976–1987. They constituted about 64% of all patients with thin melanomas who were diagnosed in the region during the study period. Information was available on age, sex, anatomic site of the tumor, histologic type of melanoma, level of invasion, tumor thickness, and tumor regression. In a Cox regression analysis, the prognostic importance of each factor was studied. By a case-control technique with individual matching for the identified independent predictors of recurrence, the additional prognostic information given by type and grade of inflammatory response, presence of vertical growth phase, mitotic rate/mm2, and histologic ulceration of the tumor was assessed. Results. After a median follow-up time of 50 months, recurrent disease developed in 26 patients (4%). There was no difference in recurrence rate between patients treated with narrow (1–2 cm) or wide (5 cm) excision. Anatomic site, tumor thickness, level of invasion, and tumor regression were found to be independent prognostic factors in the multivariate analysis. In the case-control study, only grade of inflammatory reaction added significant prognostic information. No subgroup could be identified that was without risk of recurrent disease. Conclusions. Thin melanomas do not seem to constitute a separate form of melanoma, but compose one end of a continuous spectrum of biologic behavior.