Subclassification of Desmoplastic Melanoma: Pure and Mixed Variants Have Significantly Different Capacities for Lymph Node Metastasis

Background: There is disagreement about the behavior and optimal management of desmoplastic melanoma (DM), particularly regarding the incidence of lymph node (LN) involvement. Recently, investigators have noted the frequently heterogeneous histologic composition of DM and have found significant differences between pure desmoplastic melanoma (PDM) (>or=90% comprised of histologically typical DM) and mixed desmoplastic melanoma (MDM) [>or=10% DM and >10% conventional melanoma (CM)].

Method: We reviewed 87 cases of DM comparing the histologic and clinical features of PDM (n = 44) to MDM (n = 43).

Results: At surgical staging, there were LN metastases in 5 of 23 (22%) MDM patients, whereas all 17 PDM patients had negative LN biopsies (0%) (p = 0.04). PDM was less often clinically pigmented (36% vs. 67%) and had a lower mean mitotic index (1.3 vs. 3.0).

Conclusions: There are differences between PDM and MDM, the most important of which is the incidence of LN involvement. Our findings support the clinical utility of classifying DM into pure and mixed subtypes because the negligible rate of nodal involvement in PDM does not support the routine performance of sentinel LN biopsy in this subgroup of melanoma patients. In contrast, the incidence of LN involvement in MDM is comparable to that of CM.