Primary dermal melanoma: A unique subtype of melanoma to be distinguished from cutaneous metastatic melanoma: A clinical, histologic, and gene expression-profiling study

Background Primary dermal melanoma (PDM) is a subtype of melanoma confined to the dermis that may be morphologically impossible to distinguish from cutaneous metastatic melanoma (CMM). Objective We sought to better characterize PDM by describing the clinical, histologic, and molecular features of 49 cases of PDM and determine whether a gene expression-profiling test could help distinguish PDM from CMM. Methods We describe 49 cases of PDM and determined whether any clinical or histopathologic features had a statistically significant relationship with outcome. Secondly, we performed a melanoma gene expression-profiling test on a subset of the PDM and CMM cases. Results Overall recurrence was infrequent and seen in 9 of 49 cases. Six patients had locoregional recurrences and 3 patients had distant metastasis. None of the clinical or histologic parameters showed a statistically significant relationship with recurrence. There was a statistically significant association of a class I signature by DecisionDx-Melanoma assay (Castle Biosciences Inc, Friendswood, TX) for PDM whereas CMM were more frequently class II (P value =.023). Limitations The mean follow-up time was 26 months. Conclusions Most conventional staging parameters used for prognosis in cutaneous melanoma have limited applicability to PDM. The melanoma prognostic assay may be a useful tool for distinguishing PDM from CMM.