Desmoplastic melanomas (DM) may only represent up to 4% of melanomas, however, they are responsible for 50% my insomniac nights. It is easy to lose sleep worrying if that the slightly indurated lesion that I reassured the patient was a “scar”, or the spindle cell lesion I diagnosed histologically as a “dermatofibroma”, was really a DM.
Desmoplastic melanomas may arise de novo or in association with other melanoma types, most notably lentigo maligna. They are characterized by persistent local growth and less frequent nodal metastases compared to other melanomas. Histologically there is variability in the degree of cellularity, fibrosis, and/or perineural invasion. Lesions are classified as pure (> 90% desmoplastic) or mixed (with other features of melanoma). Pure DMs are less aggressive (1).
When assessing such lesions histologically, it is essential to carefully assess the epidermis for at least subtle evidence of a lentigo maligna and searching for lymphoid aggregates around the tumor. The latter finding is not typically seen in dermatofibromas or scars. As a dermatopathologist, one of the greatest difficulties in assessing these lesions is having a small tissue sample. Under such circumstances, the lymphoid aggregates, which may be noted at the base of these lesions, may not be present. This makes the use of immunohistochemistry even more essential. According to Adreevscaia et al (2):
The application of immunohistochemistry, and particularly analysis of S-100, SOX-10 and p75, is mandatory for the diagnosis of melanoma. The distinction between paucicellular pDM and scarring may be straightforward. Immature scars may also express S-100, and pDM is mostly negative for other melanocytic markers, such as HMB45, microphthalmia transcription factor (MITF) or melan-A.
In a study of 40 cases of DM, utilizing a broad panel of histochemical stains, Plaza et al demonstrated that the optimal panel, being most sensitive and specific for DM is S-100p, WT1, SOX10, p75 and nestin (3).
Assuming others corroborate this, this panel will be most useful in accurately diagnosing DM, thereby avoiding all the ramifications of delayed diagnosis and treatment.
- Chen LL, et al. Desmoplastic melanoma: A review. J Am Acad Dermatol 2013: 68(5): 825-33.
- Adreevscaia O, et al. Diagnostic challenge of desmoplastic melanoma. Rare Tumors 2016; 8: 5713.
- Plaza JA. Desmoplastic melanoma: An updated immunohistochemical analysis of 40 cases with a proposal for an additional panel of stains for diagnosis. J Cutan Pathol 2016; 43: 313-23.