#dermpathJC July 2021 summary

Thursday, July 29th, 9pm EST

Article discussed: Lichenoid dermatoses with pseudomelanocytic nests vs inflamed melanoma in situ: A comparative study

Authors: Gauri Panse, MD, Jennifer M. McNiff, MD

Temporary free access courtesy of The Journal of Cutaneous Pathology: https://onlinelibrary.wiley.com/doi/10.1111/cup.13945

Summary prepared by: Haneen Salah, MD (@Hansalah)

Journal Club Summary:

Introduction:

Pseudomelanocytic nests (pseudonests) were first defined as group of Melan-A positive cells in non-melanocytic lesions. They have been seen in various dermatitides, such as lichen planus, lupus erythematosus, fixed drug eruption and phototoxic eruptions. They have also been reported in solitary lesions such as lichen planus-like keratosis and basal cell carcinoma with regression. Pseudomelanocytic nests arising in lichenoid dermatoses are considered a diagnostic pitfall for regressed melanoma in situ (MIS).

This paper discusses histopathologic features and important morphological clues that help dermatopathologists in differentiating this benign entity from inflamed melanoma in situ (MIS). Clinicopathological correlation is important to achieve the accuracy of the final diagnosis, particularly in challenging cases.

Summary of current study methodology and results:

The authors performed a retrospective review of the dermatopatholgy archive at their institutions. Ten biopsy specimens with pseudomelanocytic nests within lichenoid dermatoses were identified, and twenty cases of inflamed (lichenoid) or regressing MIS were retrieved for comparison.

Here is an example of a patient with cutaneous eruption compatible with graft vs host disease, and histopathologic findings that included pseudomelanocytic nests.

Cases with pseudomelanocytic nests were either a pigmented rash (n = 6) or a discrete non-melanocytic lesion, such as lichenoid keratosis (n = 4). All the cases with pseudomelanocytic nests exhibited nests of microphthalmia-associated transcription factor (MITF)-positive cells at the dermo-epidermal junction (DEJ), interface changes, and lichenoid inflammation, and did not display pagetoid scatter, confluence of solitary melanocytes at the DEJ and significant cytologic atypia. All cases of inflamed MIS showed confluence of single melanocytes at the DEJ with cytologic atypia (P < 0.001) and 18/20 cases showed pagetoid scatter of melanocytes (P = 0.001).

The comparative histopathologic features between patients with pseudomelanocytic nests and inflamed melanoma in situ are shown in Table 2.

Discussion points:

The presence of pseudomelanocytic nests in sun-damaged skin causes diagnostic challenge for dermatologists and dermatopathologits, where atypical junctional melanocytic lesions are more likely to be present.

The morphological and immunohistochemical overlap between pseudomelanocytic nests and atyical nests of melanocytes in inflamed MIS makes clinical correlation vital to rule out an atypical junctional melanocytic proliferation. The immunohistochemical staining of pseudomelanocytic nests with nuclear markers such as SOX10 and MITF adds to the diagnostic difficulty in differentiating this phenomenon from MIS. The pseudonests are composed of degenrated melanocytes, keratinocytes and macrophages. They have also been reported to stain with Mart-1/Melan-A.

This study is the largest series showing that pseudomelanocytic nests lack morphological features like pagetoid scatter and confluence of solitary atypical melanocytes at the DEJ, while these features occur in most cases of inflamed melanoma in situ. These two features should help dermatopathologists in differentiating those two entities morphologically.

Conclusion and future directions:

One limitation of this study is that the control group included unequivocal cases of inflamed MIS only, while cases where a definitive diagnosis of MIS was not rendered, were excluded.

The authors of this study suggest that the presence of pagetoid scatter and confluence of atypical solitary melanocytes at the DEJ may be a useful morphologic clue to differentiate pseudomelanocytic nests from inflamed MIS in diagnostically challenging cases.

Although immunohistochemical findings require careful interpretation, and a thorough histopathologic examination, clinical correlation is important to eliminate the possibility of melanoma in situ with lichenoid inflammation in puzzling cases.

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