Melanoma Masquerading as Xanthogranuloma: A Diagnostic Pitfall

Histiocytoid melanoma: Diagnostic pitfall and mimicker of non-Langerhans cell histiocytoses including reticulohistiocytoma

Melanoma and benign histiocytic proliferations can sometimes show considerable clinical and histopathologic overlap. Recently, cases of melanomas resembling xanthogranuloma and Rosai-Dorfman disease have been reported, and herein we report a case of melanoma closely mimicking reticulohistiocytoma. An 84-year-old man presented with a 1 cm purple-red nodule on his arm concerning for squamous cell carcinoma. While the biopsy findings resembled reticulohistiocytoma, the clinical context and regression changes at the lesion perimeter raised stronger concern for melanoma, which was confirmed with immunohistochemistry. We review prior rare reports of melanomas resembling non-Langerhans cell histiocytic proliferations and summarize helpful clinical and histopathologic clues to avoid a diagnostic pitfall when confronted with this unusual quandary.

Uncommon Histopathological Variants of Malignant Melanoma: Part 1

Despite new horizons opened by recent advances in molecular pathology, histological evaluation still remains the diagnostic gold standard regarding cutaneous melanocytic neoplasms. Several histological variants of melanoma have been described, and their knowledge is crucial for accurate diagnosis and classification of cases with unusual clinicopathological features. Uncommon histological variants of melanoma have been described based on a broad constellation of features, including architectural pattern, stromal alterations, cytological attributes, and other morphological properties. This review is aimed at providing an extensive discussion of unusual but distinctive histopathological variants of melanoma.

Metastatic melanoma with balloon/histiocytoid cytomorphology after treatment with immunotherapy: A histologic mimic and diagnostic pitfall

Epithelioid cells with foamy cytoplasm (histiocytoid features) are typical histopathologic findings among benign and malignant histiocytic neoplasms such as xanthoma and atypical fibroxanthoma. However, these changes are unusual in melanoma, which is typically composed of nested and variably pigmented atypical epithelioid cells. Here, we report a patient with metastatic melanoma in lymph nodes presenting with prominent balloon cell/histiocytoid features expressing melanocytic markers, after treatment with nivolumab. This report suggests that the spectrum of neoplasms with histiocytoid features should be expanded to include melanoma, a pattern that, to the best of our knowledge, is uncommon, especially in the setting of post-neoadjuvant therapy.

Metastatic melanoma mimicking solitary fibrous tumor: report of two cases

Malignant melanomas are known for their remarkable morphological variation and aberrant immunophenotype with loss of lineage-specific markers, especially in recurrences and metastases. Hot spot mutations in BRAF, NRAS, GNAQ, and GNA11 and mutations in KIT are oncogenic events in melanomas. Therefore, genotyping can be a useful ancillary diagnostic tool. We present one case each of recurrent and metastatic melanoma, both showing histological and immunohistochemical features of solitary fibrous tumor (SFT). Mutational analysis detected BRAF and NRAS mutations in the primary and secondary lesions, respectively. This result confirmed the diagnosis of recurrent/metastastic melanoma.