Malignant Epithelioid Neoplasm versus Dedifferentiated Malignant Melanoma: A Case Report

The metastasis of poorly differentiated epithelioid carcinoma to the axillary node is uncommon. This tumor has heterogeneous expression and is challenging to diagnose with certainty. Often, it necessitates immunoperoxidase staining to ascertain the tumor lineage, and diagnosis is prolonged due to low suspicion. Herein, we present a case involving a 75-year-old male war veteran with a prior history of a gunshot wound complicated by colostomy that presented with an axillary mass, fecal and urinary incontinence, leg weakness, fevers, night sweats, and substantial weight loss. On admission, he had heightened leukocytosis (43K), anemia (hemoglobin 6.6), and thrombophilia (1000). This patient constantly picked at his back to remove recurrent "gun shrapnel" eruptions. An excisional biopsy of the axillary mass was performed for diagnosis and lymph node removal. Notably, after excision, there was marked improvement in the presenting symptoms. Diagnostic challenges arose due to the tumor cells' inconsistent immunohistochemical marker expression. The staining patterns alluded to metastatic melanoma. Yet, the tumor displayed epithelial characteristics, supported by an immunophenotypic marker pattern indicative of poorly differentiated carcinoma. This case underscores the morphological and immunoperoxidase staining similarities between poorly differentiated carcinoma and dedifferentiated tumors of varying origins. It illustrates the intricate nature of these malignant metastatic tumors and their overlapping manifestations, which requires provider awareness. The timely diagnosis of poorly differentiated epithelial carcinoma remains paramount to early treatment and improved prognosis. Therefore, in patients manifesting with an axillary mass, fecal and urinary incontinence, and B-symptoms, poorly differentiated epithelial carcinoma should be included in the differential diagnosis.