Primary Cutaneous Signet Ring Cell/ Histiocytoid Carcinoma of the Axilla in a Young Male

Summary: Primary cutaneous signet ring cell/ histiocytoid carcinoma, is a rare and aggressive cutaneous adnexal neoplasm occurring usually on the eyelid and rarely on the axilla with middle aged male predominance. We report a young 27-year-old male presenting with a painless 1.5 cm axillary mass for 1 year. The mass was subcutaneous and firm on palpation with erythematous overlying skin. Following excision, pathology showed a diffusely infiltrative malignant cells arranged singly and in cords involving dermis and subcutis. The cells exhibited histiocytoid/signet ring cell morphology with abundant granular cytoplasm and intracytoplasmic vacuoles and enlarged, vesicular and pleomorphic nuclei with conspicuous nucleoli. Multiple atypical mitotic figures and infiltration into adjacent eccrine glandular units. Subsequent targeted next-generation DNA & RNA sequencing (NGS) analysis covering 161 genes using the ONCOMINE Comprehensive assay indicated no significant genomic alterations or copy number changes. In particular, mutations involving PIK3CA, CDH12 and ERBB2 amplification were not identified. The tumor exhibited microsatellite stability and a low tumor mutational burden (TMB). A PET scan revealed intense uptake associated with a 3 cm osteolytic lesion in the right scapula which was biopsied and showed vascular lesion and negative for similar malignancy. Primary or metastatic lesions involving other organ sites such as breast, gastrointestinal and genitourinary tracts are negative by imaging and pathological evaluation. The clinical, radiological and histopathological findings are compatible with a primary cutaneous signet ring cell/histiocytoid carcinoma.