Stage IV SMARCA4-Deficient Undifferentiated Malignant Neoplasm With Primary Cutaneous Origin and Early Distant Metastases.

SMARCA4-deficient undifferentiated malignant neoplasms (SD-UMNs) are a recently recognized group of malignant epithelioid tumors, associated with mutations in the SWItch/Sucrose nonfermentable chromatin remodeling complex. To our knowledge, there have been only three cases of SD-UMNs primary to the skin. We report a rare case of primary cutaneous SD-UMN in an 85-year-old male with a former 25-pack-year smoking history. Unlike previous cases, he notably was diagnosed as Stage IV upon presentation, with metastatic involvement of lymph nodes and liver. Our case highlights the importance of recognizing SD-UMN from other types of poorly differentiated cutaneous epithelioid malignant neoplasms, given its aggressive nature and potential for targeted therapies. It also adds to the growing but still limited understanding of the clinical and histopathological features of this rare malignancy.