Answer: Metastatic carcinoma

Histology: This is a cellular infiltrate that diffusely involves the fat. Several of the cells have an epithelioid appearance, with well-demarcated, pink cytoplasm. Other cells are spindled, and others have more vacuolated cytoplasm. On immunohistochemical analysis, these cells are diffusely immunoreactive for Cytokeratins (AE1/AE3 and CAM5.2). The patient has a history of a lung mass, highly suspicious for a primary lung carcinoma. Therefore, the present lesion likely represents a soft tissue metastasis from that primary carcinoma.

Discussion: It is important to recognize that sarcomas are not the only malignant spindle cell neoplasm. One should always consider the possibility of a sarcomatoid carcinoma, malignant melanoma, and even Anaplastic Large Cell Lymphoma. These possibilities are usually easily distinguished by immunohistochemistry. In the above case, diffuse immunoreactivity for Cytokeratin, as well as the clinical history, established the diagnosis.