Abstract

Proliferating pilar tumors (PPT) are uncommon, typically benign neoplasms arising from the outer root sheath of hair follicles. Cutaneous carcinosarcoma is an extremely rare, biphasic tumor composed of a mixture of malignant epithelial and mesenchymal components that may present at various anatomic sites. Due to the limited number of reported cases, its clinical behavior remains unclear. 

We present a case of a 43-year-old female with a left occipital nodular scalp mass first noticed in 2016. CT imaging demonstrated that the mass extended from the skin to the calvarium without invasive characteristics. The clinical differential included benign vs. malignant PPT, pilomatrical carcinoma, and basal cell carcinoma. Histological examination demonstrated two distinct populations of malignant cells. The first population was composed of sheets of atypical epithelioid cells with architectural features most consistent with a trichilemmal carcinoma. The second population consisted of enlarged, markedly pleomorphic cells, set within a fibrous stroma that intermixed with the epithelial islands. In the background, there was a lobular proliferation of bland squamous cells, consistent with a pre-existing PPT. Due to the multifaceted nature of the neoplasm, a comprehensive immunohistochemical panel was performed to further characterize the lesion. Immunohistochemical staining for SOX-10 and CD34 was negative, while Vimentin, Cytokeratin 5/6, and p63 highlighted dual populations of mesenchymal and epithelial cells, consistent with a carcinosarcoma arising within a PPT. This diagnosis underscores the importance of thorough histological and immunohistochemical evaluation in seemingly benign lesions and adds to our understanding of the potential for malignant transformation in PPTs.