Low-grade trichoblastic carcinosarcoma of the skin

We present an unusual case of cutaneous carcinosarcoma with the epithelial component closely resembling nodular basal cell carcinoma, and the mesenchymal component composed of cells constituting extended follicular papillae. A solitary tumor was excised in an 80-year-old man. Histologic sections revealed an ulcerated, asymmetric, poorly circumscribed neoplasm composed of epithelial cells arranged in lobules with peripheral palisading or in a cribriform pattern. The epithelial cells were darkly basophilic with scant cytoplasm and round or oval nuclei with an indistinct chromatin pattern and nucleoli. Nuclei crowding and mitotic figures were observed. Some lobules contained melanin. There were no shadow cells, sebaceous or apocrine glandular differentiation. Each epithelial nodule was surrounded by multiple rows of cells with pale vesicular nuclei and scant cytoplasm. Smaller epithelial aggregations were encircled by these cells concentrically; in larger ones these cells were aligned across a broad front resembling so-called "continuous papillae". Additionally, numerous small follicular germ-like structures associated with papillae were seen. The cells composing "continuous papillae" showed nuclear pleomorphism, numerous mitotic figures including atypical ones, and nuclear crowding. At foci, the transition from the multilayered arrangement of these cells into their diffuse proliferation in the stroma was seen. There were no transitions between the epithelial and stromal component; both were intermingled as though being mutually dependent, with no areas revealing a high-grade tumor or dedifferentiation. Immunohistochemically, the epithelial cell component stained with cytokeratins. The cells of the mesenchymal component tested positive for vimentin and negative for desmin and cytokeratins. The proliferation index (Ki-67) was high in both components. There were also a high number of p53-positive cells in both compartments. We propose the term "low-grade trichoblastic carcinosarcoma" for this neoplasm. We are not aware of a similar tumor published in the English literature.