Trichoblastoma (TB) is a benign cutaneous adnexal neoplasm. The lesion typically presents as a slow-growing, solitary, well-circumscribed nodule measuring up to 3 cm in diameter. On rare occasions, TB causes malignant transformation into an aggressive form described as high-grade trichoblastic carcinoma. Four such cases have been reported to date; all were described as high-grade trichoblastic carcinomas. Here, we describe the case of a 72-year-old Japanese male patient with a rapidly enlarging subcutaneous tumor on his lower back, which was diagnosed as high-grade trichoblastic carcinoma. Histopathologically, the tumor featured both benign and malignant components, and a transition zone between these states was clearly evident. In the immunohistochemical analysis, a malignant component was positive for p53 and showed stronger staining of phospho-RAC-α serine/threonine-protein kinase (AKT) Ser473 in comparison with a benign component. These results suggest that loss of p53 function and activation of phosphatidylinositol 3-kinase–AKT signaling pathways played important pathogenic roles in malignant transformation of the present case.
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