Keratotic Basal Cell Carcinoma With Matrical Differentiation on the Flank: A Case Report.

Basal cell carcinoma with matrical differentiation (BCCMD) is a rare histologic variant of basal cell carcinoma. We report a case of keratotic BCCMD on the right flank of a 57‑year‑old female, presenting as a 10 cm × 6 cm lesion that rapidly enlarged over the course of one year. Histopathology revealed basaloid lobules with central keratinization, resembling early pilar structures, a key feature of matrical differentiation. Immunohistochemical staining with BerEP4 confirmed the diagnosis. MRI demonstrated no deep or regional extension. The lesion was excised surgically and healed by secondary intent. This case is notable due to the lesion's aggressive clinical growth and uncommon anatomic location. Recognizing matrical features is essential for accurate diagnosis, especially given the overlap with adnexal tumors such as pilomatricoma and pilomatrix carcinoma. Although BCCMD rarely metastasizes, its potential for local destruction underscores the importance of early recognition and appropriate management.