Giant Pilomatricoma Mistaken for a Malignant Tumor.

Pilomatricomas are tumors originating from the matrices of hair follicles. Giant pilomatricomas, defined as pilomatricomas that are 5 cm or larger, are benign but may appear malignant clinically. We present the case of a 69-year-old man with a rapidly growing mass on his scalp. When he visited our department, the tumor measured 10.0×6.0×4.0 cm and showed inflammation and ulceration. Magnetic resonance imaging and 18F-fluorodeoxyglucose positron emission tomography-computed tomography showed findings resembling lymph node metastasis from a malignant tumor. However, upon an incisional biopsy, the tumor was diagnosed as a pilomatricoma. Therefore, we performed an excisional biopsy instead of radical surgery and lymph node dissection. The tumor was ultimately diagnosed as a giant pilomatricoma based on the excisional biopsy, and the patient received reconstruction only at the site of the defect. A giant pilomatricoma can be mistaken for a malignant tumor due to its characteristics. In such uncertain cases, it can be helpful to first perform an excisional biopsy.