Malignant proliferating trichilemmal tumor of the forearm: a case report of an unusual location of a rare cutaneous adnexal tumor

Proliferating trichilemmal tumor (PTT) is a rare cutaneous adnexal neoplasm of the hair follicle that undergoes outer root sheath differentiation in the isthmus. Histological hallmarks include trichilemmal keratinization, lack of granular layer and lobular proliferation of squamous epithelium with glycogenated clear cells. It affects predominantly elderly women, especially over the scalp. In some cases, malignant transformation can occur. However, only a few cases are reported in the literature. We hereby describe the case of a patient diagnosed with malignant PTT (MPTT) in the dorsal forearm, 2 years after undergoing surgical excision of a squamous cell carcinoma in the same topography. Thus, providing a thorough description of the clinical presentation of MPTT will assist surgeons in diagnosing and treating this rare tumor.