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Case Reports

Abstract

Malignant proliferating trichilemmal tumors (MPTTs) are rare, aggressive skin neoplasms that often arise from pre-existing trichilemmal cysts. MPTTs predominantly occur on the scalp of elderly women and can exhibit rapid growth, ulceration, and a high propensity for metastasis. We present a case of a 75-year-old African-American woman with a remote history of endometrioid adenocarcinoma, status post hysterectomy, who developed a 16 cm ulcerated, necrotic, lobulated, exophytic mass on the left scalp. The lesion, which had gradually increased in size over four years, became painful, foul-smelling and bled. CT imaging of the soft tissue of head revealed no osseous or intracranial involvement. An incisional biopsy was suggestive of squamous cell carcinoma (SCC) on frozen section. However, further histopathological examination in the context of clinical presentation favored a malignant proliferating trichilemmal tumor. The tumor exhibited variably sized lobules of squamous epithelium with abrupt trichilemmal keratinization and showed diffuse cytokeratin 5 positivity, basal epithelial CD34 positivity, increased Ki-67 proliferative index in the basal layers, and negative p63 staining. MPTTs are characterized by recent rapid growth, a size greater than 5 cm, patchy necrosis, prominent cytologic atypia, and atypical mitosis. Compared to conventional SCC, MPTTs are more aggressive and have a higher rate of metastasis (21-25%). Surgical excision with clear margins remains the primary treatment modality. Due to the typically aggressive behavior, close follow-up is crucial to monitor for recurrence or metastasis. This case underscores the importance of recognizing the malignant potential of proliferating trichilemmal tumors and differentiating them from squamous cell carcinoma to ensure appropriate treatment and follow-up.