Clinical history: 65 year old female patient consults for a 5 cm lesion in the left arm that has been growing slowly since the past year. Palpation reveals a soft consistency. Biopsy is performed and H&E pictures are shown
What is the most probable diagnosis?
Proliferating Pilar Tumor (Ppt)
Also known as proliferating pilar cyst (PPC) or proliferating trichelemmal cyst/tumor. PPT is defined as a multicystic squamous neoplasm composed of mature keratinocytes lining keratin-filled spaces. The etiopathogeny is still unknown but it is postulated that most cases arise in pre-existing pilar (tricholemmal) cyst and may be related to chronic inflammation of trauma. It typically occurs in older adults (F>M) and involves face, trunk and extremities. This lesion usually presents as a large skin tumor (6 cm or greater in diameter). Grossly, PPT often consists of a multicystic dermal-based tumor that may involve the subcutis. Microscopic examination reveals a well circumscribed multi-cystic tumor with anastomosing spaces containing abundant, dense and eosinophilic keratin. Unlike the proliferating epidermoid cyst, the lining squamous cells show keratinisation without granular layer. Peripheral palisading of basilar layer is typically present, and there may be thickened basement membrane. Squamous epithelial cells show mild enlargement and nuclear hyperchromasia. Occasional mitotic figures are present, but no high grade cytologic atypia or increased mitotic activity should be present. In case they are present, a differential diagnosis of squamous cell carcinoma should be raised. Malignant transformation of PPT into a squamous cell carcinoma is also possible and shows features of spindle cell or sarcomatoid carcinoma in some cases. Complete surgical excision is recommended in order to prevent recurrence and malignant transformation. PPTs behave in benign fashion, but malignant PPTs are aggressive tumors that have a high rate of metastasis.