Proliferating pilar tumor - Skin
Like a common trichilemmal cyst, PTT arises from the isthmus of the outer root sheath of the hair. Clinically, PTT is often large and may grow rapidly. Histologically, PTT manifests features of a common trichilemmal cyst, but there is also epithelial proliferation, variable cytologic atypia, and some mitotic activity.
The malignant potential of PTT is controversial. Only a small number of cases with documented invasion and metastasis have been reported. Some experts have promoted further stratification of PTT into benign, "low-grade" malignant, and "high-grade" malignant variants. There is some subjectivity in analysis, even among experienced dermatopathologists.
L72.12 – Trichilemmal cyst
254678009 – Proliferating pilar cyst
- Common trichilemmal cyst – with trichilemmal epithelium, but without histologic features of proliferation
- Epidermoid cyst – from upper third of follicle, with a granular layer and without features of proliferation
- Pilomatricoma – a cystic structure that arises from the lower one-third of the follicle, often with matrical elements
- Lipoma – composed of adipose tissue without epithelium
- Metastases – common on the scalp in elderly persons; excluded by histology