Proliferating pilar tumor
PTT is more common among individuals of Northern European descent and is seen most often (approximately 90% of cases) on the scalp, typically in women in their fifth or sixth decade. Isolated cases inherited in an autosomal dominant fashion have been reported.
The malignant transformation of PTT is very rare. Only a small number of malignant proliferating trichilemmal tumor (MPTT) cases with documented invasion and metastasis have been reported. Malignant transformation may be preceded by rapid growth, necrosis, and ulceration.
L72.12 – Trichilemmal cyst
254678009 – Proliferating pilar cyst
Differential Diagnosis & Pitfalls
- 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.
- Chondroid syringoma – Benign glandular neoplasm found most commonly on head (including scalp) and neck.
- Squamous cell carcinoma (SCC) – Identification of squamous eddies, trichilemmal keratinization, or possibly the remnant of a pilar cyst point toward PTT. Moreover, PTT shows positive staining for AE13, AE14, and CD34, whereas SCC tumor cells show negative staining for these markers. SCC is less circumscribed, does not "shell out," and is characterized by more cytologic atypia, with clinically more obvious broad connection to surface epidermis in setting of solar elastosis.
- Malignant proliferating trichilemmal tumor (MPTT) – Clinical features concerning for potential malignant transformation include a location off the scalp, recent and/or rapid growth, ulceration of the surface, and size greater than 5 cm. Histologic findings include infiltrative growth, significant cytologic atypia, and mitotic activity.
- Metastases – Common on the scalp in elderly persons; excluded by histology.