Malignant proliferating trichilemmal tumor
The diagnosis of MPTT is based mainly on histological features. A histological classification system has been proposed to differentiate the more common and benign proliferating trichilemmal tumor (PTT) from low-grade and high-grade MPTTs, which have potential for metastasis.
Recurrences are not uncommon. MPTT is at high risk for metastasis, with regional and distant metastatic disease in up to 25% of patients.
More than 40 cases of MPTT have been described in the literature, including 12 cases with metastatic disease and several mortalities.
C44.49 – Other specified malignant neoplasm of skin of scalp and neck
399878004 – Malignant neoplasm with pilar differentiation
446023005 – Proliferating trichilemmal tumor
Differential Diagnosis & Pitfalls
- Proliferating trichilemmal (pilar) tumor (PTT) – Differentiating between benign PTT and MPTT can be done with cytological and histological evaluations as well as immunohistochemical studies. MPTT is characterized by atypical mitoses, marked cellular pleomorphism, cytological and architectural atypia, and infiltrating margins with invasion and necrosis of surrounding tissue. MPTT shows high proliferative activity with Ki-67. Previous studies showed increased staining with Ki-67 and P53 in MPTT when compared to PTT. Moreover, proliferating cell nuclear antigen (PCNA) stains positive in MPTT and negative in PTT.
- Squamous cell carcinoma (SCC) – Key histopathological features that point to MPTT over SCC are abrupt trichilemmal keratinization (with absence of granular cell layer) and lack of a precursor premalignant epidermal lesion, such as actinic keratosis. Additionally, the expression of cytokeratins AE13 and AE14 is positive in MPTT and negative in SCC.
- Trichilemmal carcinoma – Trichilemmal carcinoma, which also exhibits differentiation towards the outer root sheath, tends to occur on sun-exposed areas and exhibits lobular proliferation of clear cells centered on the pilosebaceous unit, as well as continuity with the epidermis.
- Cutaneous metastases from an internal malignancy – Common on the scalp in elderly persons; excluded by histology.
- Mixed tumor
- Epidermal cyst
- Pilar cyst