http://www.histopathology-india.net/SAClass.htm

Proliferating trichilemmal tumour arises from the isthmus region of the outer root sheath.

Clinical presentation : Usually occurs in elderly women & presents as a slowly enlarging, painful subcutaneous scalp nodule.

Sites : Scalp is the most common site. Other sites include - forehead, nose, back, chest, abdomen, buttocks, elbow, wrist, and vulva .

Gross : Well defined and lobulated lesion ( 0.4 to 10 cm in diameter ).

Microscopic features :  Well- circumscribed lesions with islands of squamous epithelium displaying trichilemmal keratinization. Small cystic areas may be present.  Foci of calcification, glycogen rich clear cells and horn pearls or squamous eddies may be present. Mild degree of pleomorphism is common.

Differential diagnosis :  Squamous cell carcinoma-  (Proliferating trichilemmal tumour is characterized by abrupt keratinization, minimal pleomorphism, low mitotic activity, sharp circumscription, foci indistinguishable from a trichilemmal cyst, calcification and absence of a premalignant lesion such as actinic keratosis).

The tumour often recurs if not fully excised.

                                IMAGE LINK (Dr Weems)

MALIGNANT PROLIFERATING TRICHILEMMAL  TUMOUR

This malignant tumour originates from a preexisting trichilemmal cyst.

Clinical presentation:   Presents as a large ( 20- 25cm) exophytic lobulated mass usually in elderly women. Patient complains of sudden rapid growth .

Site:  Confined to the scalp, back or neck.

Microscopic features:

Areas of transformation from benign proliferation of pilar sheath epithelium to an obvious malignant neoplasm ;  it  is  characterised by poorly defined borders, clear cut infiltrative properties with cytological evidence of malignancy ;  extensive dermal lobular proliferation of pilar sheath epithelium is seen together with palisading arrangement of nuclei at the periphery of the lobes ; multiple central areas of trichilemmal keratinization with occasional areas of calcification and homogeneous  keratin cyst formation ;  epithelium shows severe disarrangement of epithelial cells with large number of cells  having irregular shaped  hyperchromatic  nuclei ;  there is brisk mitosis ;  dyskeratotic cells , shadow cells and multinucleated cells are also present ; tumour invades into the surrounding connective tissue.

Differential diagnosis:   Squamous cell carcinoma - Abrupt mode of keratinization and foci of calcification are characteristic of malignant proliferating  trichilemmal tumour.

This low grade malignant tumour shows recurrence following excision and subsequently develops metastasis.  
(Visit these sites: Trichilemmal carcinoma and Clear cell carcinoma of skin  )

Malignant proliferating trichilemmal tumour of the scalp. Report of a case and a short review of the literature.
Ann Ital Chir. 2006 Mar-Apr;77(2):179-81.

Malignant Proliferating Trichilemmal Tumor of the Lower Eyelid.Ophthal Plast Reconstr Surg. 2005 ;21(5):349-352.

Proliferating trichilemmal tumors: clinicopathologic evaluation is a guide to biologic behavior.J Cutan Pathol. 2003;30(8):492-8.

Proliferating trichilemmal tumor with a malignant spindle cell component.J Cutan Pathol. 2002;29(8):506-9.

Malignant proliferating trichilemmal tumor.Am J Clin Oncol. 2001 ;24(4) :351-3.

Pathologic quiz case. A nodule on the back. Pathologic diagnosis: proliferating trichilemmal tumor.Arch Pathol Lab Med. 2000;124(8):1237-8.

Malignant proliferating trichilemmal tumor in the skin over the breast: a case report.Breast Cancer. 2000;7(1):79-82.

A trichilemmal carcinoma arising from a proliferating trichilemmal cyst: the loss of the wild-type p53 is a critical event in malignant transformation.Hum Pathol. 1998;29(2):193-5.

Malignant transformation of a giant proliferating trichilemmal tumor of the scalp: patient report and literature review.Ann Plast Surg. 1998 ;41 (3):314-6.

Malignant proliferating trichilemmal tumor showing distant metastases. Am J Dermatopathol. 1997;19(5):536-9.

Malignant proliferating trichilemmal cyst.J Am Acad Dermatol.  1995 ;32 (5 Pt 2):870-3.

Malignant aneuploid spindle-cell transformation in a proliferating trichilemmal tumour.Acta Derm Venereol. 1993;73(6):444-6.

Malignant proliferating trichilemmal tumors--report of three cases.J Dermatol Surg Oncol. 1987 ;13(12):1339-42.

Proliferating trichilemmal cyst: a simulant of squamous cell carcinoma.Cancer. 1981;48(5):1207-14.

Trichilemmal pilar tumors.Cancer. 1980 ;45(7):1594-7.