A large mass on the right posterior scalp.

    Dr. Ekladous, 4th year Pathology resident in Miller School of Medicine, Univ of Miami, is presenting this case.


    Clinical history:


    The patient is a well-developed and well-nourished 67-year-old male who presented with a large mass on the right posterior scalp. The patient works at an aircraft repair station and has a past medical history of basal cell carcinoma of the face and multiple cystic lesions on the scalp, which were removed over 20 years ago. The mass grossly measured 4.5 x 3.3 cm and was elevated, firm and nodular with central ulceration. The surgeon performed a complete excision and closure with a full-thickness skin graft.



    Microscopic Description:


    Histological sections show a large lobulated and cystic intradermal tumor composed of pilar squamous epithelium, trichilemmal keratinization, foreign body reaction and ghost keratinocytes. An epidermal opening from the tumor is noted. The tumor shows regular pushing border. No diffuse high-grade cytological atypia is identified.


    Immunohistochemistry is performed showing CD34 and calretinin positivity, supporting a trichilemmal origin. Phosphorylated histone H3 (PHH3) highlights an increased mitotic rate (>15/10 high-power fields with area variation). Ki-67 shows high cell proliferation activity not limited to the basal layer and extends into the upper layer of tumor nests. p53 shows areas of random positivity. A conventional proliferating pilar cyst usually shows cell proliferation activity, a limited p53 expression in the basal layer and an absence or very few mitoses. In this context, this tumor was diagnosed as a mitotically-active proliferating trichilemmal cyst (PTC).