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10 May 2017 - Case of the Week #424

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Case of the Week #424

Clinical history:
A 34 year old man presented with an occipital nodule. Fine needle aspiration was performed, followed by excision.

Cytology images:

Additional cytology images:

Histopathology images:

Additional histopathology images:

What is your diagnosis?

Pilomatrixoma (pilomatricoma, calcifying epithelioma of Malherbe)

Test question (answer at the end):
Which of the following are not typical of pilomatrixoma? (choose all that are appropriate)

A. Calcification, ossification and extramedullary hematopioiesis
B. Sheets of basaloid cells, occasionally with increased mitotic activity, Shadow cells, trichilemmal-type keratinization
C. Budding nests of atypical basaloid cells associated with a desmoplastic stromal response, zonal necrosis and marked cytologic atypia
D. Recurrence following local excision


Pilomatixoma (pilomatricoma, calcifying epithelioma of Malherbe) is a benign skin appendigeal tumor with differentiation towards the follicular matrix. Lesions typically arise either in the head and neck or the upper extremities of children and young adults, with a second peak in incidence in individuals over 50 years old.

Histologic features are characteristic. Pilomatrixomas, often partially cystic in early lesions, contain solid nests/sheets of basaloid cells with trichilemmal-type keratinization transitioning into shadow cells, amorphous keratinous debris and foci of calcification more centrally. Additional features include ossification, extramedullary hematopoiesis and foreign body type inflammatory response. Variants may demonstrate pigmentation, anetoderma (possibly due to elastophagocytosis) and ulceration with extrusion of keratinous debris. Multiple pilomatrixomas may be seen in patients with myotonic dystrophy, Rubinstein-Taybi syndrome and Turner syndrome. Cysts with features of pilomatrixoma are identified in patients with Gardner syndrome (J Clin Pathol 2007;60:129).

As illustrated in this case, deeper seated lesions and those arising in the head and neck region clinically mimic enlarged lymph nodes. Fine needle aspiration (FNA) may result in an erroneous interpretation of malignancy, especially in a limited sample where only one component predominates (e.g. cohesive sheets of mitotically active basaloid cells), and is a well known pitfall. A cell block preparation, which reveals the shadow cells, may be helpful in arriving at the correct diagnosis (Malays J Med Sci 2015;22:84, report of two cases).

The vast majority of pilomatrixomas are benign and do not recur following local excision. Although not necessary for diagnosis, the majority of these lesions strongly express BCL2. Activating mutations of β‐catenin may contribute to tumorigenesis in pilomatrixomas.

Features suggestive of aggressive behaviour (and likely recurrence following excision) include large size, longstanding history and budding nests infiltrating into adjacent tissues associated with desmoplasia (proliferating pilomatrixoma). True pilomatrix carcinoma is rare and shows zones of geographic necrosis, invasive growth and marked cytologic atypia associated with recurrence and lymph node metastasis. The differential diagnosis includes basal cell carcinoma (peripheral palisading, atypia, retraction artifact, typically lacks of shadow cells).

Test Question Answer:
C & D

Image 01 Image 02