Pilomatrixoma

Pilomatrixomas are rare, slow-growing tumors that derive from hair follicle cells, which are ectodermal in origin. They develop in the lower dermis, present as irregularly shaped, firm growths that are tethered to the overlying skin but are not, however, attached to the underlying tissue. They often cause a blue or red discoloration on the skin but are otherwise non-tender [1]. Moreover, pilomatrixomas have the ability to calcify or ossify.

They most frequently occur within the first decade of life and affect females more than males [2]. Furthermore, they are usually found in the head and neck region, but have also been reported in the upper extremities. Typically, pilomatrixomas are solitary, yet multiple growths can occur simultaneously. The occurrence of the latter may be linked to various conditions, exemplified by sarcoidosis, Gardner syndrome, and myotonic dystrophy [3].

An even rarer type of tumor is the malignant form of pilomatrixomas, that is, pilomatrix carcinoma. These may present in a similar manner to pilomatrixomas, but are distinguished by their rapid growth, and involvement of the hematologic and lymphatic systems [4]. They have a tendency to recur and sometimes metastasize.

The diagnosis of pilomatrixoma is often missed, with the correct identification of the mass being less than 30% pre-operatively [5]. The reason for the high diagnostic inaccuracy may be a combination of the rarity of the tumor, its similarity to other superficial masses such as dermoid cysts, lipomas, foreign bodies and branchial cleft cysts, and atypical features such as an uncommon location [6].

The diagnosis of pilomatrixomas can be made based on clinical findings alone. The tent sign can be elicited and allows visualization of the irregularity of the mass. There is no development of lymphadenopathy. The modal location of tumors is the periorbital area, and a minority of cases are preceded by a history of trauma.

Histologically, pilomatrixomas are well circumscribed and surrounded by connective tissue. They consist of groups of metrical cells with cystic changes and central degeneration caused by ghost cells which lack a nucleus and are not exclusively found in pilomatrixomas [7]. Inflammation may be present in the surrounding tissue. Calcification occurs in the majority of tumors, whilst other changes that can occur include a local build-up of keratin, and foreign body granulomas [6].

Imaging studies are not routinely required. If X-rays are carried out, there may be visible calcifications. Pilomatrixomas are depicted on computerized tomography (CT) scans as soft tissue masses that may also contain calcifications. Magnetic resonance imaging (MRI) reveals ring-enhancing lesions, while ultrasonography may reveal peripherally hyperechoic masses with acoustic shadowing visible in the subcutaneous tissue [8].

Fine needle aspiration cytology (FNAC) remains the most accurate diagnostic test, although it also has a high margin of error [7]. On FNAC, basaloid and ghost cells are indicative of pilomatrixomas [9].

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