Diagnosis: Pilomatricoma

Discussion

Pilomatricoma, or calcifying epithelioma of Malherbe, is a benign skin neoplasm that arises from hair follicle matrix cells. These lesions are typically found in the head and neck region, but they have also been described in various upper extremity locations. These lesions present most commonly in children and young adults, and they are noted more commonly in females. ^[1] This lesion makes up around 20% of all hair follicle related tumors in most series and is therefore the most common hair-follicle tumor. The majority of patients are under 20 years of age, and females are affected more often than males. ^[2] The largest case series in the literature includes 346 pilomatricomas of which 15.3% were observed in the upper extremities. ^[1] There have been few reports of pilomatricoma occurring in the arm in the existing literature. Clinically, pilomatricoma presents as solitary painless and well circumscribed dermal or subcutaneous mass up to 3 cm in diameter. ^[3]

The histopathologic features of a pilomatricoma include a well-demarcated tumor which is often surrounded by a connective tissue capsule. Generally, it is located in the dermal or subcutaneous layer. The tumor is composed of islands of epithelial cells made up of varying amounts of uniform basaloid matrical cells and often shows cystic change. Centrally, there is degeneration of these basaloid cells as the tumor matures. This is characterized by formation of anucleated ghost (or shadow) cells due to the central unstained areas of these cells. ^[1] The central areas often calcify, and calcium can be demonstrated in the basophilic areas of the tumor. ^[2] Calcification of the tumor was observed in 80% of cases, sometimes achieving a true osteoma. ^[4]

Clinical dilemma encountered is the differentiation of this tumor from other benign masses, encountered in the clinical practice more frequently. These lesions include: Epidermal inclusion cyst, dermoid cyst, brachial cleft remnants, preauricular sinuses, foreign body reaction, lipoma, degenerating fibroxanthoma, osteoma cutis, ossifying hematoma etc. ^[5] To differentiate, inclusion cysts have a diffuse yellow color when filled with keratin and are softer and more palpable. They are rarely encountered in childhood. In addition, dermoid cysts are firmly attached to underlying tissue and show normal skin moving freely over the lesion. Neither exhibit irregular nodules on the skin whereas pilomatricoma does. Clinically, branchial cleft cysts present as a firm draining nodule. ^[5] Management of pilomatricomas typically involves marginal excision. Lesions on the extremities may be left untreated unless they become large or symptomatic; however, in many cases these are excised for definitive diagnosis. If the tumor adheres to the dermis, the overlying skin may be excised. The recurrence rate is low, ranging from 0 to 3%. ^[1] If a lesion recurs after excision or rapidly enlarges, it should be excised due to malignant potential or possible misdiagnosis. ^[1] In our case, the atypical appearance, bigger size and unusual site of pilomatricoma was seen.