Solitary Nodular Lesion on the Scalp

The Diagnosis: Pilomatricoma

Pilomatricoma, first described by Malherbe and Chenantais¹ in 1880, is a benign appendageal tumor derived from hair follicle matrix cells. It classically manifests as a solitary, asymptomatic, firm dermal nodule with a normal overlying epidermis. Less common morphologic variants include perforating, lymphagiectatic, keratoacanthomalike, pigmented, and anetodermalike surface changes.² Inflammation and erosion through the skin surface are observed in the rare perforating variant, as seen in our patient. The average size is 1 cm, and it rarely exceeds 3 cm in diameter.³ The tumors predominantly occur on the head, neck, and upper extremities, with only 9.5% on the scalp.² It may occur at any age, though it has a bimodal distribution with peaks in childhood and in adults older than 60 years. A slight preponderance in females has been observed with a female to male ratio of 1.5 to 1.² Although our patient is black, most reported cases have occurred in individuals of European descent. Because cases of pilomatricoma are not systematically reported, it is uncertain if this finding represents a publication bias or if race is an actual risk factor. Multiple pilomatricomas and familial cases have been described in association with myotonic dystrophy, Turner syndrome, Gardner syndrome, Rubinstein-Taybi syndrome, polyfactorial coagulopathy, trisomy 9, xeroderma pigmentosum, and basal cell nevus syndrome.^2,4

It has been shown that the proliferating cells of pilomatricomas stain with antibodies directed against Lef1 (lymphoid enhancer binding factor 1), a marker from hair matrix cells, providing biochemical evidence for the morphologic appearance of these neoplasms.⁵ Pilomatricomas have been associated with B-cell/chronic lymphocytic leukemia lymphoma 2 gene, BCL2, expression, a proto-oncogene that suppresses apoptosis in benign and malignant neoplasms, which may contribute to the pathogenesis of these tumors.⁶ Pilomatricomas also have been associated with β-catenin mutation, expression of Bmp2 (bone morphogenetic protein 2), and human hair keratin basic 1.^7-9

Definitive diagnosis is obtained through biopsy, looking for characteristic histopathologic findings. The lesion usually is found in the lower dermis and subcutaneous fat. However, in the perforating variant, the lesion is more superficial, located in the papillary and mid dermis, as seen in our patient.¹⁰

^{Islands of pale eosinophilic shadow cells with an intense dermal infiltrate consisting of lymphocytes, histiocytes, plasma cells, and neutrophils (H&E, original magnification ×400).}

Pilomatricomas are sharply demarcated, often surrounded by a connective-tissue capsule. Histopathologic analysis reveals islands of epithelial cells comprised of 3 subtypes: basophilic cells with scant cytoplasm, shadow cells with a central pallor (Figure), and transitional cells between the former 2 cellular types.¹¹ The number of basophilic and transitional cells is inversely related to the number of shadow cells. In older lesions, the shadow cells predominate, while the basophilic cells are few in number or absent. Calcium deposits are seen in 80% of lesions with von Kossa staining.¹²

Transformation into malignancy, known as pilomatrical carcinoma, is rare. These malignant neoplasms are characterized by aggressive biologic behavior such as recurrence, diffuse spread, or metastasis, or by cytologic abnormalities such as poor cellular organization, squamous differentiation, and conspicuous mitotic activity.¹³ The recent growth of the long-standing lesion in our patient might be interpreted as a sign of malignant transformation. However, this observation may be related to the intense inflammatory reaction supported by the histopathology.

Pilomatricomas are not associated with mortality. Pilomatrical carcinomas are uncommon but are locally invasive and can cause visceral metastases and death. Spontaneous regression has never been observed and medical treatment is ineffective. The treatment of choice is incision and curettage or surgical excision.¹⁴ Although recurrence has only been reported in 2.6% of cases from a large case series (N=228), patients should be monitored after surgical excision.¹²