Pilomatricoma

A pilomatricoma is an uncommon, benign neoplasm thought to arise from hair cortex cells. It was formerly referred to as pilomatrixoma or calcifying epithelioma of Malherbe.

Epidemiology

• the reported incidence ranges between 1 in 500 - 2000
• they make up 0.12% of cutaneous neoplasms and 20% of all hair follicle related tumours
• more common in females;  F:M ratio = 3:2 :
• more common in caucasians
• more common in children but occurrence in adults is increasingly being recognised
• there is a developmental bimodal peak during the first and sixth decades of life.
• usually develops within the first two decades of life with 40% of cases occurring before the age of 10 years and 60% before the age of 20 years
• usuall solitary but up to 3% are multiple

Clinical presentation

• painless
• slow-growing
• superficial
• mobile
• hard mass
• ranges in size from 0.5 to 5 cm in diameter
• overlying skin has a bluish-red discoloration 

Pathology

Location

Approximately 50% of cases occur in the head and neck region. This is followed by the upper extremities, trunk and the lower extremities. Less commonly seen in the frontal, temporal, cheek, periorbital and preauricular areas 

Macroscopic appearance

Well circumscribed nodulocystic tan coloured mass containing irregularly shaped, lobulated islands of cells separated by fine, fibrovascular connective tissue stroma.

Microscopic appearance

• peripheral basaloid cells are darkly stained, round or elongated, with indistinct cell borders and minimal cytoplasm ; they contain round to ovoid deeply basophilic nuclei with most showing prominent nucleoli.
• central ghost cells have abundant, pale, eosinophilic cytoplasm with well defined cell borders and central clear area.

Associations

Recognised associations include

• sarcoidosis
• Turner syndrome
• Gardner syndrome
• Steinert disease
• Rubinstein-Taybi syndrome
• myotonic muscular dystrophy (associated with multiple and recurring lesions)
• skull dysostosis

Familial forms may be present.

Variants

Pilomatrix carcinoma = malignant pilomatrixoma (rare)

Radiographic features

Plain radiography

• there may be non-specific soft tissue calcification

Ultrasound

• ovoid complex mass, often with calcification, at the junction of the dermis and subcutaneous fat with focal thinning of the overlying dermis
• appear as target lesions with a hyporeflective connective tissue capsule and central hyper-reflective epithelial cells

Computed tomography

• well-defined, subcutaneous tumours with microcalcifications.

MRI

• T1 : uniform, homogeneously low to intermediate to high signal
• T2 : variable appearance either inhomogeneous with multiple areas of intermediate signal or homogeneous with either intermediate signal intensity or high signal radiating from the centre
• T1 C+ (Gd) : variable post contrast T1 appearance, where some only enhance peripherally

Treatment

Surgical resection is often curative.

Differential diagnosis

General imaging differential considerations include

• sebaceous cyst
• ossifying haematoma
• giant-cell tumour
• chondroma
• dermoid cyst
• foreign body reaction
• degenerating fibroxanthoma
• metastatic bone formation
• osteoma cutis

Etymology

It was first described by Malherbe and Chenantias in 1880, as a subcutaneous tumour and was shown to be a derivative from hair cortex cells in 1942 by Turhan and Krainer.