Pilomatricoma is an uncommon, harmless, skin lesion derived from hair matrix cells. It is also spelled ‘pilomatrixoma’, and sometimes known as ‘calcifying epithelioma of Malherbe’.
Pilomatricoma is most often diagnosed in young children but may also affect adults. Single skin-coloured or purplish lesions arise on the head and neck, but they may occur on any site. They are characterised by calcification within the lesion, which makes it feel hard and bony, and often results in an angulated shape (the ‘tent’ sign).
The cause is unknown. However, recently some genetic changes have been found in the affected hair cells (an overactive proto-oncogene called BCL-2 suggests the normal process of cell death is suppressed and mutations in CTNNB1 suggest loss of regulation of a protein complex called beta-catenin/LEF).
Complications of pilomatricoma are rare. However, occasionally they grow to giant size (several centimetres in diameter), and pilomatrical carcinoma (cancer) has been very rarely reported.
A few cases have been reported of multiple pilomatricomas in association with the rare neurological condition myotonic dystrophy. Individual cases have also been reported of pilomatricomas arising in patients with a variety of other genetic disorders. The vast majority are not associated with any other abnormality.
How is the diagnosis made?
If the nature of the skin lesions is uncertain, ultrasound scan may be recommended. The scan of pilomatricoma is described as a doughnut within the dermis (mid layer of the skin) with a tail (the tail denotes calcification). Alternatively, the calcification may be detected by X-ray.
Quite often, a small biopsy (when a tiny piece of skin is removed under local anaesthetic) will help to establish the cause of the lesion. Alternatively the whole lesion can be removed providing both diagnosis and treatment. The histology of pilomatricoma is striking. It may show a sharply demarcated tumour surrounded by a fibrous capsule or a poorly demarcated tumour without capsule. There are darkly stained ‘basophilic’ cells and ‘shadow’ cells with missing nuclei. Calcium deposits are found in most lesions.
What is the treatment for pilomatricoma?
It is generally recommended that the lesions be cut out (excised). They do not disappear by themselves, and if incompletely removed, they may recur.