Pilomatricomas (sometimes called pilomatrixomas) are common, benign skin tumors that present as hard lumps beneath the skin’s surface. These growths originate from the cells of the hair follicle matrix, the area responsible for hair production. While they are non-cancerous, their firm feel often causes concern. This overview explains the biological nature of these tumors, how they are identified, and the standard medical approach for their resolution.
Understanding the Nature of Pilomatricomas
Pilomatricomas arise from specialized cells within the hair follicle matrix, a location deep within the skin. The development is typically linked to a somatic mutation in the \(CTNNB1\) gene, which is acquired during a person’s lifetime. This mutation results in an overactive beta-catenin protein, triggering the hair matrix cells to divide uncontrollably and form a tumor.
A defining characteristic is the subsequent process of calcification within the growing mass. As the tumor matures, the cells die and harden, creating deposits of calcium that give the lump its dense, bony feel. In later stages, ossification (the formation of actual bone tissue) may also occur within the lesion.
Pilomatricomas are most frequently observed in children and young adults, with approximately 60% of cases diagnosed before the age of 20 years. They show a slight predilection for females and most commonly appear as a single lesion. While they can arise anywhere hair follicles exist, the head, neck, and upper extremities are the most typical locations.
These masses are almost always benign. However, the presence of multiple pilomatricomas is a rare occurrence that may signal an underlying genetic condition. Syndromes such as myotonic dystrophy, Familial Adenomatous Polyposis (including Gardner syndrome), or Rubinstein-Taybi syndrome have been associated with multiple lesions. The presence of six or more pilomatricomas is highly suggestive of an underlying syndromic condition and warrants further medical investigation.
Identifying the Clinical Presentation
A pilomatricoma typically presents as a firm, solitary nodule that is slow-growing and usually measures between 0.5 and 3 centimeters in diameter. The mass is generally painless, though discomfort or tenderness can occur if the lesion becomes inflamed or infected. Because of their hard consistency beneath the skin, these tumors are often mistaken for cysts or other types of lumps.
The overlying skin may appear normal, or it can take on a reddish or bluish-red discoloration. Two specific findings during a physical examination are highly indicative of this tumor. The first is the “tent sign,” observed when the skin directly over the lesion is stretched taut.
Stretching the skin causes the firm, angulated contours of the calcified mass beneath to become visible, creating a tent-like appearance. The second characteristic sign is the “teeter-totter sign,” which demonstrates the tumor’s distinct mobility. Applying pressure to one edge of the lesion causes the opposite edge to protrude upward, much like the motion of a seesaw. These signs are helpful clues for a clinician attempting to distinguish a pilomatricoma from other subcutaneous masses, such as lipomas or dermoid cysts.
The Process of Diagnosis and Removal
Diagnosis often begins with a clinical examination where a healthcare provider assesses the physical characteristics of the lump, looking for the signs of hardness and mobility. Since the definitive diagnosis cannot be made by touch alone, imaging is sometimes utilized prior to removal.
An ultrasound is the most common imaging technique used, as it can confirm the size, depth, and internal structure of the lesion. On a scan, a pilomatricoma often appears as a well-defined mass containing bright, highly reflective spots, which represent the internal calcifications. This imaging helps the surgeon plan the most effective method for excision.
Complete surgical excision is the standard and most effective treatment, as these tumors do not spontaneously shrink or resolve on their own. Removal is generally recommended for several reasons, including the relief of discomfort and for cosmetic purposes. The most important reason for excision is to obtain a tissue sample for histopathology, which provides the definitive diagnosis.
Microscopic examination confirms the diagnosis by showing two unique cell types: darkly stained “basaloid cells” and characteristic “ghost cells.” This histological confirmation is necessary to rule out the extremely rare possibility of the malignant counterpart, pilomatrix carcinoma. The surgical procedure involves removing the entire tumor, often along with any overlying skin that is adhered to the mass, to ensure complete extraction. The risk of the pilomatricoma recurring in the same location is very low, estimated to be around 2.6%.