Could That Hard Lump Under Your Skin Be a Pilomatricoma?

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Understanding Pilomatricoma Through Imaging and Pathology

Keywords: Pilomatricoma, skin nodule, pediatric skin tumor, calcified lump, benign skin tumor, pilomatricoma ultrasound, pilomatricoma CT, ghost cells, cutaneous adnexal tumor, hard lump under skin

What Is Pilomatricoma?

Pilomatricoma (also known as calcifying epithelioma of Malherbe) is a rare, benign skin tumor that originates from hair matrix cells. Most frequently seen in children and adolescents, this tumor presents as a firm, subcutaneous nodule and typically grows slowly. Though generally harmless, it may resemble more concerning conditions, making proper diagnosis crucial.

Clinical Case Snapshot

A 10-year-old boy presented with a slowly enlarging, firm mass on his right cheek, which had first been noticed a year prior. The lesion was painless, with normal overlying skin and no signs of inflammation. Imaging via ultrasound and computed tomography (CT) revealed a well-defined nodule with internal calcifications. Following complete surgical excision, histopathologic evaluation confirmed the diagnosis of pilomatricoma.

What Causes Pilomatricoma?

While the exact cause remains unclear, pilomatricomas arise from the abnormal proliferation of hair matrix cells. Some studies have identified mutations in the β-catenin gene, which plays a critical role in cell adhesion and signaling, as a contributing factor.

Pathophysiology

Pilomatricomas are composed of two main cell types:

  • Basophilic cells resembling immature hair matrix cells

  • Ghost cells, which are enucleated, eosinophilic cells that represent keratinized remnants.

Over time, the lesion undergoes calcification and fibrosis, leading to a hard, palpable mass beneath the skin.

Epidemiology

  • Age: Primarily affects children and young adults under 30, with a peak incidence in the first decade of life.

  • Sex: Slight female predominance.

  • Location: Common sites include the face, neck, upper arms, and shoulders.

Clinical Features

  • Firm, painless nodule under the skin

  • Normal or slightly bluish-gray overlying skin

  • Typically measures 0.5–3 cm

  • Rarely associated with inflammation or ulceration

Imaging Characteristics

Ultrasound (US)

  • Hypoechoic or heterogeneous echotexture

  • Presence of high echogenic foci indicating internal calcification

  • No invasion into adjacent soft tissues

Computed Tomography (CT)

  • Well-demarcated, high-density lesion

  • Internal calcifications are hallmark features

  • Confined to the subcutaneous fat layer

Treatment Options

The gold standard for treatment is complete surgical excision.

  • Recurrence rates are very low (0–3%) when the condition is adequately removed.

  • Radiation or chemotherapy is not necessary.

  • Incomplete excision may lead to regrowth, emphasizing the importance of clear surgical margins.

Prognosis

Pilomatricoma is a benign tumor with an excellent prognosis.

  • Malignant transformation is extremely rare.

  • No documented cases of metastasis.

  • With proper treatment, patients typically recover without recurrence.

Quick Reference Summary

CategoryDetails
ConditionPilomatricoma
OriginHair matrix cells
Genetic Linkβ-catenin mutations
Peak Age GroupChildren, adolescents
Key Imaging SignsCalcified nodule on CT, hyperechoic spots on US
TreatmentSurgical excision
PrognosisExcellent

Diagnostic Images

  • CT Scan: High-density lesion with central calcification


  • Ultrasound: Punctate echogenic foci within a hypoechoic mass

  • Pathology Slide: Clear ghost cells indicating keratinized degeneration


Pilomatricoma Quiz Questions

1. What is the hallmark histological feature of pilomatricoma?

A. Keratin pearls
B. Multinucleated giant cells
C. Spindle cells
D. Ghost cells
Explanation: Ghost cells are anucleated keratinized cells found in the center of pilomatricomas and are pathognomonic for this tumor.

2. Which imaging feature is most commonly associated with pilomatricoma?
A. Hypodense lesion without enhancement
B. Fluid-fluid levels
C. Calcification
D. Central necrosis
Explanation: Pilomatricomas frequently show internal calcification on both ultrasound and CT, making this a key imaging clue.

3. What is the standard treatment for pilomatricoma?

A. Radiation therapy
B. Chemotherapy
C. Surgical excision
D. Watchful waiting
Explanation: Complete surgical excision is curative for pilomatricoma, and recurrence is rare when the entire lesion is removed.

4. Which statement about malignant transformation is accurate?

A. Malignant pilomatricomas are common in children
B. It is extremely rare and the tumor is typically benign
C. All pilomatricomas eventually become cancerous
D. Pilomatricomas often metastasize to lymph nodes
Explanation: Pilomatricomas are almost always benign. Malignant transformation into pilomatrix carcinoma is extremely rare.

5. Which age group is most affected by pilomatricoma?

A. Elderly adults
B. Middle-aged individuals
C. Children and adolescents
D. Neonates
Explanation: The peak incidence of pilomatricoma is in the first two decades of life, especially among pediatric and adolescent patients.

Final Thoughts

Pilomatricoma is a benign but often misdiagnosed tumor that can be confidently identified with modern imaging techniques. Early evaluation and complete surgical removal lead to excellent outcomes with minimal recurrence risk. When a child or adolescent presents with a firm, subcutaneous lump, consider pilomatricoma in your differential, especially when imaging reveals calcification. Collaboration with a radiologist and dermatologic surgeon can significantly enhance diagnostic accuracy and patient satisfaction.

References

  1. Julian CG, Bowers PW. A clinical review of 209 pilomatricomas. J Am Acad Dermatol. 1998;39(2 Pt 1):191–195. doi:10.1016/S0190-9622(98)70174-6

  2. Moehlenbeck FW. Pilomatrixoma: A statistical study. Arch Dermatol. 1973;108(4):532–534.

  3. Kaddu S, et al. Morphological stages of pilomatricoma. Am J Dermatopathol. 1996;18(4):333–338.

  4. Lan MY, et al. Pilomatricoma of the head and neck: A review of 179 cases. Arch Otolaryngol Head Neck Surg. 2003;129(12):1327–1330.

  5. Yoshimura Y, et al. Imaging of pilomatricoma: Diagnostic value of ultrasound and CT. Br J Radiol. 2018;91(1083):20180025.

  6. Smith VC, Hession MT. Pilomatricoma: Clinical features and diagnostic challenges. Dermatol Clin. 2020;38(1):69–76.

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