What is the likely diagnosis of a small, hard subcutaneous mass that has been present for many years and is growing very slowly?

Likely Diagnosis: Lipoma or Epidermoid Cyst

A small, hard subcutaneous mass that grows very slowly over many years is most likely a benign lipoma or epidermoid cyst, both of which are common benign lesions that present with these exact characteristics. 1, 2

Most Common Diagnoses

Lipoma

• Lipomas are the most common benign subcutaneous tumors, typically occurring on the trunk and proximal extremities 1, 2
• They present as slow-growing, painless masses that can persist for years without significant change 1
• On examination, lipomas feel soft to firm, mobile, and well-circumscribed beneath the skin 2

Epidermoid (Epidermal Inclusion) Cyst

• Epidermoid cysts are slow-growing, painless masses that elevate the skin and frequently occur on the face, scalp, neck, and trunk 3, 4
• They typically present as firm, skin-colored nodules filled with keratinous material 3
• These cysts often have a central punctum (visible pore opening) which is a distinguishing feature 4
• They are round to oval, well-circumscribed, avascular masses located in subcutaneous tissue 4

Recommended Diagnostic Approach

Initial Evaluation

• Ultrasound is the most useful initial imaging modality for superficial/subcutaneous masses, with sensitivity 94.1% and specificity 99.7% 5
• Ultrasound can characterize lipoma features such as hyperechoic or isoechoic appearance, well-circumscribed margins, and minimal to no internal vascularity 5
• For epidermoid cysts, ultrasound shows a round to oval structure, well-circumscribed, avascular mass with dorsal acoustic amplification and lateral shadowing 4

Red Flags Requiring Further Investigation

Any mass with the following features requires MRI and/or biopsy 5, 6:

• Size >5 cm 5, 6
• Deep location (beneath the deep fascia) 6
• Rapid growth or increasing size 6
• Pain, especially night pain 6
• Atypical ultrasound features: heterogeneous echotexture, increased vascularity, or irregular margins 5

Advanced Imaging When Indicated

• MRI without and with contrast is indicated for masses with concerning features 5
• Deep lesions should be evaluated with MRI or CT to exclude involvement of underlying structures and/or liposarcoma 1

Management Based on Characteristics

For Small, Typical Benign Lesions

• Small, superficial, asymptomatic lipomas with typical imaging features can be observed, with clinical follow-up at 6-12 months to assess for growth 5
• Small lipomas can sometimes be managed with serial injections of midpotency steroids 1

Indications for Surgical Excision

Complete surgical excision is recommended for 5, 1:

• Symptomatic masses 5
• Rapidly growing masses 5
• Large masses (>5 cm) 5, 1
• Atypical masses on imaging 5
• Masses compressing other structures 1
• Any suspicion for malignancy 5, 1

Specific Considerations for Epidermoid Cysts

• The only definitive management is surgical excision with complete removal of the cyst wall or capsule 1
• All surgically excised cysts should undergo pathologic evaluation to exclude rare malignant transformation 3
• Malignant transformation into squamous cell carcinoma is rare but possible, making histological examination crucial 3

Critical Pitfalls to Avoid

• Do not assume all slow-growing masses are benign—recent data show that as many as half of the fat cells in lipomas are atypical 1
• Never dismiss masses >5 cm as simple lipomas without advanced imaging, as they have significantly higher malignancy risk and could represent liposarcoma 6, 1
• Do not rely solely on clinical examination—ultrasound confirmation is essential for accurate diagnosis 5, 1
• Avoid incomplete excision of epidermoid cysts, as leaving the cyst wall leads to recurrence 1