What is the most likely diagnosis and recommended management for a 21‑year‑old male with a 5‑month‑old, 1 cm × 1 cm, firm, movable, non‑tender, non‑warm nodular mass on the right occiput that has remained stable?

Most Likely Diagnosis and Management

The most likely diagnosis is a benign subcutaneous lesion—either an epidermoid cyst, lipoma, or less commonly nodular fasciitis—and the next step is ultrasound imaging to characterize the mass before considering excisional biopsy if features are atypical or the patient desires removal. 1

Initial Diagnostic Approach

Clinical Features Favoring Benignity

• The 5-month stability without change in size or shape strongly suggests a benign process, as malignant soft tissue masses typically demonstrate progressive growth 2, 3
• The movable, non-tender, non-warm characteristics are consistent with benign subcutaneous lesions rather than inflammatory or malignant processes 2, 3
• The firm, nodular texture at 1×1 cm in a young adult (21 years old) most commonly represents an epidermoid cyst, lipoma, or reactive process 2, 4

Key Historical Points to Clarify

• Inquire specifically about preceding trauma or infection, as nodular fasciitis can follow minor injury in approximately 10-15% of cases 2, 4
• Document any recent growth velocity—even benign lesions like nodular fasciitis can enlarge rapidly over weeks to months before stabilizing 2, 4
• Assess for pain with palpation or functional impairment, though absence does not exclude pathology 2

Recommended Diagnostic Workup

First-Line Imaging: Ultrasound

• Ultrasound should be the initial imaging modality to characterize the mass as cystic versus solid, assess vascularity, and evaluate depth of extension 1
• Simple cystic appearance on ultrasound (anechoic, well-defined, posterior acoustic enhancement) confirms a benign epidermoid or sebaceous cyst and requires only reassurance unless symptomatic 1
• Solid, hypoechoic lesions with well-defined margins and minimal vascularity suggest lipoma or fibroma 1

When to Escalate to MRI

• If ultrasound shows irregular margins, solid components with internal vascularity, or deep extension beyond the subcutaneous plane, obtain MRI before any surgical intervention 1, 5
• MRI is particularly valuable for distinguishing benign reactive processes (nodular fasciitis) from rare low-grade malignancies like dermatofibrosarcoma protuberans, which can present as firm subcutaneous masses in young adults 5
• T1-weighted and T2-weighted sequences with contrast help characterize tissue composition and enhancement patterns 3, 5

Differential Diagnosis by Likelihood

Most Common (>90% probability)

• Epidermoid (sebaceous) cyst: Most common subcutaneous mass in young adults, typically mobile and firm 1
• Lipoma: Soft to firm, mobile, slow-growing; ultrasound shows homogeneous hyperechoic mass 1

Less Common but Important to Consider

• Nodular fasciitis: Benign reactive proliferation of fibroblasts that can mimic sarcoma; occurs in head/neck in 10-20% of cases, often in children and young adults 2, 4

  • Typically presents as rapidly enlarging firm mass over 2-8 weeks, though can stabilize 2, 4
  • Histologically shows spindle cells in myxoid stroma; requires excisional biopsy for definitive diagnosis 2, 4
  • Surgical excision is curative with no recurrence in most series 2

• Pilomatricoma: Calcifying epithelial tumor, more common in children but can occur in young adults 2

Rare but Must Not Miss

• Dermatofibrosarcoma protuberans (DFSP): Low-grade malignant spindle cell tumor with high local recurrence risk 5

  • MRI shows ill-defined margins, infiltration of adipose tissue, irregular shape, and high enhancement (type-III time-intensity curve) 5
  • Requires wide surgical excision with 2-3 cm margins 5

• Masson's tumor (intravascular papillary endothelial hyperplasia): Rare benign vascular lesion that can mimic angiosarcoma 3

  • Imaging shows well-defined mass with heterogeneous signal; only histopathology distinguishes from malignancy 3

Management Algorithm

If Ultrasound Shows Simple Cyst

• Reassure patient; no further workup needed 1
• Offer excision only if symptomatic (pain, cosmetic concern, recurrent infection) 1

If Ultrasound Shows Solid Mass with Benign Features

• Proceed directly to excisional biopsy for definitive diagnosis and treatment 2, 4
• Excisional biopsy is both diagnostic and curative for most benign subcutaneous masses 2, 4
• Ensure complete excision with preservation of surrounding normal tissues 2

If Ultrasound Shows Atypical Features

• Obtain MRI with contrast before biopsy or excision 1, 5
• Atypical features include: irregular margins, deep extension, significant vascularity, or infiltration of adjacent structures 1, 5
• If MRI suggests possible malignancy (DFSP), coordinate with surgical oncology for wide excision 5

Common Pitfalls to Avoid

Do Not Skip Imaging

• Never proceed directly to excision without ultrasound characterization, as imaging may reveal features requiring wider surgical planning or specialist referral 1, 5

Do Not Assume Stability Equals Benignity in All Cases

• While 5-month stability is reassuring, low-grade malignancies like DFSP can have indolent growth patterns over months to years 5
• Nodular fasciitis can stabilize after initial rapid growth, mimicking a stable benign lesion 2, 4

Do Not Rely on Clinical Examination Alone

• Firm, mobile masses can represent both benign (lipoma, cyst) and malignant (DFSP) processes 5
• Absence of pain or warmth does not exclude pathology requiring excision 2, 3

Ensure Adequate Histopathologic Evaluation

• If excisional biopsy is performed, submit the entire specimen for pathology to avoid missing focal areas of concern 2, 4
• Nodular fasciitis can be mistaken for sarcoma on frozen section; ensure experienced pathologist reviews permanent sections 2, 4

Follow-Up After Excision

• Mean follow-up of 6-12 months is appropriate to monitor for recurrence, though recurrence is rare (<2%) for benign lesions after complete excision 2, 3
• If nodular fasciitis is diagnosed, reassure patient that recurrence is extremely rare after complete excision 2, 4
• If DFSP is diagnosed, long-term surveillance is mandatory due to high local recurrence risk even after wide excision 5