What is the most likely diagnosis and appropriate next steps for a 41‑year‑old man with a 0.5 × 0.3 cm soft, movable, painless fleshy mass on the medial posterior left thigh just above the buttock, present for one year with slow growth and no other symptoms?

Most Likely Diagnosis: Benign Soft Tissue Lesion (Fibroma, Lipoma, or Skin Tag)

The most likely diagnosis is a benign soft tissue lesion such as a fibroma, lipoma, or skin tag, given the small size (0.5 × 0.3 cm), superficial location, mobility, slow growth over one year, and complete absence of symptoms in this 41-year-old man.

Clinical Reasoning

Why This is Likely Benign

• Size criterion: The lesion measures only 0.5 × 0.3 cm, which is well below the 5 cm threshold that triggers concern for soft tissue sarcoma 1
• Superficial location: The mass is described as a "fleshy movable mass" on the skin surface of the posterior thigh, not arising from deep soft tissues where sarcomas typically originate 1
• Mobility: The freely movable nature suggests it is not fixed to underlying structures, which would be concerning for malignancy 1
• Symptom profile: Complete absence of pain, particularly night pain (a red flag for bone or soft tissue malignancy), makes sarcoma unlikely 1
• Growth pattern: While the lesion has grown slowly over one year, the absolute size remains very small, inconsistent with aggressive behavior 1

When to Worry About Malignancy

Referral to a sarcoma center would be indicated if this patient had 1, 2:

• A deep soft tissue mass (below the muscular fascia)
• Any mass >5 cm in diameter, even if superficial
• Rapid growth over weeks to months
• Fixed or immobile mass
• Night pain or constitutional symptoms

Recommended Next Steps

Immediate Management

Perform a simple excisional biopsy in your office or outpatient setting, as this small superficial lesion can be both diagnostic and therapeutic in a single procedure 1

• For superficial lesions <5 cm, excisional biopsy is the most practical diagnostic approach and provides definitive treatment if benign 1
• Send the entire specimen for histopathologic examination to confirm the diagnosis 1
• Document the exact anatomic location and ensure complete excision with clear margins 1

If Excisional Biopsy is Not Immediately Feasible

Consider clinical observation with the following parameters:

• Re-examine in 4-6 weeks to assess for any change in size, consistency, or development of symptoms
• Educate the patient about red flag symptoms: rapid growth, development of pain (especially night pain), fixation to underlying structures, or size increase beyond 5 cm 1
• If any concerning features develop, obtain MRI of the thigh and refer to a sarcoma center before any biopsy 1, 2

Critical Pitfalls to Avoid

• Do not ignore this lesion entirely: While likely benign, tissue diagnosis provides definitive reassurance and prevents delayed diagnosis if it were an unusual presentation of malignancy 1
• Do not perform core needle biopsy for this small superficial lesion: Core biopsy is reserved for larger or deep masses where excisional biopsy would be morbid; for small superficial lesions, excisional biopsy is preferred 1, 2
• Do not refer to a sarcoma center at this stage: This would represent inappropriate resource utilization for a lesion that does not meet referral criteria 1
• If pathology returns with any suggestion of sarcoma or atypical features: Immediately refer to a specialized sarcoma center for expert pathology review and management planning, as the initial excision may have compromised oncologic margins 1, 2

Documentation for Pathology

When submitting the specimen, include 2:

• Patient age (41 years)
• Exact anatomic location (medial posterior left thigh, 1 cm from buttock crease)
• Duration of lesion (one year)
• Clinical impression (benign soft tissue lesion vs. skin tag)
• Request routine histopathologic examination with assessment of margins