What are the possible causes and treatments for a lump on the back of the skin?

Evaluation and Management of a Skin Lump on the Back

For any unexplained lump on the back, obtain an urgent ultrasound within 2 weeks to assess for soft tissue sarcoma if the lump is increasing in size, and refer to a specialist sarcoma multidisciplinary team if ultrasound findings are suggestive or uncertain with persistent clinical concern. 1

Initial Clinical Assessment

When evaluating a back lump, immediately assess for high-risk features that determine urgency:

Red Flag Features Requiring Urgent Evaluation 1

• Size greater than 3 cm - significantly increases malignancy risk 2
• Deep location - fixed to or located deep to fascia 2
• Rapid or progressive growth 2
• Firm consistency 2
• Skin ulceration or bleeding 3
• Pain 3
• Onset in neonatal period (for pediatric patients) 2

Specific Skin Changes Requiring Imaging 1

• Hairy patch - may indicate spinal dysraphism 1
• Skin discoloration or capillary malformation - associated with underlying spinal cord malformations 1
• Dermal sinus tract or draining sinus - requires MRI to evaluate for intraspinal lesions 1
• Subcutaneous mass with overlying skin changes - consider spinal cord malformation 1

Diagnostic Pathway

For Adults with Soft Tissue Lumps 1

Step 1: Urgent Ultrasound (within 2 weeks)

• Ultrasound effectively discriminates benign from malignant soft tissue masses 1
• Most soft tissue lumps are benign lipomas, which can be safely reassured after ultrasound 1
• Ultrasound is highly user-dependent; if diagnostic uncertainty exists, proceed to MRI 1

Step 2: If Ultrasound Suggests Malignancy or Remains Uncertain

• Refer via suspected cancer pathway (appointment within 2 weeks) 1
• MRI of the affected region provides the most accurate diagnostic information for soft tissue tumors of the trunk 1
• Do not perform excisional biopsy before specialist evaluation - obtain core/punch biopsy to guide surgical planning 1

For Pediatric Patients with Associated Skin Findings 1

Neonates and Infants <4 months:

• Ultrasound of the spine is the initial modality for evaluating spinal dysraphism when skin abnormalities are present 1

Older Children:

• MRI spine of the area of interest without IV contrast is appropriate initial imaging 1
• MRI with and without IV contrast if suspected neoplasm or infected dermal sinus tract 1

Common Benign Diagnoses

Lipomas 1, 4

• Most common soft tissue mass seen in primary care 1
• Typically superficial, soft, mobile, and slow-growing 4
• Atypical lipomatous tumors (well-differentiated liposarcomas) are less common but tend to be larger, deep-seated, and require specialist evaluation 1
• MRI can differentiate lipomas from atypical lipomatous tumors in up to 69% of cases based on nodularity, septations, and stranding 1

Epidermoid Cysts 4

• Common benign tumors typically treated with excision 4
• May become inflamed or infected

Dermatofibromas 4

• Firm dermal nodules, often treated with excision if symptomatic 4

Malignant Considerations

Soft Tissue Sarcomas 1

• Approximately 1% of superficial lumps in children are malignant 2
• Five risk factors identify 80% of malignant lesions: neonatal onset, rapid/progressive growth, ulceration, deep fixation, and firm mass >3 cm 2
• In absence of all five risk factors, 99.7% accuracy for benign diagnosis 2

Cutaneous Malignancies 3, 5

• Cutaneous horns on sun-exposed areas in elderly males with bleeding, pain, or rapid growth require complete excisional biopsy with 2-4 mm margins 3
• Melanoma risk factors include UV exposure, fair skin, dysplastic nevi, and family history 5
• Any suspicious pigmented lesion requires biopsy with deep scoop shave, saucerization, or full-thickness excision 5

Critical Management Principles

Biopsy Technique for Suspected Malignancy 1, 3:

• Obtain core/punch biopsy before definitive surgery to guide surgical planning 1
• Avoid shave biopsies or partial removal of keratotic projections - these prevent accurate diagnosis 3
• Complete excisional biopsy with margins is required for cutaneous horns and suspicious pigmented lesions 3, 5

When to Observe vs. Excise 2, 4:

• Approximately 6% of benign superficial lumps spontaneously regress 2
• In absence of risk factors, observation is reasonable with patient reassurance 2
• Elective excision indicated for cosmetic concerns, prevention of late infection, or diagnostic certainty 2, 4

Common Pitfalls to Avoid

• Do not perform excisional biopsy of large or deep masses before imaging - this compromises subsequent surgical planning 1
• Do not reassure patients with lipomatous masses >5 cm or deep-seated lesions without specialist evaluation - these may be atypical lipomatous tumors requiring different management 1
• Do not ignore skin markers in pediatric patients - hairy patches, discoloration, or dimples may indicate serious underlying spinal cord malformations requiring MRI 1
• Do not delay ultrasound for growing lumps - the 2-week window for urgent assessment is critical for early sarcoma detection 1