What is the differential diagnosis for a bony superficial swelling in the foot of a child?

Differential Diagnosis for Bony Superficial Swelling in the Foot of a Child

In children presenting with a bony superficial swelling in the foot, the differential diagnosis is age-dependent: before age 5, consider metastatic neuroblastoma or eosinophilic granuloma; above age 5, primary bone sarcomas (particularly osteosarcoma and Ewing sarcoma) become more likely, though benign bone tumors remain far more common. 1

Age-Stratified Differential Diagnosis

Children Under 5 Years

• Metastatic neuroblastoma is the most common destructive bone lesion in this age group 1
• Eosinophilic granuloma represents the other primary consideration for destructive lesions 1

Children Over 5 Years

Malignant Lesions (Rare but Critical)

• Osteosarcoma is the most frequent primary bone cancer, with peak incidence in adolescents (0.8-1.1/100,000/year at age 15-19), accounting for >10% of all solid cancers in this age group 1
• Ewing sarcoma is the third most common primary malignant bone tumor, with median age at diagnosis of 15 years; approximately 50% involve extremity bones 1
• Note: Osteosarcomas of the foot are easily misdiagnosed (50% misdiagnosis rate in one series) because they often occur in adults and may mimic benign conditions 2

Benign Lesions (Most Common)

• Osteochondroma accounts for 30-35% of benign bone tumors and is the most common overall 3, 4
• Giant cell tumor represents 20% of benign bone tumors 3
• Bone cysts (simple or aneurysmal) are among the most frequent benign lesions in pediatric foot bones 4
• Osteoid osteoma accounts for 12% of benign bone tumors 3
• Fibrous dysplasia is a common benign bone-forming tumor 4
• Enchondroma is a cartilage-forming benign tumor that is typically asymptomatic 3

Critical Clinical Features to Assess

Red Flags Suggesting Malignancy

• Persistent non-mechanical pain lasting more than a few weeks should cause immediate concern and warrant further investigation 1
• Night pain is particularly concerning for malignancy 1
• Visible swelling indicates the tumor has progressed through the cortex and distended the periosteum 1
• Recent trauma does not rule out malignancy and must not prevent appropriate diagnostic procedures 1

Physical Examination Priorities

• Assess size, consistency, location, and mobility of the swelling 1
• Evaluate the relation of swelling to the involved bone 1
• Check for regional or local lymph nodes 1

Diagnostic Algorithm

Step 1: Initial Imaging

Obtain conventional radiographs in two planes as the first investigation in all cases 1, 5

• Look for bone destruction, periosteal reaction, new bone formation, and soft tissue swelling 6
• Benign features include: well-marginated borders, no periosteal reaction, no bone destruction 6

Step 2: Advanced Imaging (When Malignancy Cannot Be Excluded)

Proceed to MRI of the whole compartment with adjacent joints if radiographs cannot exclude malignancy with certainty 1

• MRI is the best modality for local staging of extremity tumors 1
• CT should be reserved for diagnostic problems or to visualize calcification, periosteal bone formation, or cortical destruction more clearly 1

Step 3: Referral Before Biopsy

All patients with suspected primary malignant bone tumor must be referred to a bone sarcoma reference center BEFORE biopsy 1, 6

• Bone sarcomas are frequently difficult to recognize as malignant by clinicians, radiologists, and pathologists 1
• Biopsy should only be performed by the surgeon who will perform definitive resection if malignancy is confirmed 6
• Inappropriate biopsy can compromise treatment outcomes 6

Critical Pitfalls to Avoid

Diagnostic Errors

• Do not dismiss persistent pain and swelling as benign without proper imaging workup, as osteosarcomas of the foot are misdiagnosed in 50% of cases (mistaken for osteoblastoma, chondroblastoma, osteoid osteoma, osteomyelitis, or other benign conditions) 2
• Do not assume adult-age presentation excludes osteosarcoma in the foot, as foot osteosarcomas occur more commonly in adults (mean age 33 years) compared to osteosarcomas at other sites 2
• Do not allow recent trauma history to delay diagnostic evaluation, as this does not rule out malignant tumors 1

Management Errors

• Never perform or arrange biopsy before referral to a bone sarcoma center if there is any diagnostic uncertainty about malignancy 6
• Do not delay referral for suspected malignancy, as late diagnosis is common (mean 28 months from symptoms to diagnosis in foot osteosarcomas) and worsens outcomes 2

When to Refer Urgently

• Patients under 40 years with suspected malignancy or persistent pain/palpable mass require urgent referral to a bone sarcoma center 6
• Any case where malignancy cannot be excluded with certainty on radiographs warrants referral before biopsy 1
• High-grade tumors of the foot are as aggressive as osteosarcomas elsewhere and require safe-margin surgical procedures plus chemotherapy 2