What is the step‑by‑step investigative hierarchy for pediatric lymphoma and when should one proceed to the next step?

Investigative Hierarchy for Pediatric Lymphoma

The diagnostic workup for pediatric lymphoma follows a systematic approach starting with tissue diagnosis via excisional/incisional biopsy, followed by comprehensive staging workup including bone marrow evaluation, CNS assessment, and imaging—with each step completed before initiating treatment unless the patient is critically ill. 1

Step 1: Tissue Diagnosis (DO NOT SKIP)

Obtain excisional or incisional biopsy of the most accessible site immediately. 1

• Send fresh tissue in saline to pathology (not formalin) to preserve viability
• Perform touch preparation for cytologic examination
• Minimum requirements to start treatment: morphology + flow cytometry 1
• Complete workup includes: immunohistochemistry, immunophenotyping, and cytogenetics

When to proceed: If patient is critically ill with oncologic emergency (tumor lysis syndrome, superior vena cava syndrome, respiratory compromise, spinal cord compression), you may begin treatment based on morphology and flow cytometry alone while awaiting complete immunophenotyping and cytogenetics. 1 Otherwise, complete the full pathologic evaluation before moving forward.

Key Pathologic Markers:

• Immunophenotyping: CD20+, surface immunoglobulin+, CD10+ (most cases), CD3-, TdT- 1
• Cytogenetics: FISH for C-MYC rearrangement is essential 1
• Ki-67 ≥95% supports Burkitt lymphoma diagnosis 1

Step 2: Staging Workup (Simultaneous with Step 1)

Once tissue diagnosis is underway, immediately initiate comprehensive staging evaluation. 1

Required Components:

Laboratory Analysis:

• Complete blood count, comprehensive metabolic panel
• LDH, uric acid (tumor burden markers)
• Coagulation studies

Bone Marrow Assessment:

• Bilateral bone marrow aspirate AND biopsy (both required) 1
• Determines stage IV disease if positive

CNS Evaluation:

• Lumbar puncture with CSF cytology 1
• CNS is positive if ANY of the following: 1
  • Any lymphoma cells in CSF by cytology
  • Any CNS tumor mass on imaging
  • Cranial nerve palsy (unexplained by extracranial tumor)
  • Clinical spinal cord compression
  • Parameningeal extension (cranial/spinal)

Imaging:

• FDG-PET/CT or FDG-PET/MRI if available 1
• High-resolution CT or MRI of involved sites
• Critical caveat: Do NOT delay treatment to obtain PET scan 1
• PET does not replace diagnostic-quality CT/MRI 1

Cardiac Assessment:

• Baseline echocardiogram or MUGA scan (required before anthracycline therapy) 1

When to proceed: Complete all staging elements unless patient has life-threatening emergency requiring immediate treatment. The staging determines risk stratification and treatment intensity.

Step 3: Risk Stratification

Use the International Pediatric NHL Staging System (IPNHLSS) published in 2015, NOT the older Murphy/St. Jude system. 1

The IPNHLSS better accounts for:

• Extranodal involvement (skin, bone, kidney, ovarian)
• Improved detection of bone marrow and CNS disease
• Newer histologic entities

Stage IV disease is defined by:

• CNS involvement (any of the criteria above)
• Bone marrow involvement
• Distant organ spread

Step 4: Treatment Initiation

Begin treatment immediately after completing staging workup, or sooner if oncologic emergency exists. 1

Special Populations:

• Adolescents/Young Adults (17-21 years): Treat with pediatric protocols, not adult regimens—pediatric approaches show superior progression-free survival despite similar radiation exposure 2
• Double/triple-hit lymphomas: Treat identically to other pediatric BL/DLBCL despite MYC + BCL2/BCL6 rearrangements 1
• EBV-positive disease: Does not change treatment approach 1

Critical Pitfalls to Avoid:

1. Never perform fine needle aspiration alone—insufficient tissue for complete immunophenotyping and cytogenetics
2. Never delay treatment for PET scan—clinical urgency supersedes optimal imaging 1
3. Never send tissue in formalin if fresh tissue evaluation is needed—send in saline 1
4. Never skip bilateral bone marrow biopsy—unilateral sampling is inadequate
5. Never assume CNS is negative without LP—approximately 9% of Burkitt and 3% of DLBCL have CNS involvement 1
6. Never treat AYA patients with adult protocols when pediatric protocols are available—10% reduction in progression-free survival with adult approaches 2

The only acceptable reason to skip or abbreviate steps is life-threatening oncologic emergency, in which case begin treatment after obtaining minimum diagnostic tissue (morphology + flow cytometry) while completing remaining workup concurrently. 1