A Mediastinal Mass in a 3-Year-Old
A 3-year-old girl presents with a 3-week history of cough, mild dyspnea, and a visibly swollen neck. Chest radiograph shows a large anterior mediastinal mass with tracheal deviation. CT shows a homogeneous mass encasing the great vessels without calcification. CBC is normal. A biopsy is performed. The pathologist must decide which ancillary studies to order and how to handle the specimen to avoid delay.
1. What is the differential for an anterior mediastinal mass in a young child, and what specimen-handling steps should be taken at the time of biopsy?
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Differential in a young child:
- T-cell lymphoblastic lymphoma/leukemia - most common in this age group; often presents with respiratory compromise.
- Hodgkin lymphoma - less common under 5 years but possible.
- Germ cell tumor (teratoma, yolk sac tumor) - anterior mediastinum; check AFP/β-hCG.
- Thymoma - rare in children.
- Anaplastic large cell lymphoma and other non-Hodgkin lymphomas.
- Neuroblastoma - usually posterior mediastinum but can occur.
- Langerhans cell histiocytosis and infectious/granulomatous processes.
Specimen handling at biopsy (critical for lymphoma):
- Fresh tissue in RPMI for flow cytometry and cytogenetics.
- Touch imprints for rapid morphology.
- Formalin for histology and IHC.
- Glutaraldehyde if electron microscopy might be needed (rarely).
- Molecular testing (clonality, translocations) on fresh or FFPE tissue.
- Do not place all tissue in formalin before flow is considered.
Given the risk of airway compromise, the biopsy should be coordinated with anesthesia and oncology, and the pathologist should be available for frozen section to confirm adequacy.
2. Flow cytometry shows a population of immature T cells (CD3+, CD4/CD8 double positive, CD1a+, TdT+, surface CD3 negative). How do you interpret this, and what molecular findings would you seek?
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The immunophenotype - CD1a+, TdT+, surface CD3 negative, CD4/CD8 double positive - is that of cortical thymocytes, i.e., T-lymphoblastic lymphoma/leukemia. Normal thymus also contains such cells, so the diagnosis depends on architecture and clonality: effacement of architecture, sheets of blasts, and absence of normal maturation.
Molecular/cytogenetic findings to seek:
- T-cell receptor (TCR) gene rearrangement - clonal rearrangement supports neoplasia, though TCR rearrangement can be seen in normal thymocytes; clonality is supportive, not absolute.
- NOTCH1 mutations are common in T-ALL/LBL.
- Translocations involving TCR loci (7q34, 14q11) with partners such as TAL1, LMO2, TLX1, TLX3, MYC.
- CDKN2A/B deletions and other copy-number changes.
Practical point: In a child with a mediastinal mass and this phenotype, the distinction between T-lymphoblastic lymphoma and T-ALL is largely clinical (marrow/blood involvement). Bone marrow and CSF evaluation are needed for staging. The pathologist should state the immunophenotype and recommend staging studies rather than overinterpreting a single marker.
3. The biopsy is limited and shows crush artifact with few viable cells. How should the pathologist proceed, and what are the risks of a repeat biopsy in this patient?
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With crush artifact and limited viable cells, the pathologist should:
- Use flow cytometry on any fresh sample - it can salvage a diagnosis even when morphology is poor.
- Perform IHC on the limited tissue (CD3, CD1a, TdT, CD20, CD30, keratins, etc.) to characterize the population.
- Consider FISH or PCR on the available material for clonality/translocations.
- Communicate directly with the clinical team about adequacy; a definitive diagnosis may require repeat biopsy.
Risks of repeat biopsy:
- Airway compromise under anesthesia - the mass encases great vessels and deviates the trachea; anesthesia risk is substantial.
- Bleeding and tumor seeding (rare but relevant).
- Delay in starting therapy.
Alternatives to repeat biopsy:
- Bone marrow and CSF evaluation - if marrow is involved, the diagnosis of leukemia can be established without a second mediastinal biopsy.
- Pleural/pericardial fluid cytology and flow.
- Empiric therapy in a critically ill child with a characteristic presentation is sometimes undertaken by oncology, though histologic confirmation is preferred.
The pathologist's job is to extract maximum information from the available tissue, state the limitations clearly, and participate in a multidisciplinary decision about repeat sampling versus empiric treatment.