# Pediatric Hematopathology: Unique Entities in Children

## Introduction

Pediatric hematopathology encompasses hematologic malignancies and reactive conditions with distinct biologic features, molecular profiles, and clinical behaviors compared to adult counterparts. Understanding these age-specific entities is essential for accurate diagnosis, risk stratification, and treatment planning.

## Pediatric Acute Lymphoblastic Leukemia

### Overview

Acute lymphoblastic leukemia is the most common childhood malignancy, accounting for approximately **25%** of all pediatric cancers. Peak incidence occurs at **2-5 years** of age, with B-lymphoblastic leukemia (B-ALL) representing approximately 85% of cases and T-lymphoblastic leukemia (T-ALL) approximately 15%. Overall survival exceeds **90%** with modern risk-adapted therapy. Diagnosis requires a bone marrow with **more than 20% blasts**, flow cytometry immunophenotyping, and cytogenetic and molecular studies that are essential for classification.

### Key Genetic Subtypes in B-ALL

**ETV6::RUNX1** [t(12;21)] is the most common pediatric translocation, present in approximately 25% of cases, and carries an excellent prognosis. It is cryptic on conventional karyotype and requires FISH or molecular testing for detection. **Hyperdiploidy** (51-67 chromosomes) accounts for approximately 25% of cases and confers a favorable prognosis, particularly with trisomies of chromosomes 4, 10, and 17. **KMT2A (MLL) rearrangements** are common in infants under one year, are aggressive, and carry a poor prognosis. **BCR::ABL1** [t(9;22)], the Philadelphia chromosome, occurs in approximately 3-5% of pediatric ALL, with outcomes improved by TKI incorporation. **TCF3::PBX1** [t(1;19)] carries an intermediate prognosis with CNS relapse risk. **Philadelphia-like (Ph-like) ALL** harbors ABL-class fusions, JAK/STAT alterations, and CRLF2 rearrangements, carries a poor prognosis, and is targetable with TKIs or JAK inhibitors. **iAMP21** (intrachromosomal amplification of chromosome 21) is adverse and requires intensified therapy. **DUX4-rearranged ALL** is associated with ERG deletion and carries a favorable prognosis.

| Genetic Subtype | Frequency | Prognosis | Key Features |
|---|---|---|---|
| ETV6::RUNX1 [t(12;21)] | ~25% | Excellent | Cryptic; requires FISH |
| Hyperdiploidy (51–67) | ~25% | Favorable | Trisomies 4, 10, 17 best |
| KMT2A rearrangements | ~5% (>75% in infants) | Poor | Infant ALL |
| BCR::ABL1 [t(9;22)] | 3–5% | Improved with TKI | Philadelphia chromosome |
| Ph-like ALL | ~15% | Poor | Targetable (TKI, JAK inhibitors) |
| TCF3::PBX1 [t(1;19)] | ~5% | Intermediate | CNS relapse risk |
| iAMP21 | ~2% | Adverse | Requires intensified therapy |
| DUX4-rearranged | ~5% | Favorable | ERG deletion |

### Minimal Residual Disease (MRD)

MRD is the most powerful prognostic factor in pediatric ALL. It is measured by **multiparameter flow cytometry** (sensitivity 10^-4) or **PCR/NGS-based** methods (sensitivity 10^-5 to 10^-6). Day 29 (end of induction) MRD guides risk stratification and treatment intensity, and MRD-negative status (less than 0.01%) is associated with excellent outcomes.

![Flow cytometry dot plots showing B-ALL immunophenotype with CD19+, CD10+, TdT+ blast population](images/ball-flow-cytometry.jpg)

## Pediatric Acute Myeloid Leukemia

### Key Differences from Adult AML

Pediatric AML comprises approximately 15-20% of pediatric leukemias, with overall survival of approximately **65-70%**. **Core binding factor AML** (RUNX1::RUNX1T1, CBFB::MYH11) is favorable and accounts for approximately 25% of pediatric AML. **KMT2A-rearranged AML** is more common in infants and young children. **NUP98 rearrangements** (NUP98::NSD1, NUP98::KDM5A) are adverse and more prevalent in the pediatric population. The **RAM immunophenotype** (CD56+, dim CD45, dim CD38, low or absent HLA-DR) is associated with NUP98::KDM5A and adverse prognosis. Myeloid sarcoma (chloroma), an extramedullary form of AML, is more common in children and may precede bone marrow disease.

### Transient Abnormal Myelopoiesis (TAM)

TAM is unique to neonates with **Down syndrome** (trisomy 21). Peripheral blood blasts display megakaryoblastic morphology and harbor GATA1 mutations. The condition is **self-resolving** in most cases within 1-3 months without treatment. However, **10-20%** of affected infants subsequently develop myeloid leukemia of Down syndrome (ML-DS) within 1-4 years. ML-DS is GATA1-mutated, highly chemosensitive, and carries an excellent prognosis with reduced-intensity therapy.

## Pediatric Lymphomas

### Burkitt Lymphoma

Burkitt lymphoma is the most common pediatric non-Hodgkin lymphoma, with endemic (EBV-associated, jaw mass) and sporadic (abdominal) forms. The hallmark is **MYC translocation**, with t(8;14) being most common and t(2;8) and t(8;22) as variants. Histologically, there are diffuse sheets of intermediate-sized cells with multiple nucleoli, a high mitotic rate, and a "**starry sky**" pattern created by tingible body macrophages. The Ki-67 proliferation index approaches **100%**. Despite being highly aggressive, Burkitt lymphoma is very chemosensitive, with cure rates exceeding 90% with intensive short-duration chemotherapy.

### Anaplastic Large Cell Lymphoma (ALCL)

ALCL is predominantly a pediatric and young adult disease. The **ALK-positive** subtype carries the t(2;5) NPM1::ALK translocation most commonly, is CD30+ and ALK+, and has an excellent prognosis (approximately 90% overall survival). Hallmark cells are large cells with eccentric horseshoe or kidney-shaped nuclei. Presentation may include skin involvement, lymphadenopathy, and systemic symptoms. **Breast implant-associated ALCL** is a distinct ALK-negative entity seen in adults.

### Pediatric Hodgkin Lymphoma

Hodgkin lymphoma accounts for approximately 6% of childhood cancers with peak incidence in adolescence. **Nodular lymphocyte-predominant Hodgkin lymphoma (NLPHL)** features LP cells (popcorn cells) in a nodular B-cell background that are CD20+, CD15-, and CD30-. It is indolent and more common in young boys. **Classic Hodgkin lymphoma** features Reed-Sternberg cells that are CD30+, CD15+, weak PAX5+, and usually CD20 negative, with a strong EBV association in younger children.

![Burkitt lymphoma with starry sky pattern and high-grade morphology on H&E](images/burkitt-starry-sky.jpg)

## Pediatric Histiocytic Disorders

### Langerhans Cell Histiocytosis (LCH)

LCH is a clonal proliferation of **Langerhans-type cells** with BRAF V600E mutation in approximately 50-60% of cases. The clinical spectrum ranges from single-system disease (bone, skin) to multisystem involvement (liver, spleen, bone marrow, CNS). Histologically, the cells have characteristic **grooved (coffee-bean) nuclei** with eosinophilic cytoplasm, accompanied by a mixed inflammatory infiltrate with eosinophils. Immunohistochemistry shows **CD1a+, langerin (CD207)+, and S100+** staining. Electron microscopy demonstrates Birbeck granules (pentalaminar rod-shaped structures), which were historically diagnostic but are rarely needed with current immunohistochemical panels. Treatment ranges from observation to chemotherapy depending on the extent of disease.

### Juvenile Xanthogranuloma (JXG)

JXG is the most common non-LCH histiocytic disorder of childhood. It presents as benign, often self-resolving skin nodules that are rarely systemic. Histologically, **Touton giant cells** (characterized by a ring of nuclei with foamy cytoplasm) are present alongside histiocytes and an inflammatory infiltrate. The immunophenotype is CD1a-negative, CD207-negative, and factor XIIIa-positive, which distinguishes it from LCH.

### Hemophagocytic Lymphohistiocytosis (HLH)

HLH is a life-threatening hyperinflammatory syndrome that may be primary (familial, genetic: PRF1, UNC13D, STX11) or secondary (triggered by infection, malignancy, or autoimmune disease). Diagnostic criteria include fever, splenomegaly, cytopenias, hyperferritinemia, hypertriglyceridemia, low or absent NK cell activity, elevated soluble IL-2 receptor, and **hemophagocytosis** in bone marrow or other tissues. Bone marrow biopsy shows activated macrophages engulfing blood cells, which may be focal and require careful search. Early recognition and treatment with etoposide-based protocols and dexamethasone is critical, with stem cell transplant indicated for familial forms.

## Unique Reactive Conditions

### Autoimmune Lymphoproliferative Syndrome (ALPS)

ALPS results from defective **FAS-mediated apoptosis** caused by germline FAS, FASL, or CASP10 mutations. It presents with chronic lymphadenopathy, splenomegaly, and autoimmune cytopenias. The characteristic finding is an expansion of **double-negative T cells** (CD3+, CD4-, CD8-, TCR alpha-beta+) exceeding 2.5% of lymphocytes. Patients have an increased risk of both Hodgkin and non-Hodgkin lymphoma.

### Infectious Mononucleosis and EBV-Related Lymphoproliferations

**EBV-positive reactive lymphadenopathy** shows paracortical expansion with immunoblasts and may mimic lymphoma. **Post-transplant lymphoproliferative disorder (PTLD)** is EBV-driven in most pediatric cases and spans a spectrum from reactive hyperplasia to monomorphic lymphoma. **Chronic active EBV infection** involves persistent EBV infection with T-cell or NK-cell proliferation and is more prevalent in East Asian populations.

![Langerhans cell histiocytosis showing cells with grooved nuclei, eosinophils, and CD1a positivity](images/lch-histology.jpg)

## Clinical Pearls

ETV6::RUNX1 translocation is the most common genetic abnormality in pediatric B-ALL and carries an excellent prognosis, but it is cryptic on conventional karyotyping and requires FISH or molecular methods for detection. Transient abnormal myelopoiesis in neonates with Down syndrome is a self-resolving condition driven by GATA1 mutations, but 10-20% of affected infants will later develop myeloid leukemia of Down syndrome. Langerhans cell histiocytosis is a clonal neoplastic disorder with BRAF V600E mutations in approximately half of cases, and the diagnosis rests on the immunophenotype (CD1a+, langerin+) rather than electron microscopy. Minimal residual disease assessment at end of induction is the single most powerful prognostic factor in pediatric ALL and directly guides risk-adapted therapy intensity.

## References

1. Hunger SP, Mullighan CG. Acute lymphoblastic leukemia in children. *N Engl J Med*. 2015;373(16):1541-1552.
2. Rasche M, et al. Pediatric acute myeloid leukemia: from diagnosis to treatment. *J Clin Med*. 2021;10(20):4686.
3. Allen CE, et al. Langerhans-cell histiocytosis. *N Engl J Med*. 2018;379(9):856-868.
4. Cairo MS, et al. Childhood and adolescent non-Hodgkin lymphoma: new insights in biology and critical challenges for the future. *Pediatr Blood Cancer*. 2019;66(4):e27594.