The immune system, a host defense system, recognizes foreign antigens and defends against invading microorganisms. It consists of primary (central) and secondary (peripheral) lymphoid organs. Primary lymphoid organs include the bone marrow and thymus, which contain lymphoid precursors and support initial antigen-independent differentiation from the immature stage to the mature stage. Secondary lymphoid organs comprise the lymph nodes, spleen, and sites of mucosa-associated lymphoid tissue, including the tonsils, appendix, and Peyer patches of the gastrointestinal tract, where antigen-dependent proliferation and differentiation of B- and T-lymphocytes take place [1].

Benign enlargement of the lymph nodes or other lymphoid organs (e.g., tonsil, spleen, thymus) is a consequence of hyperplasia of some or all of the cellular components, reflecting stimulation of the lymphoid cells or other cells by a variety of pathogens. The common causes include autoimmune disorders, drug or allergen exposures, foreign body reaction, infection with microorganisms, or undetermined etiologies. Its incidence and etiology vary with age. In the majority of children, lymph node enlargement is transient and reversible. When a biopsy is needed for clinical management purposes, the primary goal of histologic evaluation is to determine the nature of the process: whether it is a benign or malignant, hematopoietic or non-hematopoietic cell proliferation.

Bone marrow failure syndromes comprise a heterogeneous group of disorders that may be inherited or acquired; may be static or progressive; and may affect any one hematopoietic lineage individually, multiple lineages, or all lineages collectively. The unifying feature consists of the failure of the bone marrow to supply the body with normal quantities of the mature forms of any of the three hematopoietic-lineage elements. Although bone marrow failure syndromes may engender a predisposition to the development of associated malignancies, and primary neoplastic processes involving the bone marrow and peripheral blood may meet the criteria of impaired hematopoiesis, discussion of neoplastic processes associated with cytopenias will be largely covered elsewhere.

Langerhans cell histiocytosis (LCH), juvenile xanthogranuloma (JXG), and Rosai-Dorfman-Destombes disease (RDD) can each manifest as a focal lesion, as multiple lesions, or as a widespread systemic disorder with organ involvement. Erdheim-Chester disease (ECD) is a rare systemic disease process in children, with more frequent adult presentations. New distinct emerging entities covered include ALK-positive histiocytosis and post-leukemia/lymphoma histiocytic lesions. These histiocytic lesions are now best classified as myeloid-derived inflammatory neoplastic disorders composed of clonal dendritic- or macrophage-/monocyte-derived cells that infiltrate tissues and are driven by recurrent kinase-activating alterations, most often in the mitogen-activated protein kinase (MAPK), PI3K-AKT, and receptor tyrosine kinase (RTK) signaling pathways, which have all had a long history of being associated with human neoplasia, with ERK overexpression noted in many of these neoplasms (Fig. 28.1; Table 28.1) (1–3). Furthermore, these oncological signaling cascades are critical to the intranuclear regulation of transcription factors that serve as key factors influencing cellular proliferation, survival, and differentiation (4).

Non-hematopoietic tumors can involve the bone marrow (BM). BM metastasis represents advanced-stage disease and is associated with inferior outcomes [1]. Examination of the BM is a common procedure in the initial evaluation of children with solid tumors for tumor staging, subsequent treatment monitoring, and documentation of recurrent disease [1]. Less frequently, it provides a diagnosis from an occult primary site. The frequency of BM metastasis depends on the tumor type. Neuroblastoma, Ewing sarcoma, and alveolar rhabdomyosarcoma are more frequently present with BM metastases, whereas metastases from other soft tissue sarcomas, brain tumors, retinoblastomas, and hepatoblastomas are much less common. Some tumors, such as Wilms tumor, almost never metastasize to the BM and do not require BM examination in general.

Clinical presentations of solid tumor metastasis to the BM are variable. Bone pain, pathologic fractures, and cytopenia are frequently recorded, but they are non-specific.

Blastic plasmacytoid dendritic cell neoplasms (BPDCNs) are an aggressive hematopoietic malignancy derived from precursors to plasmacytoid dendritic neoplasms, with a predilection toward involvement of the skin, bone marrow, and peripheral blood.1

Childhood myelodysplastic syndromes (MDSs) are clonal myeloid neoplasms characterized by impaired production of circulating hematopoietic elements (generally, hemoglobin concentration < 10 g/dL, platelet count < 100 × 109/L, or absolute neutrophil count < 1.8 × 109/L), frequently accompanied by morphologic dysplasia and recurrent genetic abnormalities and lacking features of overproduction of additional hematopoietic lineage elements. In light of its rarity in comparison to its clinicopathologic mimics, minimal diagnostic criteria have been proposed (see Table 13.1).

Acute myeloid leukemia (AML) is a heterogenous group of diseases representing a clonal expansion of immature, non-lymphoid, bone marrow–derived cells that involve the bone marrow and blood and may also be present in other tissues [1,2]. In the pediatric population, AML is less common than lymphoblastic leukemia, accounting for approximately 18% of childhood leukemia diagnoses [3]. Although the cause of AML in many cases is unknown [1], as molecular genetics has been evolving over the last couple of decades, new molecular technology has enhanced our knowledge of the underlying genetic defects and gene mutations associated with the development of AML. It has been demonstrated that AML occurs more commonly in children with pre-existing genetic disorders, such as Down syndrome (DS) or Fanconi anemia, and familial cases of AML are now recognized [1]. A subset arises from pre-existing myelodysplastic syndrome (MDS) or after therapy for another neoplasm.

Hematolymphoid malignancies represent an area in which ancillary studies offer particularly valuable information for diagnosis, classification, and prognosis, as well as risk-stratified and targeted therapy. The results of multiple test modalities, including flow cytometry, immunohistochemistry, and cytogenetic and molecular genetic analyses, should be integrated and interpreted within the context of morphologic evaluation. In this chapter, the principles, general technical aspects, and clinical applications of these ancillary tests are discussed.

Anemia is defined as a hemoglobin (Hb) level or red blood cell (RBC) mass of less than the 5th percentile for age [1]. Approximately 20% of American children have anemia [1]. Most children with anemia are asymptomatic, and the condition is often discovered incidentally on routine laboratory testing.

Hemoglobin is a tetrameric protein composed of two pairs of globin chains complexed with four heme groups and is responsible for the transportation of oxygen of RBCs [2,3]. Normal adult RBCs contain 97% hemoglobin A (HbA; α2β2), 2% HbA2 (α2δ2), and 1% fetal Hb (HbF; α2γ2). At birth, HbF is predominant. Within the first year of life, it is largely replaced by HbA [3].

Peripheral (mature) T-cell lymphomas (PTCLs) are much less common than B-cell lymphomas. Derived from post-thymic T-cells, PTCLs generally arise in lymphoid tissues “peripheral” to the thymus, such as the lymph nodes, spleen, gastrointestinal tract, and skin. Because natural killer (NK) cells and T-cells arise from a common progenitor cell and have some overlapping properties, NK-cell lymphomas are considered together with PTCLs in the World Health Organization (WHO) classification system [1].

PTCLs occur most frequently in adults and less frequently in children and adolescents. They comprise about 10–15% of all non-Hodgkin lymphomas (NHLs) in pediatric age groups [2]. NK-cell neoplasms are even rarer. Unlike adults, where there is a broad spectrum of T-cell neoplasms, the most common type of PTCL in pediatric patients is anaplastic lymphoma kinase (ALK)-positive anaplastic large cell lymphoma, with other subtypes of PTCL (0.9%) being observed much less frequently [3].

Chronic myeloid leukemia (CML), BCR-ABL1+, is a clonal myeloproliferative neoplasm (MPN) with a hyperabundance of granulocyte forms, defined by the presence of the BCR-ABL1 fusion gene, most frequently due to the chromosomal translocation t(9;22)(q34.1;q11.2) resulting in the Philadelphia (Ph) chromosome.

Lymphoblastic leukemias/lymphomas constitute a family of malignant lymphoid neoplasms that morphologically and immunophenotypically recapitulate the features of early lymphoid precursors of either B- or T-cell origin [1–4]. The distinction between acute lymphoblastic leukemia (ALL) and lymphoblastic lymphoma (LBL) is arbitrary [4]. The term LBL is used when the disease is confined to a predominant extramedullary mass lesion with no or minimal involvement (<25%) of peripheral blood and bone marrow (BM). The cases with ≥25% blasts in either peripheral blood or BM are staged and treated as ALL [1,4].

ALL and LBL have many characteristics in common. They are morphologically indistinguishable, share similar immunophenotypes, and have overlapping cytogenetic profiles. It is generally believed that the two entities are biologically equivalent, represent a clinical continuum or different stages of the same disease, and are categorized as such by the 2017 World Health Organization (WHO) classification scheme [2–5].

Non-Hodgkin lymphomas (NHLs) of childhood and adulthood differ considerably in biology, incidence, treatment, and outcome [1]. Pediatric NHLs are nearly evenly split between B-cell and T-cell neoplasms, whereas in adults, nearly 80% of NHLs are of B-cell origin [2]. The vast majority of mature B-cell NHLs in children and adolescents are high-grade, including Burkitt lymphoma/leukemia (BL), diffuse large B-cell lymphoma (DLBCL), and other less common histologies. Extranodal disease is more common in children than in adults [2]. Pediatric-type follicular lymphoma and nodal and extranodal marginal-zone lymphoma are uncommon in the pediatric population, accounting for <10% of mature B-cell lymphomas among patients <18 years of age [1]. The current classification of mature B-cell lymphoma is reviewed comprehensively in the 2017 WHO Classification published by the World Health Organization (WHO). Some of these entities occur only in adults. The entities that can occur in pediatric patients are discussed in the following sections [2].

Myeloid/lymphoid neoplasms with eosinophilia and gene rearrangement are clonal neoplasms with characteristically prominent eosinophilic proliferation and defining cytogenetic rearrangements. (See Table 11.1.)

Hodgkin lymphoma (HL) is a family of unique B-lineage lymphoma subtypes and encompasses two morphologically, immunophenotypically, and clinically distinct categories: classic Hodgkin lymphoma (CHL) and nodular lymphocyte-predominant Hodgkin lymphoma (NLPHL). HL usually affects the lymph nodes and is composed of a small number of large, dysplastic mononuclear and multinucleated neoplastic cells (Hodgkin/Reed-Sternberg [HRS] cells, or lymphocyte-predominant [LP] cells/lymphocytic and histiocytic [L&H] cells) set in the background of benign inflammatory elements with or without abundant band-like and/or more diffuse collagen fibrosis. The neoplastic cells are often surrounded by T-lymphocytes [1,2].

The difference between NLPHL and CHL is that the B-cell program is generally preserved, and the neoplastic cells exhibit the overt B-lineage phenotype in NLPHL, but the neoplastic cells in CHL display an aberrant differentiation program with a characteristic CD15+, CD30+, and CD45– immunophenotype [1,2].