2. ABBREVIATED WHO CLASSIFICATION AS APPLIED TO PEDIATRICS
Precursor B- and T-cell neoplasms
Precursor B-lymphoblastic leukemia/lymphoma
Precursor T-lymphoblastic leukemia/lymphoma
Mature B-cell neoplasms
Marginal zone B-cell lymphoma
Follicular lymphoma
Diffuse large B-cell lymphoma
Mediastinal (thymic) large B-cell lymphoma
Burkitt lymphoma
Mature T-cell and NK-cell neoplasms
Hepatosplenic T-cell lymphoma
Primary cutaneous CD30+ T-cell lymphoproliferative disorders
Primary cutaneous anaplastic large cell lymphoma (C-ALCL)
Lymphomatoid papulosis
Angioimmunoblastic T-cell lymphoma
Peripheral T-cell lymphoma, unspecified
Anaplastic large cell lymphoma

3. Immunophenotypic analysis is always required for an accurate diagnosis, and cytogenetic studies in non-Hodgkin lymphomas are frequently helpful. Malignant adenopathy can be categorized morphologically by the architectural changes seen in affected lymph nodes. Within each lineage of the WHO classification, distinct diseases are defined based on a combination of clinical, morphologic, immunophenotypic, and molecular genetic features

4. The vast majority of pediatric cases of non-Hodgkin lymphoma fall into one of the four categories: diffuse large B-cell lymphoma, Burkitt lymphoma, lymphoblastic lymphoma (T-cell or B-cell), and anaplastic large cell lymphoma. Indolent lymphomas composed of small lymphocytes (e.g., small lymphocytic lymphoma, marginal zone lymphoma, mantle cell lymphoma, follicle center cell lymphoma) are extremely rare in children, and they should be diagnosed with caution.

5. Precursor B Lymphoblastic Lymphoma
15% of all cases of lymphoblastic lymphoma
most common in older children and young adults
present with rapidly enlarging lymph nodes or soft tissue masses.
In contrast to T-cell lymphoblastic lymphoma, the B-lymphoblastic lymphoma rarely involves the mediastinum.

6. The distinction between B-cell lymphoblastic lymphoma and precursor B-cell acute lymphoblastic leukemia is made through examination of the bone marrow biopsy specimen; in cases in which fewer than 25% of the marrow cells are blasts, a diagnosis of B-cell lymphoblastic lymphoma should be made.

7. some cases of B-ALL/LBL have specific recurrent genetic abnormalities that are associated with distinctive clinical and phenotypic properties, have important prognostic implications, or demonstrate other evidence that they are mutually exclusive of other entities.

8. These cases should not be classified as B-ALL/ LBL, NOS, but rather according to their genetic abnormalities. There are currently nine genetically defined BALL/LBLs.

9. One feature typical of lymphoblastic lymphoma (B or T) is infiltration through perinodal fat and linear infiltrates in the capsular collagen .The nodal architecture is effaced by a diffuse proliferation of small and intermediate cells (12 to 14 mm) with fine or speckled chromatin, small or indistinct nucleoli, and scant cytoplasm

10. Precursor B lymphoblastic lymphoma demonstrating diffuse architectural effacement of the node in infiltration into adjacent perinodal fat and linear infiltrates of the capsular collagen. (Hematoxylin and eosin stain 10×.)

11. Coarse azurophilic granules are present in some lymphoblasts in approximately 10% of cases. In some cases, the lymphoblasts have cytoplasmic pseudopods(hand-mirror cells). Normal B-cell precursors (haematogones) can mimic lymphoblasts, but they typically have even higher nuclear:cytoplasmic ratios, more-homogeneous chromatin, and no discernible nucleoli.

12. The chromatin is fine and nucleoli are indistinct in lymphoblastic lymphoma

13. Cytochemistry
Cytochemistry seldom contributes to the diagnosis of ALL. Lymphoblasts are negative for MPO. Granules, if present, may stain light grey with Sudan Black B but are less intense than myeloblasts. Lymphoblasts may show periodic acid-Schiff (PAS) positivity, usually in the form of coarse granules. They may react with nonspecific esterase, with a multifocal punctate or Golgi region pattern that shows variable inhibition with sodium fluoride.

14. A CD45 (dim to negative), terminal deoxynucleotidyl transferase (TdT) positive,CD19+, CD20-, sIg- phenotype sets these tumors apart from lymphomas composed of mature (nonblastoid) Bcells, including Burkitt lymphoma
The myeloid-associated antigensCD13 and CD33 may be expressed
In tissue sections, CD79a and PAX5 are most frequently used to demonstrate B-cell differentiation, but CD79a is positive in many cases of TALL and is not specific

15. Almost all cases are positive for CD10 (common acute lymphocytic leukemia antigen or CALLA) .
Differential diagnosis: other small blue cell tumors including granulocytic sarcoma/chloroma, Ewing sarcoma/primitive neuroectodermal tumor , embryonal rhabdomyosarcoma
Distinguished by phenotype, although care should be taken not to overvalue results from any single stain .For example, like Ewing sarcoma, lymphoblastic lymphomas are often CD45- and CD99+.

16. Genetic profile Antigen receptor genes
Nearly all cases of B-ALL have clonal rearrangements of IGH. In addition, T-cell receptor (TR) gene rearrangements may be seen in a substantial proportion of cases (as many as 70%) Therefore,IGH and TR gene rearrangements are not helpful for lineage assignment

17. Cytogenetic abnormalities and oncogenes
del(6q), del(9p ), and del(12p): do not have an impact on prognosis.
t(17;19)(q22;p13.3) resulting in TCF3-HLF is associated with a very poor prognosis,

18. Diffuse Large B-Cell Lymphoma
present with steadily enlarging peripheral lymphadenopathy or extranodal disease (soft tissue, bone, oropharynx).
Although rare, a primary mediastinal B-cell diffuse large cell lymphoma has also been described in children

19. DLBCL has a diffuse growth pattern and may have intermixed fibrosis (particularly in the mediastinum). Cytoplasm is abundant and may be either amphophilic or densely eosinophilic .The cytology of the tumor cells ranges from that of reactive immunoblasts (round nucleus, thick nuclear membrane, single large eosinophilic nucleolus, abundant cytoplasm), to polylobate and even frankly anaplastic multilobate cells.

21. Immunophenotypically, tumor cells mark with pan B-cell markers (CD19, CD20, CD79A, CD22) and are surface Ig+ (except in mediastinal large B-cell lymphoma). CD10 is positive in a minority.

22. Burkitt Lymphoma Burkitt lymphoma takes three epidemiologic forms:
Endemic Burkitt lymphoma
Sporadic Burkitt lymphoma
Immunodeficiency related Burkitt lymphoma

23. Burkitt lymphoma diffusely effaces the nodal architecture
Burkitt lymphoma diffusely effaces the nodal architecture. A monomorphic proliferation of intermediate-sized cells is seen (nuclear size similar to that of histiocytes or endothelial cells); the round or oval nuclei have a thick nuclear membrane and two to four nucleoli, and the cytoplasm is moderately amphophilic

24. Many Burkitt lymphomas have a somewhat cohesive appearance, and the cell borders maintain a molded contour, particularly at the periphery. The mitotic rate is high (MIB-1/KI-67 is positive in >95% of cells), and necrosis is often present,particularly at the periphery. Evenly distributed macrophages containing cellular debris give a mottled (“starry sky”) appearance to Burkitt lymphoma at low power.

26. immunophenotype
Mature surface Ig+ B-cell, both CD19 and CD20 CD10 :positive. BCL-2 expression :not present. TdT expression is lacking.

27. Differential diagnosis: lymphoblastic lymphoma and rapidly replicating large cell lymphomas.
Cytogenetic analyses play a key role in confirming the diagnosis.
FISH studies are very helpful in demonstrating translocations that deregulate expression of the protooncogene c-myc (chromosome 8) paired with either the heavy-chain loci (chromosome 14) or light-chain locus (chromosome 2 and 22)

28. Other Rare B-Cell Lymphoma in Children
rarely seen in children:
Follicular lymphoma
Nodal marginal zone lymphoma

29. Follicular lymphoma grade 2 or 3 many are curable.
Morphologically, they range from the typical low-power pattern of crowded monomorphic small-to-medium sized follicles to large expansive follicles and even “floral variant”
Phenotypically like adults, most cases are CD10+ and BCL-6+, but unlike adults, are often BCL-2- and most do not have underlying BCL-2/IgH (14;18) translocation.
The small minority with BCL-2 translocations appear to have a worse prognosis

31. Pediatric nodal marginal zone lymphoma, like its adult counterpart, can be particularly difficult to diagnose in that these lymphomas often only partially efface architecture with an interfollicular distribution.
Follicular colonization may be present as well as follicles resembling those of progressive transformation of follicular centers.
Most cases have CD5-CD10-phenotype with monotypic surface Ig. Many have monotypic cytoplasmic immunoglobulin in plasma cells on paraffin immunoperoxidase stains.

32. T-Lymphoblastic Leukemia/Lymphoma
commonly seen in adolescents and young adults
approximately 30% of all pediatric non-Hodgkin lymphomas
distinguished from T-cell acute lymphoblastic leukemia by the demonstration of limited bone marrow disease (<25% involvement)
frequently located in the mediastinum

33. Typically composed of small to medium-sized blast cells with scant cytoplasm, moderately condensed to dispersed chromatin, and inconspicuous nucleoli, involving bone marrow and blood (T-ALL) or presenting with primary involvement of the thymus or of nodal or extranodal sites (T-LBL).
The lymphoblasts in T-ALL/LBL are morphologically indistinguishable from those of B-ALL/LBL.
The number of mitotic figures is reported to be higher in T-ALL than in B-ALL.

35. A starry sky effect may be present, sometimes mimicking Burkitt lymphoma, although the nucleoli and cytoplasm are typically less prominent in T-LBL
Cases with histological findings of T-LBL with a significant infiltrate of eosinophlis among the lymphoma cells may be associated with eosinophilia, myeloid hyperplasia, and an 8p11.2 cytogenetic abnormality involving the FGFR1 gene

36. Cytochemistry
T-lymphoblasts frequently show focal acid phosphatase activity in smear and imprint preparations, although this is not specific.

37. Immunophenotype: is that of an immature T-cell with CD45 (dim-to- negative), TdT+, cytoplasmic CD3+, usually surface CD3-, CD2+, CD7+ with variable expression of CD1a, CD4, CD5, and CD8. HLA-DR is negative. CD10 is expressed in 25% of cases.

38. differential diagnosis
B-cell lymphoblastic lymphoma,
Ewing sarcoma/primitive neuroectodermal tumor
embryonal rhabdomyosarcoma,
Tumor karyotype is less helpful in the prognosis of T-cell lymphoblastic lymphoma

39. Peripheral T-Cell Lymphoma
Peripheral T-cell lymphomas including angioimmunoblastic T-cell lymphoma represent only a small fraction of lymphomas in children and heterogeneity is their histologic hallmark

40. At low power,the lymph node architecture is either diffusely effaced or shows interfollicular expansion. Thick-walled vessels are more prominent in peripheral T-cell lymphomas than in B-cell non-Hodgkin lymphomas, and the even mixture of atypical small, intermediate and large lymphoid elements is another clue to the lineage of this type of lymphoma

41. Scattered eosinophils and plasma cells may also be seen.
Irregularity of the nuclear contour may be particularly prominent, and, in further contrast to lymphomas of B-cell lineage, these tumors in many cases have cells with a clear cytoplasm.
Virally mediated or drug hypersensitivity immunoblastic reactions may mimic peripheral T-cell lymphoma, as can Hodgkin lymphoma, B-cell lymphomas rich in T-cells(“T-cell-rich B-cell lymphomas”), and Fas mutation-related (ALPS).

42. Immunophenotype :
TdT-, CD45+ with most having a CD3+, CD45Ro+, CD4+, CD8- helper T-cell phenotype
Other T-cell antigens (e.g., CD2, CD5, CD7) are variably expressed, and the loss of these markers is characteristic of peripheral T-cell lymphoma

43. Anaplastic Large Cell Lymphoma
a special type of peripheral T-cell lymphoma
Cervical lymphadenopathy is particularly common in some series and the skin, bone, and soft tissue may be secondarily involved
Involved lymph nodes may exhibit either a diffuse or a sinusoidal pattern of tumor cell infiltration, and the latter may be mistaken for metastases of a solid tumor.

44. The tumor cells of T-cell anaplastic large cell lymphoma are often very large (>20mm) and have bizarre, lobulated, or wreath-like nuclei (hallmark cells) with small-to-large nucleoli and abundant eosinophilic cytoplasm
Pleomorphic ,sarcomatoid, histiocyte-neutrophil-rich, and even monomorphic/small cell variants

46. The common denominator in most pediatric cases being the t(2 ;5) karyotype abnormality with ALK positivity.
Membranous staining (usually associated with Golgi positivity) for CD30 is required for the diagnosis
variable expression of pan Tcell markers CD3, CD7, and CD5
Epithelial membrane antigen (EMA) and CD45 are usually but not always positive
CD4, CD2, CD43, and CD45 RO are often helpful.

47. Most cases are positive for cytotoxic molecules like TIA-1, granzyme B, and perforin.
Only a minority are CD8+.
Stains for EBV-like LMP and EBER are consistently negative .
Staining for EBV, ALK, PAX-5, and a battery of T-cell and cytotoxic granule markers is helpful in evaluating CD45-, CD30+ cases of ALCL in which the major consideration is Hodgkin lymphoma with a syncytial growth phase.

49. Having sufficient tissue for approach is critical
Having sufficient tissue for approach is critical. Great caution is advised when core needle biopsies are used for the primary diagnosis of lymphoma; fine-needle aspiration is generally inadequate for this purpose. Morphology and immunophenotype are sufficient for the diagnosis of most lymphoid neoplasms. However, no one antigenic marker is specific for any neoplasm, and a combination of morphological features and a panel of antigenic markers are necessary for correct diagnosis

50. . Most B-cell lymphomas have characteristic immunophenotypic profiles that are very helpful in diagnosis. However, immune profiling is somewhat less helpful in the subclassification of Tcell lymphomas.

51. Although certain antigens are commonly associated with specific disease entities, these associations are not entirely disease-specific. CD30 is a universal feature of anaplastic large cell lymphoma, but can also be expressed in other T-cell and B-cell lymphomas and in classic Hodgkin lymphoma (CHL).

52. CD56 is a characteristic feature of nasal NK/T-cell lymphoma, but it can also be found in other T-cell lymphomas, in plasma cell neoplasms, and in nonlymphoid cells such as blastic plasmacytoid dendritic cell neoplasms.

53. Within a given disease entity, variation in immunophenotypic features can be seen. Most hepatosplenic T-cell lymphomas are of the gamma delta T-cell phenotype, but some cases are of alpha beta T-cell derivation. Some follicular lymphomas are CD10-negative. An aberrant immunophenotype may suggest or help to confirm a diagnosis of malignancy

54. Genetic features are playing an increasingly important role in the classification of lymphoid malignancies. However, the molecular pathogenesis of many forms of lymphoma remains unknown. Genetic studies, in particular PCR studies and FISH, are valuable diagnostic tools for the determination of clonality in B-cell and T-cell proliferations (aiding in the differential diagnosis with reactive hyperplasia) and for the identification of translocations associated with some disease entities.