Precursor T Lymphoblastic Leukemia / Lymphoma
Differential Diagnosis
| Type B1 Thymoma |
Precursor T Lymphoblastic Leukemia / Lymphoma |
| Scattered epithelial cells (may require anti-keratin stain) |
Only keratin positive cells should be overrun thymus |
| Scattered pale medullary foci |
Lacks medullary foci |
| Thick fibrous capsule and septa produce large lobules |
Lacks thick fibrous capsule and septa and lobular growth pattern |
| Typically older adult but may occur in children |
Typically adolescent or pediatric |
| No circulating blasts |
Circulating blasts |
| Variable expression of surface CD3, CD4, CD8 in a given case |
Uniform phenotype in a given case |
Lymphocytes have same basic phenotype in both: predominantly immature T cells with mature T cells restricted to medullary foci
May be a significant problem on small biopsies and in cases analyzed by flow
| Normal Thymus |
Precursor T Lymphoblastic Leukemia / Lymphoma |
| No circulating blasts |
Circulating blasts |
| Histopathology shows normal epithelial component |
No neoplastic epithelial component on histopathology (effaced epithelial component may be present) |
| Histopathology shows lobularity with cortical / medullary differentiation |
Histopathology shows no normal architecture |
Flow cytometry of normal thymus will show a preponderance of precursor T cells with a phenotype that may be indistinguishable from leukemia / lymphoma
Both are TdT+ and may be CD34+
| Burkitt Lymphoma |
Precursor T Lymphoblastic Leukemia / Lymphoma |
| Presents as soft tissue mass, rarely mediastinal |
Frequently presents as mediastinal mass |
| B cell phenotype (usually CD19+, CD20+, CD79a+) |
T cell phenotype (usually CD3+, CD7+ at a minimum) |
| Mature phenotype (TdT and CD34 negative) |
Immature phenotype (usually TdT+, sometimes CD34+) |
| Translocation involving myc gene |
No myc translocation |
| CD4+ CD56+ Hematodermic Neoplasm (Blastic NK Cell Lymphoma) |
Precursor T Lymphoblastic Leukemia / Lymphoma |
| Skin lesions usually predominate |
Frequently presents as mediastinal mass |
| Usually negative for T cell markers (rare cases may be positive for TdT) |
Immature T cell phenotype (usually CD3+, CD7+, TdT+ at a minimum) |
