Hematologic Malignancies — Leukemia, Lymphoma, Myeloma, and Related Complications

The recognition of hematologic malignancies as distinct disease entities evolved alongside advances in microscopy, immunology, and molecular biology. Before the mid-nineteenth century, patients who presented with massive splenomegaly, diffuse lymphadenopathy, or unexplained bone pain were described in vague clinical terms without a unifying pathologic framework. The transformation of hematology from descriptive morphology to precision medicine represents one of the most dramatic arcs in the history of oncology, and understanding this trajectory provides essential context for the classification systems tested on USMLE Step 2.

The central clinical question that unifies this topic is deceptively simple: a patient presents with cytopenias, lymphadenopathy, a monoclonal protein, or an incidental blast count on a CBC — how do you systematically classify the malignancy, anticipate its complications, and initiate appropriate management? Answering this question requires fluency in cell lineage, immunophenotyping, cytogenetics, and staging — all of which are high-yield for boards.

Hematologic malignancies arise from the clonal proliferation of cells at various stages of hematopoietic differentiation. The clinical behavior of each neoplasm — whether it presents acutely with blast crisis or indolently with a monoclonal protein — is determined by the cell of origin, the degree of differentiation arrest, and the genetic mutations driving clonal expansion. These three variables form the conceptual scaffold for understanding every hematologic cancer.

Understanding where each malignancy arises in the hematopoietic tree is fundamental to classification and clinically predictive of behavior. The following diagram traces normal hematopoiesis from the pluripotent stem cell through committed progenitors to mature effector cells, annotating where clonal arrest and expansion produce each category of hematologic cancer.

Notice how the anatomic compartment of disease correlates with differentiation stage. Acute leukemias represent early arrest: the malignant clone never matures past the blast stage, leading to marrow failure and circulating blasts. Chronic leukemias involve more mature cells that retain some function but accumulate relentlessly. Lymphomas form tissue-based masses because the malignant lymphocytes home to and expand within nodal architecture. Myeloma's clinical manifestations — lytic bone lesions, renal failure, hypercalcemia — are direct consequences of the plasma cell's unique biology: it secretes vast quantities of immunoglobulin and resides in the bone marrow niche.

Each hematologic malignancy is driven by specific genetic lesions that confer a proliferative advantage, block differentiation, or both. Understanding these drivers is clinically important because they serve as diagnostic markers, prognostic indicators, and therapeutic targets. The following section highlights the most board-relevant genetic associations across leukemias, lymphomas, and myeloma.

Acute Leukemias

In acute myeloid leukemia (AML), the WHO defines the blast threshold at ≥ 20% in bone marrow or peripheral blood, though certain recurrent genetic abnormalities (e.g., t(8;21), inv(16), t(15;17)) are diagnostic regardless of blast percentage. The two-hit model posits that AML requires both a Class I mutation (conferring proliferative advantage, e.g., FLT3-ITD, RAS) and a Class II mutation (blocking differentiation, e.g., PML-RARA, RUNX1-RUNX1T1). Acute promyelocytic leukemia (APL) with t(15;17) PML-RARA deserves special attention: it presents with DIC and is a medical emergency, but responds dramatically to all-trans retinoic acid (ATRA) plus arsenic trioxide, achieving cure rates exceeding 90%.

In acute lymphoblastic leukemia (ALL), the most common malignancy in children, the prognosis varies dramatically by cytogenetics. Hyperdiploidy (> 50 chromosomes) and t(12;21) ETV6-RUNX1 confer a favorable prognosis in children. Conversely, the Philadelphia chromosome t(9;22) BCR-ABL1 is associated with poor prognosis and requires addition of a tyrosine kinase inhibitor (TKI) to chemotherapy. Notably, Ph+ ALL increases in incidence with age and is present in approximately 25% of adult ALL cases.

Chronic Leukemias & MPNs

Chronic myeloid leukemia (CML) is the paradigmatic example of targeted therapy success. The BCR-ABL1 fusion oncoprotein resulting from t(9;22) constitutively activates tyrosine kinase signaling. Imatinib and subsequent-generation TKIs (dasatinib, nilotinib, ponatinib) achieve major molecular responses in >80% of patients in chronic phase. CML progresses through three phases — chronic, accelerated, and blast crisis — and transformation to blast crisis resembles acute leukemia (either myeloid or lymphoid) with a markedly worse prognosis.

Chronic lymphocytic leukemia (CLL) is the most common leukemia in adults in Western countries. The malignant clone consists of mature-appearing CD5+ B lymphocytes. Rai and Binet staging systems stratify by lymphadenopathy, organomegaly, and cytopenias. Key prognostic markers include del(17p)/TP53 mutation (poor prognosis) and del(13q) as sole abnormality (favorable). CLL may transform to diffuse large B-cell lymphoma (Richter transformation), a life-threatening complication.

Lymphomas

Hodgkin lymphoma is characterized by the Reed-Sternberg cell (CD15+, CD30+, usually CD45−), a large binucleated cell surrounded by a reactive inflammatory milieu. HL typically presents in a bimodal age distribution (15–35 and >55) with contiguous nodal spread, B symptoms, and mediastinal involvement. The Ann Arbor staging system (I–IV) plus the Cotswold modification (A/B, X for bulky disease) guides therapy. Among non-Hodgkin lymphomas, diffuse large B-cell lymphoma (DLBCL) is the most common aggressive NHL, treated with R-CHOP. Follicular lymphoma is the most common indolent NHL, associated with t(14;18) and BCL2 overexpression, and follows a relapsing-remitting course. Burkitt lymphoma harbors t(8;14) MYC-IgH, has the highest proliferation rate of any human tumor (Ki-67 ≈ 100%), and presents with rapidly growing masses — often in the jaw (endemic/EBV-associated) or abdomen (sporadic).

Multiple Myeloma

Multiple myeloma is defined by clonal plasma cell proliferation (≥ 10% in bone marrow or biopsy-proven plasmacytoma) plus end-organ damage summarized by the mnemonic CRAB: Calcium elevation, Renal insufficiency, Anemia, Bone lesions. Updated diagnostic criteria also include biomarkers (SLiM criteria): Sixty percent or more clonal plasma cells, involved/uninvolved Light chain ratio ≥ 100, and >1 focal lesion on MRI. Serum protein electrophoresis (SPEP) typically shows a monoclonal M-spike, and immunofixation identifies the heavy and light chain isotype. Light chain deposition in the kidney produces myeloma cast nephropathy. Hypercalcemia results from osteoclast-activating factors such as RANKL and MIP-1α secreted by the malignant plasma cells.

A systematic approach to the diagnostic workup of hematologic malignancies is essential for Step 2. The workup varies by suspected disease but generally combines peripheral blood analysis, bone marrow biopsy, immunophenotyping by flow cytometry, cytogenetics/FISH, molecular studies, and imaging. The following table summarizes the key diagnostic features and immunophenotypic markers for the most board-relevant entities.

The following clinical vignette mirrors the format of USMLE Step 2 CK questions and demonstrates the systematic approach to diagnosing and managing a hematologic malignancy.

Hematologic malignancies frequently present with or develop life-threatening complications that require immediate recognition and management. These oncologic emergencies are among the most commonly tested clinical scenarios on USMLE Step 2 CK. The following table summarizes the major complications, their associations, and first-line management.

Management of hematologic malignancies has been transformed by the development of targeted and immunologic therapies. While Step 2 does not require memorization of every regimen, understanding the principles behind treatment selection, the role of stem cell transplantation, and the mechanism of major targeted agents is essential. The following table contrasts traditional chemotherapy approaches with modern targeted strategies.

Looking ahead, the field is moving toward measurable residual disease (MRD)-guided therapy, in which treatment intensity is adjusted based on the depth of molecular response. Bispecific T-cell engagers (e.g., blinatumomab for ALL) and CAR-T cell therapies (e.g., tisagenlecleucel, axicabtagene ciloleucel) represent the frontier of immunotherapy, offering potentially curative options for patients with relapsed or refractory disease who have exhausted conventional approaches.