Acute myeloid leukaemia (AML) is a highly complex and heterogeneous disease. Proper classification according to the 2016 World Health Organization (WHO) classification requires a systematic approach and integration of key clinical, laboratory, pathologic and genetic information [1, 2]. Great advances in our understanding of the pathogenesis and molecular underpinnings of AML have been realized since the original AML classification using the French–American–British (FAB) system (1976). This genetic revolution not only contributes to enhanced disease diagnosis and prognostication but also to ongoing improvements in therapeutic strategies.

Introduction

Immunosecretory and plasma cell disorders cover a broad spectrum of clinical and pathologic entities. Some of the processes, such as monoclonal gammopathy of undetermined significance (MGUS), have relatively indolent behavior, while others, such as plasma cell leukemia, are associated with very poor prognosis and high mortality. This group of disorders includes lymphoplasmacytic lymphoma (LPL), a neoplastic entity which overlaps both with B-cell lymphoma and with the immunosecretory disorders group.

Benign plasma cells and reactive plasmacytosis

Plasma cells are a normal component of adult bone marrows. They typically represent about 0–1% of the overall cellularity seen in bone marrow aspirate smears (Foucar, 2001). Levels above 5% are considered abnormal in immunologically unstimulated marrows. In normal bone marrow biopsies, plasma cells are most typically located in perivascular locations.

In Wright–Giemsa-stained preparations, plasma cells have a distinctive, light to dark blue cytoplasm with an eccentrically placed nucleus. The nuclear chromatin is quite dense, and in appropriately thin histologic sections is classically described as “clockface chromatin.” Adjacent to the nucleus is a clearing in the cytoplasm, referred to as a hof, which represents the Golgi apparatus of the cell.

The cytologic features of benign and malignant plasma cells can overlap (Fig. 12.1). Inclusions which may be seen in plasma cells include Russell bodies, which are globular eosinophilic collections of immunoglobulin in the cytoplasm. Dutcher bodies are not true nuclear inclusions but rather cytoplasmic inclusions that overlie the nucleus. Dutcher bodies are only rarely seen in benign proliferations of plasma cells.

Introduction

Myelodysplastic syndromes (MDS) are a clinically heterogeneous group of clonal hematopoietic disorders characterized by ineffective hematopoiesis associated with dysplastic changes in one or more marrow lineages together with progressive cytopenias. MDS may occur as primary diseases, or may follow toxic exposures or therapy (Jaffe et al., 2001). Primary MDS is more often seen in elderly patients and frequently progresses to marrow failure, or evolves to acute leukemia, usually myeloid. The overall rate of transformation to acute leukemia depends on the subtype of MDS, ranging from 10% to over 60%. As a rule, the degree of trilineage dysplasia and percentage of blast cells correlate with the aggressiveness of the disease. It should be stressed that dysplastic features of hematopoietic elements is not unique to MDS and may occur in many reactive conditions, such as inflammatory states, HIV infection, endocrine dysfunctions, autoimmune disorders, and may also be associated with certain medications.

MDS have been traditionally subdivided according to the French–American–British (FAB) system (1982) into five major categories, primarily based on the percentage of blast cells in the peripheral blood and bone marrow (Table 7.1), the presence of ringed sideroblasts in the marrow, and the absolute count of monocytes in peripheral blood (Bennett et al., 1982). These subgroups show different rates of progression to acute myeloid leukemia (AML) and overall survival. In particular, refractory anemia and refractory anemia with ringed sideroblasts are associated with much longer median survival and a lower incidence of progression to acute leukemia than the other three FAB subtypes and can be considered “low-grade” MDS.

Fibrosis

Fibrosis of the bone marrow is caused by a reactive (non-clonal) proliferation of fibroblasts which may occur in association with a variety of neoplastic and non-neoplastic conditions (McCarthy, 1985). Fibrosis is usually of the reticulin type, as detected by silver stains, in the early stages, but it may progress to a collagen fibrosis that is detectable by trichrome stains (Fig. 6.1). Normally, reticulin staining is minimal but increases slightly with age (Beckman et al., 1990). Extensive marrow fibrosis is typically associated with a leukoerythroblastic reaction in the peripheral blood that is characterized by the presence of immature granulocytes (usually myelocytes and metamyelocytes), late-stage erythroblasts, and teardrop-shaped red blood cells and enlarged platelets. Diffuse fibrosis usually results in the inability to aspirate the bone marrow. Bone marrow fibrosis is common in chronic myeloproliferative disorders, and extensive fibrosis with peripheral blood leukoerythroblastosis is typical of chronic idiopathic myelofibrosis with myeloid metaplasia. Patchy areas of fibrosis are also seen with bone marrow involvement by mast cell disease (Horny et al., 1985), which may accompany other hematologic malignancies at diagnosis or relapse. Many other neoplasms involving the marrow, including some acute leukemias, malignant lymphomas, and metastatic tumors, result in focal or diffuse marrow fibrosis (Table 6.1).

Marrow fibrosis may also be associated with non-neoplastic conditions, especially inflammatory diseases, reparative changes, or metabolic disorders. Among these reactive cases, a particularly severe degree of fibrosis can be seen in patients with autoimmune conditions (Bass et al., 2001) (Fig. 6.2).

Introduction

Lymphoproliferative disorders frequently involve the peripheral blood and bone marrow, and bone marrow studies may be performed for primary diagnosis or as a staging procedure in patients with lymphoproliferative disorders. A primary diagnosis with accurate classification can often be made on bone marrow samples alone, if a combined morphologic and immunophenotypic approach is used. The addition of molecular or cytogenetic studies can resolve some of the low percentage of cases that are equivocal after morphologic and immunophenotypic analysis.

Many of the low-grade B-cell lymphomas will involve the bone marrow, but the precise morphologic classification of these diseases is complicated by the fact that characteristic architectural patterns seen in lymph nodes involved by these diseases are not present in the bone marrow. Despite this, many of the low-grade B-cell lymphomas have characteristic immunophenotypes that allow for proper classification. Flow cytometric immunophenotyping of involved peripheral blood or bone marrow aspirate material allows for evaluation of the largest number of antigens, as well as confirming aberrant co-expression of antigens that are characteristic of certain disease types. However, paraffin section immunophenotyping, which can be performed on core or clot biopsy material, can also be used successfully to detect many antigens of interest in these cases. The characteristic immunophenotypic features of the various small B-cell lymphoid proliferations are summarized in Table 11.1.

Molecular genetic or cytogenetic studies may also be useful in selected cases to confirm the presence of a clonal population, when the differential diagnosis is between reactive and neoplastic lymphoid proliferations (Arber, 2000).

Introduction

The chronic myeloproliferative disorders (CMPD) are a clinically heterogeneous group of clonal proliferations of stem cell origin characterized, at least initially, by marrow hypercellularity with varying degrees of marrow fibrosis and an increase in the production of one or more terminally differentiated cell types (George & Arber, 2003). These differentiated elements may accumulate in the marrow, in peripheral blood, and in other organs (e.g., spleen). All types of CMPD have a variable tendency to undergo disease progression that may terminate in bone marrow failure due to myelofibrosis or in transformation to an acute leukemic phase, occasionally preceded by a brief myelodysplastic phase. In CMPD, a definite diagnosis usually cannot be made by morphologic examination alone (Anastasi & Vardiman, 2000). The evaluation of bone marrow histology, however, holds an important role in confirming the diagnosis and excluding unsuspected pathology. Expert opinion should be sought if bone marrow histology is to be used as a major diagnostic criterion, since the changes which are specifically associated with the various subtypes of CMPD (see later) are often subtle and difficult to recognize in morphologically less-than-optimal processed samples. This greatly reduces the value of bone marrow histopathology in inexperienced hands (Pearson, 2001).

Classification of these disorders benefits from the integration of morphologic features with clinical, hematologic, and cytogenetic findings (Harris et al., 1999). Of major importance is the presence or absence of the Philadelphia chromosome (BCR/ABL or translocation 9;22), the defining feature of chronic myelogenous leukemia (CML).

Introduction

Bone marrow biopsies performed to evaluate for metastatic disease are among the most common samples received. While advances in radiologic techniques have reduced the number of such bone marrow examinations, this method is still valuable for selected patients. Determination of the tumor type is usually not difficult in patients with a known primary tumor. However, unexpected malignancies may be identified, for example during an anemia work-up (Wong et al., 1993), or bone marrow biopsy may be performed prior to a primary tumor biopsy. For example, a bone marrow biopsy may be performed as the initial procedure in a child with an abdominal mass. The ability to diagnose a tumor based on a bone marrow examination may reduce the need for further procedures or alter the approach of a second procedure. In the setting of an unknown primary, immunohistochemical studies are often essential for characterization. Even with these studies, clinical correlation is often needed to confirm the primary diagnosis. The most common non-hematologic malignancies to involve the bone marrow are prostate, breast, and lung carcinomas, neuroblastoma, Ewing sarcoma/peripheral neuroectodermal tumor (PNET), rhabdomyosarcoma, and malignant melanoma (Anner & Drewinki, 1977; Papac, 1994).

Peripheral blood and bone marrow aspirate

Peripheral blood involvement by metastatic tumor, termed carcinocythemia, is extremely uncommon and usually represents a late event with short survival (Gallivan & Lokich, 1984) (Fig. 13.1). Other, non-specific abnormalities of the blood are more common in patients with metastatic carcinoma. These usually manifest as anemia, leukoerythroblastosis, leukocytosis, and microangiopathic hemolytic anemia.

Indications for bone marrow examination

Bone marrow examination, including both aspiration and biopsy sampling, can be performed on virtually any patient. However, patients with coagulation deficiencies or profound thrombocytopenia may experience prolonged bleeding, which cannot be controlled by pressure bandages. In these rare cases, specific treatment (e.g., platelet transfusion) may be indicated. Indications for performing bone marrow examination are summarized in Table 1.1. In the vast majority of cases, both a bone marrow aspiration and biopsy should be performed. Bone marrow aspiration and bone marrow biopsy are complementary (Bain, 2001a, 2001b). Bone marrow aspiration provides excellent cytologic detail; however, marrow architecture cannot be assessed. Bone marrow core biopsy allows for an accurate analysis of architecture; however, cytologic details may be lost. Table 1.2 shows the accepted indications for performing a bone marrow biopsy. This includes cases with inadequate or failed aspiration, need for accurate assessment of cellularity, cases in which the presence of focal lesions (e.g., granulomatous disease or metastatic carcinoma) is suspected, suspected bone marrow fibrosis, need to study bone marrow architecture, need to study bone structure, bone marrow stroma, or assessment of bone marrow vascularity. In general, patients with hypocellular marrows or bone marrow fibrosis are likely to need a trephine biopsy for adequate assessment. In such patients, an aspirate would probably be inadequate or even impossible. Unexplained pancytopenia and unexplained leukoerythroblastic blood pictures are further indications for a biopsy, because they are likely to indicate the presence of bone marrow metastatic disease or fibrosis.