The diagnostic yield of trephine biopsies in plasma cell myeloma is usually excellent but can be affected by the size and quality of the specimen. Focal lesions may be irregularly distributed and widely spaced. Occasionally only one or two small myeloma lesions are found in a trephine biopsy with no evidence of a plasma cell infiltrate in the remainder of the section or in specimens from the contralateral posterior iliac spine. The pattern of the plasma cell infiltrate may be interstitial, focal, or diffuse (Fig. 8) [2, 6, 7]. The extent of marrow involvement varies from a small increase in plasma cells to complete replacement. The pattern of involvement is largely related to the extent of disease. With interstitial and focal patterns, there is generally considerable marrow sparing and preservation of normal hematopoiesis. With diffuse involvement, expansive areas of the marrow are replaced and hematopoiesis may be markedly suppressed. Typically, interstitial and focal disease in early myeloma progresses to diffuse involvement in advanced stages of the disease [6].

Bartl and associates proposed a staging system based on percentage of marrow space replaced by neoplastic plasma cells in marrow trephine biopsies [6, 14]. In Stage I less than 20 % of the marrow is replaced, Stage II, 20–50 % and Stage III, more than 50 %. In most instances the extent of involvement in biopsy sections seems to reflect overall tumor burden. In the Bartl and associates’ study there was good correlation between histologic stage, clinical stage, and prognosis [6].

Myelomas with atypical plasma cell morphology may be difficult to recognize in trephine biopsies (Fig. 7d). Plasma cell myeloma with plasmablastic features, lymphoid appearing plasma cells, convoluted plasma cells, or markedly pleomorphic or anaplastic plasma cells are particularly problematic. Examination of the neoplastic plasma cells in aspirate smears is often essential for diagnosis in these cases. Numerous cytoplasmic inclusions in the myeloma plasma cells may be a distracting morphologic feature on the bone marrow section. The inclusions are often found in large plasma cells that are distorted by crystalline or globular material hiding the identity of the plasma cell neoplasm .

Reticulin or collagen fibrosis may be present in a minority (approximately 10 %) of cases of myeloma [2, 15]. In many of these there is extensive reticulin or collagen fibrosis. A disproportionate number of fibrotic myelomas produce monoclonal light chains only [15]. Coarse fibrosis has been correlated with diffuse marrow involvement and aggressive disease [6]. Fibrosis with osteosclerotic changes is found in plasma cell neoplasms associated with polyneuropathy, organomegaly, endocrinopathy, M protein, skin changes (POEMS) syndrome, discussed later in this chapter.

Bone changes are a frequent finding in trephine biopsies from patients with plasma cell myeloma. Marked osteopenia and evidence of increased osteoclastic activity with reabsorption of bone may be identified (Fig. 9) .

Immunohistochemistry is an important complement to the morphologic assessment of plasma cell neoplasms . Selected immunohistochemical stains can supplement flow cytometry immunophenotyping or provide the primary immunophenotypic assessment for plasma cell neoplasms when a specimen is not obtained for flow cytometry or contains inadequate numbers of plasma cells for flow cytometric analysis. The plasma cells in myeloma typically express CD79a, VS38c, CD138 , CD38, and monotypic cytoplasmic immunoglobulin and lack surface immunoglobulin. Unlike normal plasma cells they are nearly always CD19 negative, and CD56 is aberrantly expressed in about 75 % of cases (Fig. 10) [16–18]. Other aberrantly expressed antigens include CD117, CD20, CD52, and CD10, in decreasing order of frequency; occasionally, myeloid and monocytic antigens are found [16, 19, 20] .

A more detailed description of the immunophenotype of normal and neoplastic plasma cells is found in the chapter on flow cytometry. The remainder of this discussion deals specifically with the utility of immunohistochemistry in diagnosis and in following patients with plasma cell myeloma.

Immunohistochemical stains on marrow biopsies or other tissues are valuable in the following:

Plasma cells may be difficult to recognize and quantify in sub-optimally prepared sections and when scattered interstitially in the marrow. Several plasma cell-associated antigens with variable sensitivity and specificity may be assessed by immunohistochemical stains to quantify plasma cells in biopsy specimens (Figs. 7 and 10) . Immunohistochemistry for CD138 , CD38, CD79a, kappa and lambda will usually stain plasma cells brilliantly on biopsy sections, allowing easy quantification. CD138 is a commonly used marker, expressed on normal plasma cells and in 60–100 % of myelomas [21, 22], and useful in identifying and quantifying plasma cells in biopsy sections. Generally, from 70 to 100 % of the cells within an individual neoplastic plasma cell population are CD138 positive [22]. CD138 appears to be plasma cell-specific among normal hematopoietic cells in the marrow, however; other neoplastic B-cell diseases such as chronic lymphocytic leukemia and primary effusion lymphomas react with some anti-CD138 antibodies. CD79a is positive in the plasma cells in most cases of myeloma but is a pan B lymphocyte antigen and is found in most B-cell neoplasms. It may be helpful in distinguishing myeloma from non-B-cell hematopoietic neoplasms and metastatic tumors .

CD20 is positive in 15–20 % of myelomas, mostly those with a t(11;14) [23]. CD79a and CD38, although not plasma cell-specific, are usually expressed on neoplastic plasma cells. CD56 is expressed in about 75 % myelomas and is a marker of aberrancy in plasma cell proliferations.

Immunohistochemical stains and in situ hybridization for kappa and lambda light chains are useful in characterizing plasma cell neoplasms and differentiating them from reactive plasma cell proliferations as may be found with connective tissue disorders, chronic liver disease, chronic infections, and metastatic tumors [4]. Normal/reactive plasma cells and myeloma plasma cells are both rich in cytoplasmic immunoglobulin and generally react strongly with antibodies to kappa or lambda light chains. In cases of myeloma the plasma cells express a monoclonal pattern of reactivity [24, 25]. In normal marrow and in reactive plasma cell proliferations, there is a polyclonal pattern of kappa and lambda staining plasma cells, usually with a slight to moderate kappa predominance (Fig. 11) . In some cases of MGUS the kappa–lambda ratio is normal, in others it is skewed but generally less, than in myeloma. In one large study a kappa–lambda staining ratio of 16:1 or higher on marrow biopsies distinguished myeloma from MGUS in nearly all instances; other investigators found a ratio of 8:1 to be as effective [24, 26]. Neither the number of marrow plasma cells nor the quantity of M-protein correlates well with the light chain ratio [26]. Kappa and lambda stains are particularly useful in cases with a relatively low percentage of marrow plasma cells, as it is often encountered when evaluating for residual/recurrent disease following chemotherapy or stem cell transplantation .

Some plasma cell myelomas have extreme atypical morphologic features that disguise their identity. Several hematopoietic and non-hematopoietic neoplasms may occasionally exhibit features that mimic a plasma cell neoplasm. Immunohistochemistry is often helpful in differentiating these cases. Stains for plasma cell-associated antigens and a panel of appropriate antigens associated with other neoplasms are useful in differentiating myeloma from other hematopoietic neoplasms or a metastatic tumor.