Chapter 29. The Lymphoid System: II. Malignant Lymphomas

Malignant lymphomas—primary neoplasms of lymphoid tissue derived from lymphocytes—occur as solid tumors, usually within lymph nodes and less often in extranodal lymphoid tissues such as the tonsil, gastrointestinal tract, and spleen.

Malignant lymphomas are classified as non-Hodgkin's lymphomas, which are derived from lymphocytes, and Hodgkin's lymphomas. Hodgkin's lymphomas have retained their eponymous designation because the cell of origin remains uncertain.

Lymphoma versus Leukemia

Neoplastic lymphocytes circulate (mimicking normal lymphocytes) and frequently are found widely distributed throughout the lymphoid tissues. If bone marrow involvement and circulating cells predominate or if they constitute the first recognized manifestation of the disease, the process is termed leukemia. If the proliferation dominantly affects the lymphoid tissues or if a tissue mass is the presenting feature, the process is termed lymphoma. This distinction is arbitrary, and in children the term lymphoma-leukemia is sometimes used for two reasons. The two forms may coexist and lymphoma frequently evolves toward a leukemic state (Table 29-1).

Malignant Lymphoma versus Benign Lymphoma

The observation that neoplastic lymphocytes circulate extensively—coupled with widespread tissue involvement in many cases—has resulted in classification of all of these conditions as malignant lymphomas. This obscures the fact that many malignant lymphomas have relatively benign biologic behavior (ie, are slow-growing and compatible with long survival). The widespread distribution of the disease in the body represents mimicry of normal lymphocyte circulation rather than metastatic potential. Neoplastic proliferation of lymphocytes confined to one area of the body without the potential for dissemination—to which the term benign may truly be applied—occurs rarely, if at all.

Incidence of Non-Hodgkin's Lymphomas

Leukemias and lymphomas, including Hodgkin's lymphoma, account for approximately 8% of all malignant neoplasms and together represent the sixth most common type of cancer. Leukemia-lymphoma is the most common malignant neoplasm of children in the United States. About 25,000 cases of non-Hodgkin's lymphoma occur annually in the United States. The relative incidence of the subtypes of lymphoma varies greatly with age (Table 29-2) and to a lesser extent with sex and geographic factors.

Etiology of Non-Hodgkin's Lymphomas

In many animals, lymphoma and leukemia have a viral etiology. Murine, feline, and avian leukemias all are caused by retroviruses (type C), while Marek's disease, a leukemia-like process in chickens, is caused by a herpesvirus. In humans, the situation is less clear.

Oncogenes

The recognition that many of the genes responsible for controlling cell growth (proto-oncogenes) may act as potent cancer-causing genes (oncogenes) in lymphoma promises a better understanding of the etiology of many forms of cancer. Many of these proto-oncogenes have their homologues in retroviruses that have been shown to cause cancer in animals (viral oncogenes; see Chapter 18: Neoplasia: II. Mechanisms & Causes of Neoplasia).

A homologue of the c-myc oncogene is present in a retrovirus causing leukemia and sarcoma in chickens. In Burkitt's lymphoma, a characteristic translocation places c-myc (normally on chromosome 8) adjacent to a highly active immunoglobulin gene, most often on chromosome 14 (heavy chain), less often on chromosome 2 (kappa) or 22 (lambda) (see Figure 18-2). The gene c-myc is thereby abnormally activated, transcription is deregulated, and there is uncontrolled cell proliferation.

The c-abl oncogene homologue is found in murine leukemia virus. The gene c-abl is rendered dysfunctional in the 9;22 translocation that forms the Philadelphia chromosome (see Chapter 19: Neoplasia: III. Biologic & Clinical Effects of Neoplasms). The result is an abnormal abl fusion transcription protein (designated p210) that is capable of transforming hematopoietic cells.

The recently described bc1-2 proto-oncogene operates by a different mechanism: Its normal function is to inhibit programmed cell death (apoptosis) in certain categories of long-lived cells (eg, memory cells). Inappropriate activation of bc1-2 is believed to occur in many follicular lymphomas as a result of a t(14;18) translocation that juxtaposes bc1-2 (on 18) with the immunoglobulin heavy chain gene (on 14). In this instance, tumor growth is due to excessive accumulation of abnormally long-lived neoplastic lymphocytes rather than to an increase in proliferative activity.

Epstein-Barr Virus Infection

Epstein-barr virus (EBV) infection shows a close association with Burkitt's lymphoma in Africa. Burkitt's lymphoma is distributed in Africa in malaria-endemic areas. It has been postulated that EBV infection initiates B cell proliferation, which in the presence of chronic malarial infection escapes control, leading to lymphoma (see Figure 18-2).

Human T Leukemia Virus I (HTLV-1)

HTLV-I shows a strong association with T cell lymphoma-leukemia in Japan and is considered to be causal. A viral cause of Japanese T cell lymphoma was first suspected on the basis of observed clustering of cases in southern Japan. HTLV-II, a related retrovirus, has been linked to hairy cell leukemia, and HTLV-V has been implicated in cutaneous T cell lymphomas.

Autoimmune & Immunodeficiency Diseases

Autoimmune and immunodeficiency diseases are associated with an increased incidence of lymphoma, presumably related to the sustained or abnormal lymphoproliferation that occurs under these circumstances. The immunodeficiency induced by drugs in transplant recipients is also associated with malignant lymphoma. There is a greatly increased incidence of non-Hodgkin's lymphomas in persons with acquired immune deficiency syndrome (AIDS).

Diagnosis of Non-Hodgkin's Lymphomas

Neoplastic proliferations of lymphoid cells must be distinguished from the proliferation that occurs normally as part of the immune response (reactive hyperplasia). This distinction is often difficult. Loss of lymph node architecture and evidence of monoclonality are most helpful.

Cytologic Criteria

The usual cytologic criteria for distinguishing malignant neoplasms are not reliable in identifying a lymphoid proliferation as neoplastic. The degree of cellular pleomorphism in lymphocytes involved in the immune response is often marked. Some lymphocytic neoplasms are pleomorphic; others are not. Cytologic nuclear features such as dispersed chromatin, primitive-appearing nuclei, and the presence of nucleoli are seen both in neoplastic and in reactive immunoblasts (transformed lymphocytes).

Mitotic Rate

The number of mitotic figures is very high in reactive lymph nodes. Malignant lymphomas may or may not have a high mitotic rate.

Degree of Differentiation

In most other tissues, the presence of poorly differentiated cells raises a suspicion of malignancy. In the lymphoid system, cells that were once termed poorly differentiated in that they show little resemblance to normal small lymphocytes are now known to include normal immunoblasts.

Evidence of Spread in the Body

Both normal and neoplastic lymphocytes circulate extensively. The extent of spread does not correlate, therefore, with the degree of malignancy.

Loss of Normal Lymph Node Architecture

Loss of the normal nodal architecture, particularly if coupled with a lack of the normal admixture of cell types seen in the immune response, is evidence that the lymphoproliferation is neoplastic.

Monoclonality

If the proliferation of lymphoid cells is part of an immune response, several clones of lymphoid cells are involved (polyclonal). The vast majority of neoplastic lymphoid proliferations arise from one clone of lymphocytes (monoclonal). The monoclonal nature of lymphoid proliferation is currently the best evidence for lymphocytic neoplasia and can be established by showing any of the following:

1.  The presence of monoclonal light or heavy chain immunoglobulin on the cell surface or in the cytoplasm of a B cell population. The most common technique utilizes the fact that a polyclonal population of lymphoid cells will have nearly equal numbers of kappa- or lambda-positive cells, whereas a monoclonal population will express exclusively kappa or lambda light chain (Figure 18-1A).

2.  Clonal immunoglobulin gene rearrangement (B cell) (Chapter 4: The Immune Response).

3.  Clonal T receptor gene rearrangement (T cell) (Chapter 4: The Immune Response).

4.  Presence of a clonal chromosomal marker (eg, the 8;14 translocation in Burkitt's lymphoma or t(14;18) in follicular lymphomas).

Nomenclature & Classification of Non-Hodgkin's Lymphomas

It is still true, as Willis wrote in 1948, that “Nowhere in pathology has the chaos of names so clouded clear concepts as in the subject of lymphoid tumors.”

Lymphomas show enormous variation in clinical behavior and response to therapy. The aim of classification is to identify homogeneous subgroups that behave in a predictable way. The lymphomas, like neoplasms of other tissues, are named according to the normal cell they most closely resemble (Figure 29-1); confusion has arisen because the nomenclature of the normal cells of lymphoid tissues has changed several times during this century.

The classification of non-Hodgkin's lymphomas has changed many times in the past several years and will continue to evolve as our understanding of the lymphoid system improves. The following represents a summary of the evolution of the nomenclature, some elements of which are still in use.

Morphologic Classifications

Prior to 1950, the large, primitive-appearing cells (now called immunoblasts) in lymphoid tissues were termed reticulum cells, and it was thought that they served as stem cells for all lymphoid cells—indeed, for all hematopoietic cells. This concept is now acknowledged to be false, and the term reticulum cell sarcoma, which was used for many large cell lymphomas, has become obsolete.

In 1956, Rappaport and colleagues published a new classification of non-Hodgkin's lymphomas (Table 29-3) based upon the belief that the reticulum cell was actually a form of histiocyte. The corresponding neoplasms were therefore called histiocytic lymphomas. The Rappaport classification was relatively simple and was of clinical value in separating some of the lymphomas into more aggressive and less aggressive types. While still used in many hospitals for its clinical relevance, the Rappaport classification is scientifically inaccurate and will eventually be replaced by one of the immune-based classifications.

Immune-Based Classifications

As modern immunologic concepts developed, several different immune-based classifications were proposed. Of these, that of Lukes and Collins (1974) is widely used in the United States, while the Kiel classification (developed by Karl Lennert) is prevalent in Europe. These classifications are conceptually similar, and the simpler Lukes-Collins classification is presented here in modified form (Table 29-4). While these immune-based classifications fit the theoretic concepts of development of lymphoid cells, they have not gained universal acceptance because of their complexity and the need for ancillary tests to reach precise diagnoses.

At present, newly developed immunologic techniques such as surface marker phenotyping with monoclonal antibodies, gene rearrangement studies in B and T lymphocytes, and studies relating to oncogenes are being extensively used (Table 29-5) in the study of malignant lymphomas. It is expected that the information provided by these studies will lead to further evolution in the classification of these neoplasms.

The Working Formulation Classification

(Table 29-6)

In 1975, in an attempt to resolve the many conflicts in classifying non-Hodgkin's lymphomas, the National Cancer Institute sponsored a multi-institutional study that resulted in a classification system called “The Working Formulation of Non-Hodgkin's Lymphoma for Clinical Usage.” This classification is currently in use in the United States.

The Working Formulation sorts non-Hodgkin's lymphomas into three main prognostic groups: (1) low-grade lymphomas, which are clinically indolent diseases with long median survival times but rarely cured by therapy; (2) intermediate-grade lymphomas; and (3) high-grade lymphomas, which have an aggressive natural history but are responsive to chemotherapy and, as more effective treatment protocols are developed, are potentially curable.

The Working Formulation is primarily a morphologic and clinical classification. As immunologic information accumulates, there is no doubt that this classification will change. Furthermore, as newer lymphoma subtypes are recognized, they will be added to the basic classification in the appropriate prognostic group, depending on their clinical course.

Specific Types of Non-Hodgkin's Lymphomas

The different types of non-Hodgkin's lymphomas have greatly differing clinical and histologic features. The more important types are set forth in Table 29-7 and illustrated in Figures 29-2, 29-3, 29-4, and 29-5.

B Cell Lymphomas

B cell lymphomas are common. The most common types arise from the follicular center in lymph nodes (follicular center cell lymphomas). Histologically, they replace the lymph node in which they arise and may have either a diffuse or follicular pattern. In general, follicular lymphomas have a better prognosis than do diffuse lymphomas (Figure 29-6).

B cell lymphomas arise from cells that display the full range of transformation of B cells and are classified as low-grade, intermediate-grade, and high-grade according to the cell type involved. The morphologic and clinical features of these specific types of B cell lymphomas are considered in Table 29-7.

Grades (predicted clinical behavior patterns) according to the Working Formulation are set forth in Table 29-6.

T Cell Lymphomas

T cell lymphomas are less common than B cell lymphomas. A low percentage of small lymphocytic lymphomas (chronic lymphocytic leukemia) are T cell, and this represents the only low-grade T cell lymphoma. T immunoblastic sarcoma and lymphoblastic T cell lymphoma, which includes convoluted T cell lymphoma, are the two common types of T cell lymphoma, and they are both high grade lymphomas. Cutaneous T cell lymphoma (mycosis fungoides) is a special extranodal type of T cell lymphoma (Figure 29-5). The morphologic and clinical features of these specific types of T cell lymphomas are considered in Table 29-7.

Factors Determining Prognosis in Non-Hodgkin's Lymphomas

Histologic Type

The prognosis varies markedly with different histologic types of non-Hodgkin's lymphoma (Tables 29-6 and 29-8). It is useful to classify malignant lymphomas as low-grade, intermediate-grade, and high-grade because the grades correlate well with survival. In general, lymphomas with a follicular histologic pattern are of lower grade (longer survival times) than those with a diffuse pattern. A follicular pattern occurs only in follicular center cell lymphomas (Figure 29-6).

Stage of Disease

The stage of the disease is an expression of the extent of spread of the neoplasm. Specific criteria have been developed for staging Hodgkin's lymphoma and the non-Hodgkin's lymphomas. The aim is to define the extent of disease precisely as a basis for rational decisions about therapy (surgery, radiotherapy, chemotherapy, etc). The staging procedure given here (Table 29-9) was developed for Hodgkin's lymphoma and has been extended for use in management of non-Hodgkin's lymphomas.

The stage may be the clinical stage, which is determined by the history and physical examination, radiologic studies, isotopic scans, laboratory tests of urine, blood, and liver, and the initial biopsy results; or the pathologic stage, which is based on histologic findings in tissue removed by biopsy or laparotomy, with symbols indicating the tissue samples taken (N = node, H = liver, S = spleen, L = lung, M = marrow, P = pleura, O = bone, D = skin) and the results of histopathologic examination (+ indicates involved, − indicates not involved).

It must be emphasized that the clinical and pathologic staging classifications are unique to the main subtypes of lymphomas and apply only at the time of disease presentation—before definitive therapy is started.

Treatment of Non-Hodgkin's Lymphoma

Survival depends on rational choice of treatment. The use of combined (multiple-agent) chemotherapy (Table 29-10) has favorably influenced the prognosis of these neoplasms. Paradoxically, treatment of previously poor prognostic types of lymphoma (lymphoblastic and immunoblastic) is sometimes more successful in providing complete remission than treatment of the less aggressive histologic types. This probably reflects the fact that most chemotherapeutic agents act only on dividing cells, which are plentiful in high-grade lymphomas but sparse in low-grade lymphomas. Cyclic chemotherapeutic regimens are used so that neoplastic cells that are not in the dividing phase during one dose and therefore survive are treated by the next dose, when they may have entered the dividing phase. Radiotherapy is useful for patients with localized disease.

Hodgkin's lymphoma (also called Hodgkin's disease) is a malignant lymphoma characterized by the presence of Reed-Sternberg cells in the involved tissue (Figures 29-7 and 29-8). It accounts for 15% of all lymphomas and is usually considered separately from the other (non-Hodgkin's) lymphomas. Eight thousand cases occur annually in the United States. Hodgkin's lymphoma shows a bimodal age incidence, with a peak in early adulthood and another in old age.

Etiology of Hodgkin's Lymphoma

The cause of Hodgkin's lymphoma is not known. Early controversy over whether the disease was a peculiar infection (once thought to be a form of tuberculosis) or a form of cancer has given way to general acceptance of Hodgkin's lymphoma as a neoplastic process. That the neoplastic cells are often so sparsely distributed among reactive lymphoid cells has led to the hypothesis that the histologic features reflect some form of host response against the neoplastic Reed-Sternberg cells (Figure 29-9). The demonstration of EBV viral deoxyribonucleic acid (DNA) in Reed-Sternberg cell nuclei has led to a renewed belief that EBV may have a causative role.

Immunologic responses are abnormal in Hodgkin's lymphoma, and mice with chronic low-grade graft-versus-host reactions may show similar histologic features. Cell-mediated (T cell) immunity is depressed in Hodgkin's lymphoma, but it is not clear whether that is a cause or a consequence of the disease. Clusters of cases of Hodgkin's lymphoma in restricted geographic areas have been suggestive of some infective or other environmental agent—EBV was implicated in the same studies.

The cellular origin of the Reed-Sternberg cell, whether lymphocytic, histiocytic, or other, has long been a matter of debate. Recent immunologic and molecular data appear increasingly to favor the notion that the initial neoplastic event involves an early lymphoid progenitor cell prior to definitive commitment to the B or T cell pathways. The different subsets of Hodgkin's disease may then be explained on the basis of divergent differentiation with the acquisition of some B or T cell features (such as immunoglobulin gene rearrangement or phenotypic markers). In this context, lymphocyte predominant Hodgkin's disease is increasingly set apart. It is clearly a B cell process, but it shows features that are more consistent with a progressive hyperplasia (or dysplasia) than with a truly malignant neoplasm.

Pathologic Features of Hodgkin's Lymphoma

Although the diagnosis of Hodgkin's lymphoma depends upon the finding of classic Reed-Sternberg cells, these cells show little evidence of nucleic acid synthesis or proliferative activity. Large mononuclear cells (called Hodgkin's cells) that resemble Reed-Sternberg cells are the proliferative cells in Hodgkin's lymphoma (Figure 29-9).

The histologic picture of Hodgkin's lymphoma is particularly distinctive in that the neoplastic Reed-Sternberg cells are few in number and are admixed with variable numbers of lymphocytes, plasma cells, histiocytes, eosinophils, neutrophils, and fibroblasts, all of which are considered to be reactive (Figures 29-7 and 29-9). Yet the lymph node may be totally destroyed, and an identical process may progress to involve many lymph nodes, spleen, liver, bone marrow, and extralymphatic tissues. This is a contrast with other malignant neoplasms, in which the malignant cells predominate in the involved tissues.

Staging, both clinical and pathologic, is similar to that for non-Hodgkin's lymphoma (Table 29-9).

Classification of Hodgkin's Lymphoma

The varying relative proportions of Reed-Sternberg cells (and mononuclear variant cells), lymphocytes, histiocytes, and areas of fibrosis have permitted subclassification of Hodgkin's lymphoma into 4 subtypes that have epidemiologic, prognostic, and therapeutic differences (Table 29-11 and 29-12; the so-called Rye classification).

Lymphocyte-Predominant Hodgkin's Lymphoma (< 10%)

This subtype, which is characterized by the presence of numerous lymphocytes and few classic Reed-Sternberg cells, has the best prognosis. It tends to affect young adult males. It may occur in nodular or diffuse form and may include a conspicuous component of reactive histiocytes (earlier known as the lymphocytic and histiocytic (L&H) form of Hodgkin's lymphoma). The presence of large polyploid variants of the Reed-Sternberg cell with lobulated nuclei (popcorn cells) is characteristic. It typically presents as stage I or II disease and progresses slowly.

Lymphocyte-Depleted Hodgkin's Lymphoma (< 10%)

This form has the worst prognosis and typically presents as stage III or stage IV disease in elderly patients. Lymph nodes are replaced by a destructive process containing numerous pleomorphic mononuclear and classic Reed-Sternberg cells, variable amounts of diffuse fibrosis, and very few lymphocytes. Lymphocyte-depleted Hodgkin's lymphoma is often refractory to therapy.

Mixed-Cellularity Hodgkin's Lymphoma (20–40%)

This subtype has an intermediate histologic appearance with numerous lymphocytes, plasma cells, eosinophils, and Reed-Sternberg cells (Figure 29-7 and Figure 29-9). The prognosis is intermediate between that of lymphocyte-predominant and lymphocyte-depleted lymphoma. The response to therapy is usually good. It is more common after age 50 years.

Nodular Sclerosing Hodgkin's Lymphoma (30–60%)

This subtype has a good prognosis, usually presenting as early stage disease. Young women are particularly affected, and mediastinal involvement is common. Nodular sclerosis is histologically characterized by broad bands of collagen circumscribing nodules of involved tissue and by the presence of large Reed-Sternberg cell variants that have multilobate nuclei and abundant pale cytoplasm (lacunar cells).

Diagnosis & Treatment of Hodgkin's Lymphoma

In spite of intensive research and a wealth of immunologic data, the diagnosis of Hodgkin's lymphoma is still based entirely upon histologic examination—the finding of the classic Reed-Sternberg cell in pathologic tissue is considered essential for diagnosis. Phenotypic markers that facilitate recognition of Reed-Sternberg cells include CD15 and CD30 and BLA 36 (detected by monoclonal antibodies). Subclassification is then accomplished by examination of other histologic parameters (Table 29-11). Note that although Reed-Sternberg cells are characteristic of Hodgkin's lymphoma, morphologically similar cells may be seen occasionally in non-Hodgkin's lymphomas and in reactive hyperplasias (such as Epstein-Barr virus infection).

Selection of therapy depends not only upon the histologic type but on the stage and other clinical parameters. Localized forms of Hodgkin's lymphoma may be treated with either radiation or chemotherapy. Chemotherapy is highly effective when multiple agents are used and may lead to cures even in patients with disseminated (late stage) disease (Table 29-10). The evolving treatment of Hodgkin's lymphoma has provided a model of the team approach to management of neoplastic diseases, requiring close consultation among pathologists, radiologists, oncologists, surgeons, and radiotherapists (Table 29-13).

As noted above, the term histiocytic lymphoma (in the Rappaport classification) is a misnomer, and almost all of these tumors are neoplasms of large lymphocytes (large follicular center cells and immunoblasts). True histiocytic neoplasms do occur but are uncommon. They fall into three main categories, discussed below.

True Histiocytic Lymphoma

This tumor is rare, accounting for less than 5% of primary neoplasms of lymph nodes. The neoplastic cells are large and pleomorphic, typically with granular pink cytoplasm. Distinction from large cell lymphomas is difficult without immunologic tests. The malignant histiocytes may show phagocytosis and variable reactivity with anti-monocyte/histiocyte antibodies such as CD68 and CD11 (Mo 1) (Table 4-1). Histiocytic lymphomas are usually aggressive and refractory to treatment. Early disease is confined to lymph nodes and appears clinically like malignant lymphoma. Advanced disease is difficult to distinguish from histiocytic medullary reticulosis (see below).

Malignant Histiocytosis & Histiocytic Medullary Reticulosis

These two terms are used interchangeably to denote a highly malignant systemic neoplasm of histiocytes that involves lymph nodes and soft tissue. The malignant histiocytes first involve medullary sinuses within lymph nodes but rapidly spread to destroy lymph nodes and other tissues, producing hepatosplenomegaly, lymphadenopathy, and pancytopenia. Typically there is extensive erythrophagocytosis by the neoplastic cells. The prognosis is poor.

Distinction from recently recognized hemophagocytic syndromes associated with infection or T cell lymphomas is difficult.

Histiocytosis X

The term histiocytosis X is used to denote three related diseases:

1. Eosinophilic granuloma is a relatively benign unifocal disease that involves bone, particularly the skull and ribs of children and young adults, although long bones are sometimes involved. Radiologically, it presents as a well-demarcated lytic lesion. Histologically, the lesion is seen as a diffuse infiltrate composed of histiocytes, giant cells, and eosinophils.

2. Hand-Schüller-Christian disease is morphologically similar to eosinophilic granuloma but is multifocal and has a less favorable prognosis. The base of the skull is characteristically involved, producing the triad of proptosis, lytic bone lesions in skull, and diabetes insipidus—the last due to destruction of the posterior pituitary.

3. Letterer-Siwe disease (generalized histiocytosis) appears to represent the aggressive end of the spectrum, with widespread lesions of bone and lymphoid tissue. The condition is uncommon and occurs only in young children. Lymphadenopathy and skin lesions are due to infiltration by large pale neoplastic histiocytes.

In all of these conditions, the neoplastic histiocytes show a resemblance to the Langerhans cells of the skin (ie, react with monoclonal antibody OKT6 and contain tennis racket-shaped Birbeck granules on electron microscopy). Langerhans cells are thought to be antigen-handling cells.

Metastatic neoplasms—most commonly carcinomas and malignant melanoma—are a common cause of lymph node enlargement. Not infrequently, an enlarged lymph node is the method of clinical presentation of a carcinoma, the primary tumor being occult; cervical lymphadenopathy is a common mode of presentation of nasopharyngeal carcinoma.

The histologic diagnosis of metastatic neoplasms is easy when the neoplasm is well differentiated. When it is poorly differentiated, the distinction between large-cell (histiocytic) lymphoma, poorly differentiated carcinoma, and amelanotic malignant melanoma is very difficult to make on histologic examination. The demonstration of specific markers (common leukocyte antigen for lymphomas; keratin for carcinomas; and S100 protein and melanosomes [HMB 45] for melanomas) by immunoperoxidase techniques is essential for accurate diagnosis.