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trachea, subcutaneous tissue

There is a diffuse infiltrate of neoplastic plasma cells in the subepithelial tissue, accompanied by a scant vascularized stroma, and rarely blood lakes. There can be deposits of amyloid or immunoglobulin in the stroma. The tumour can be well, moderately or poorly differentiated {18,1267}. Well-differentiated EMP is characterized by uniform normal-looking to mildly atypical plasma cells. Intracytoplasmic crystals can be abundant in some cases. Dutcher bodies are sometimes seen. Moderately-differentiated EMP comprises moderately atypical plasma cells that vary in size. Poorly-differentiated (anaplastic) EMP comprises large cells that are often barely recognizable as being plasma cells. The nuclei often show significant variation in size, and can be round or irregularly folded. The chromatin pattern ranges from vesicular to finely granular to coarsely clumped. Nucleoli can be prominent. The cytoplasm is amphophilic and eccentrically located, and a paranu-clear hof (Golgi zone) may be present. Mitotic figures are frequent. Some tumour cells can be multinucleated.

Immunohistochemistry

Immunohistochemically, the plasma cells express cytoplasmic immunoglobulin with light chain restriction. CD20 is negative in most cases, and some cases express CD79a. PAX-5 is negative, while Oct-2 and Bob.1 are frequently positive. There is usually expression of CD38, CD138 and VS38, markers characteristically positive in but not specific for plasma cells. Epithelial membrane antigen is commonly positive, and rare cases can show cytok-eratin immunoreactivity (often with a dot pattern). Leukocyte common antigen, CD31 or CD56 is sometimes positive.

Differential diagnosis

Well-differentiated EMP should be distinguished from reactive plasma cell proliferations, either non-specific or associated with specific disorders, such as rhinoscleroma or Rosai-Dorfman disease. Reactive plasmacytic proliferations show a polyclonal pattern of immunoglobulin staining. Moderately or poorly-differentiated EMP can cause significant difficulties in distinction from large cell lymphoma, carcinoma, melanoma, extramedullary myeloid sarcoma and olfactory neurob-lastoma. The occasional positive staining for cytokeratin can lead to a misdiagno-sis of carcinoma. A high index of suspicion for EMP should be raised for any poorly differentiated neoplasm occurring in the upper aerodigestive tract. Features suggestive of the diagnosis include eccentrically placed nuclei, coarsely clumped "clock-face" chromatin in some nuclei, and amphophilic cytoplasm with a paranuclear hof. The diagnosis can be confirmed by immunohistochemistry or in-situ hybridization for immunoglobulin mRNA to look for monotypic light chain expression {18}.

Prognosis and predictive factors

The mainstay of treatment for primary EMP is radiotherapy. The prognosis of primary EMP is far more favourable than that associated with myeloma {1267}. Approximately 20% of patients with primary EMP will develop multiple myeloma, but it is not possible to predict which cases will progress.

Extramedullary myeloid sarcoma

ICD-O code

9930/3

Fig. 1.82 Extramedullar myeloid sarcoma of nasal cavity. A In this case, some eosinophilic myelocytes (right field) are evident among primitive myeloid cells which show a fine cytoplasmic granularity. B This example comprises a uniform population of primitive myeloid cells (blasts). The nuclear membranes are delicate and the chromatin pattern is fine. Distinction from malignant lymphoma can be difficult on morphologic grounds. C The neoplastic cells show strong and uniform immunostaining for myeloperoxidase, supporting the presence of granulocytic differentiation (same case as B).

Extramedullary myeloid sarcoma, also known as granulocytic sarcoma, is a tumour mass of myeloblasts or immature myeloid cells occurring outside the bone marrow or bone. It can precede, co-exist with or follow the presentation of acute myeloid leukaemia. It can also arise as blastic transformation of an underlying chronic myeloproliferative disease or myelodysplastic syndrome. The most common sites for occurrence of extramedullary myeloid sarcoma are lymph node and skin, but involvement of the nasal cavity and paranasal sinuses has also been reported {1701,2204}. The tumour mass comprises diffuse sheets of blast cells, which often show a single file pattern of infiltration in some areas. The blast cells have round or ovoid nuclei, very fine chromatin, small but distinct nucleoli, and a small to moderate amount of lightly eosinophilic cytoplasm. There can be better-differentiated cells with eosinophilic cytoplasmic granules. Intermingled eosinophilic myelocytes and metamyelocytes, if present, can provide an additional clue to the diagnosis. Giemsa-stained touch preparations are excellent for identification of cytoplasmic azurophilic granules as well as Auer rods, if present. Not uncommonly, extramedullary myeloid sarcoma is mis-diagnosed as malignant lymphoma.

Immunohistochemistry

The tumour cells show chloroacetate esterase activity in approximately 75% of cases. Immunohistochemically, they express various myeloid markers (such as myeloperoxidase, CD13, CD33, CD117, CD68/KP1, neutrophil elastase and lysozyme), with myeloperoxidase being most sensitive and specific. Myeloid sarcoma with monocytic differ entiation shows a myeloperoxidase -, CD68/PGM1+ immunophenotype. The pan-T marker CD43 is commonly expressed and may lead to a misdiagno-sis of T-cell lymphoma.

Histiocytic sarcoma

ICD-O code 9755/3

Histiocytic sarcoma, defined as a malignant proliferation of cells showing morphologic and immunophenotypic features of mature tissue histiocytes, is a rare tumour that can occasionally present in the nasal cavity {2043}. The large pleomorphic tumour cells have eccentrically-located round, ovoid, indented or grooved nuclei, and abundant eosinophilic cytoplasm that may show fine vacuolation. Phagocytosis is rare. Histologic distinction from large cell lymphoma is difficult, except that the cytoplasm tends to be voluminous and eosinophilic.

The diagnosis depends on the demonstration of histiocytic differentiation (granular staining for CD68 and lysozyme), in the absence of expression of pan-B markers (e.g. CD19, CD20, CD22, CD79a), pan-T markers (e.g. CD3), myeloid markers (e.g. MPO), Langerhans cell marker CD1a, and follicular dendritic cell markers (e.g. CD21, CD35). Since CD68 or lysozyme per se is not totally specific for histiocytic lineage, it is preferable to demonstrate additional haematolymphoid markers such as LCA/CD45, CD4,CD43 or CD163 to confirm the diagnosis. The frequent expression of CD43 may lead to a misdiagnosis of T-cell lymphoma. A small proportion of cases can express S100 protein.

Langerhans cell histiocytosis

ICD-O code 9751/1

Langerhans cell histiocytosis may occasionally present with nasal obstruction due to facial bone involvement {1183}. For details see Chapters 4 on tumours of the oral cavity and oropharynx' and 7 on tumours of the ear.

Juvenile xanthogranuloma

This histiocytic proliferation may mimic tumours of Langerhans cells. It commonly presents as skin nodules in infants and children, but rare extracutaneous cases involving the nasal cavity and paranasal sinuses have also been reported {568, 2245}. Some of the histiocytes have foamy cytoplasm, and frequently there are scattered Touton giant cells and spindly cells within the infiltrate of nondescript mononuclear cells. The histiocytes in juvenile xanthogranuloma express CD68 and factor XIIIa. In contrast to Langerhans cells, they are negative for S100 protein and CD1a.

Rosai-Dorfman disease

Rosai-Dorfman disease (sinus histiocytosis with massive lymphadenopathy) is an uncommon reactive condition of unknown etiology, characterized by proliferation of distinctive histiocytes that usually exhibit emperipolesis of lymphocytes. The tumour masses can mimic lymphoma or other malignancies both clinically and histologically. The patients are commonly children or young adults who present with massively enlarged

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