Final Diagnosis & Discussions: Fine Needle Aspiration of a Parotid Mass on 57-Year-Old Man

FINAL DIAGNOSIS: High grade neoplasm. Resection: Malignant fibrous histiocytoma—Pleomorphic type

FNA cell block
Positive Stains
None		 Negative Stains
CK7
CK20
HMB45
S100
MelanA
CD45

DISCUSSION
Malignant fibrous histiocytomas (MFH) are the most common soft tissue sarcomas of late adulthood occurring primarily between the 6th and 7th decade. Located most often in the deep tissues of the extremities and retroperitoneum, they are uncommonly found in the head and neck region. When found in this area, however, they most often are found in the parotid gland. They present as rapidly enlarging masses. There is a slight male predominance and may be induced by radiation.

Subclassification of malignant fibrous histiocytomas include, in order of their incidence: storiform-pleomorphic type (which has some giant cells), myxoid type, giant cell type (which contains osteoclast-like giant cells), and finally an inflammatory type. The more common pleomorphic type is also the more common type found in a younger age group, down to 40 years of age. Pleomorphic and giant cell types have a more aggressive course where myxoid and inflammatory type MFH have a more indolent less aggressive. In addition to type, depth and location will affect prognosis.

Cytologic preparations of pleomorphic MFH are cellular unlike myxoid type, and more commonly have a hemorrhagic and necrotic background, although some myxoid or fibrocollagenous material may also be present. Capillary vessels may be found with neoplastic cells attached giving the appearance of pseudopapillary forms. The morphology of pleomorphic MFH is variable, with no distinctive features as to cell origin. Variable size and shape including spindled, polygonal and pleomorphic forms are present. The large amount of cytoplasm on some cells may mimic that of epithelial cells but other cells resemble histiocytes, which are in fact fibroblasts. This is further complicated by erythrophagocytosis by tumor cells. Xanthoma cells are also common. The pleomorphic multinucleated cells themselves are non-specific. These are not to be confused however with the giant cells found in the giant cell type of MFH which have osteoclast-like giant cells and are composed of multiple benign appearing nuclei that are S100 positive. A storiform pattern may be appreciated in tissue sections.

Due to their wide range of appearance and lack of morphologically differentiating features, pleomorphic MFH have a wide differential initially. Efforts can be directed towards broadly categorizing the lesion by noting some cohesiveness, a lack of pigment, and perhaps a lack of lymphoglandular bodies and floret cells. However, immunohistochemical stains for CD30, ALK, S100, HMB45, melanA, may still be undertaken to assist in the diagnosis of a large cell anaplastic lymphoma or malignant melanoma. Absence of keratin reactivity points away from a poorly differentiated carcinoma as source. An immunohistochemical panel can include muscle markers (actin, desmin, MyoD1, and myogenin) when there is consideration for rhabdomyosarcoma. As MFH lack features of definitive tissue differentiation, immunohistochemistry is typically nonreactive except for vimentin.

Many lesions classified as MFH in the past are now believed to be de-differentiated variants of sarcoma. A pleomorphic liposarcoma may have recognizable mulberry lipoblasts and be S100 positive. They also have cytoplasmic vacuoles which indent the nucleus and displace it to the periphery of the cell. Rhabdomyosarcoma may be identified by bizarre tumor cells which contain striations. Cytogenetics and molecular diagnostics are beginning to help further characterize these high grade sarcomas.

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References

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