Page 162 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

CHAPTER 7 Diagnostic Cytopathology in Clinical Oncology 141

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• Fig. 7.30 Fine-needle aspirate of a melanoma in a dog. Note the fine • Fig. 7.31 Fine-needle aspirate of a melanoma in which melanin granules
melanin granules in the tumor cells and in the background. are not visible (amelanotic melanoma).

granules are common, and nuclei are round with finely stippled pigmented melanomas contain melanophages, which are mac-
chromatin and variably prominent nucleoli. rophages containing abundant melanin; in the authors’ experi-
ence, this is especially true after surgical removal or biopsy of the
Periarticular Tumors primary mass. These melanophages may be mistaken for meta-
Periarticular tumors are predominantly histiocytic sarcomas (HSs) static cells, but they differ from neoplastic melanoblasts as mela-
with other sarcomas (synovial myxoma, pleomorphic sarcoma, nophages typically contain coarse collections of melanin within
fibrosarcoma, and undifferentiated sarcoma) diagnosed less fre- phagolysosomes rather than the fine granulation typically found
11
quently. Descriptions of synovial cell sarcoma histologically as within melanoblasts.
monophasic (spindle) or biphasic (epithelioid and spindle) may Most dermal tumors of melanocytic origin are benign and
encompass these different tumor types or a periarticular tumor of are termed melanocytomas. They consist of polygonal or spindle
undefined cell lineage. 12,13 Cytomorphologic appearances of peri- cells containing black cytoplasmic granules. Several of the adnexal
articular tumors correspond to the specific tumor type described tumors may contain melanin pigment and must be differentiated
elsewhere under mesenchymal tumors with round or spindle cells from melanocytomas. These typically are of epithelial origin and
or under tumors with frequent multinucleated cells. should be distinguished by their cellular arrangement in cohesive
sheets (see Fig. 7.9).
Tumors of Melanocytes
In malignant melanoma, the cells can adopt the appearance of Mesenchymal Tumors Composed of Cells Arranged in
epithelial (sheets of cohesive cells), mesenchymal (individual- Dense Aggregates
ized oval or spindle cells), or discrete round cell tumors, and all Cells aspirated from some mesenchymal tumors, including rhab-
three cytologic appearance may be evident in the same tumor. domyosarcoma, perivascular wall tumor, PNST, amelanotic mela-
Individual melanoblasts are round, oval, or spindle cells with noma, and the epithelioid form of HSA, form dense aggregates
moderately high N:C ratios, lightly basophilic cytoplasm, and and clusters that are more characteristic of epithelial cells. Careful
round or oval nuclei with fine chromatin and distinct nucleoli. examination typically reveals some spindle cells with indistinct
Criteria of malignancy consist primarily of anisokaryosis and margins and spaces between the closely packed cells indicating
nucleolar pleomorphism. Highly pigmented tumors do not the lack of intercellular junctions. Slide preparation by imprinting
present a diagnostic challenge, and fine black melanin gran- and scraping also may yield clusters and sheets of mesenchymal
ules may be so numerous that they obscure all cellular detail. cells that mimic epithelial populations.
Cells with varying degrees of pigmentation are typically found
within the same tumor, and melanin granules may be sparse Mesenchymal Tumors with Frequent Multinucleated Cells
in some cells (Fig. 7.30). Amelanotic tumors present a greater Although any neoplasm can have a few multinucleated cells,
diagnostic challenge. Usually, a faint scattering of fine gray- multinuclearity is especially common in certain sarcomas. These
black melanin granules is found in a few cells to support a include HSs, feline ISSs, pleomorphic sarcoma (also called malig-
diagnosis, but cells may be completely devoid of pigmentation nant fibrous histiocytosis or anaplastic sarcoma with giant cells),
(Fig. 7.31). In these circumstances, moderately pleomorphic rhabdomyosarcoma, and some plasmacytomas in which the mul-
tumor cells aspirated from masses on the digits or in the oral tinucleated cells are part of the tumor population. In OSA, mul-
cavity should alert the clinician to the possibility of this highly tinucleated osteoclasts are often present and are not part of the
malignant tumor. Because melanocytes are of neuroectodermal neoplastic population of cells.
origin, the cells may express certain neural markers, such as
S-100 and neuron-specific enolase, in addition to vimentin and Discrete Round Cell Tumors
often Melan A.
An additional cytologic challenge is the identification of meta- The majority of discrete round cell tumors are of hematopoi-
static lesions within lymph nodes. Most lymph nodes draining etic origin, including neoplasms of mast cells, plasma cells,