Practitioners should be able to confidently diagnoseseveral common skin masses with cultivation ofcytologic and microscopic skills.

Peer ReviewedTop 5 Diagnostics / Cytology

Skin Masses Diagnosed with In-House Cytology

Katie M. Boes, DVM, MS, DACVPVirginia-Maryland Regional College of Veterinary Medicine

ytology, a relatively noninvasive means for rapid in-clinic diagnosis, can be useful for identifying sampledmasses, fluids, or lesions, frequently without undue

patient harm. Fine-needle aspiration of internal lesions mayrequire ultrasonographic assistance and confirmation of normalcoagulation status (eg, clotting times, platelet numbers). Skincytology samples can be acquired via fine-needle aspiration,impression smear, or skin scrape, all of which have few clinicalcomplications. For some masses, cytologic features can be diffi-cult to interpret, but general practitioners should be able to con-fidently diagnose several common skin masses with cultivationof cytologic and microscopic skills.

Vaccine reactionVaccine reactions may occur at vaccination sitesweeks or months following administration. Oncytologic examination, there is a mixed inflamma-tory infiltrate of mostly lymphocytes and macro-

phages with some plasma cells, neutrophils, and eosinophils.Macrophages become activated with increased cytoplasmicbasophilia, foamy cytoplasm, binucleated forms, and multinu-cleated giant cells. The key diagnostic feature is macrophagescontaining phagocytized adjuvant (visualized as a bright pink,purple, or blue globular or granular material; Figure 1, nextpage). If inflammation is not self-limiting and the mass doesnot resolve, complete surgical excision should be curative.

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TOP 5

TOP 5 Skin Masses Diagnosed withIn-House Cytology

1. Vaccine reaction

2. Follicular cyst

3. Lipoma

4. Histiocytoma

5. Mast cell tumor

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Top 5

Follicular cystFollicular cysts (ie, epidural inclusion cysts, epider-moid cysts) are nonneoplastic, noninflammatory,sac-like lesions lined by epithelium. Most canine andfeline skin cysts are follicular cysts (ie, arise from

hair follicles) and include several histologic subtypes that arenot cytologically distinguishable. These histologic subtypes havelittle clinical significance; they are benign and can be completelyexcised surgically.

On cytologic examination, follicular cysts have abundant keratinocytes (either anucleate or containing karyolytic nuclei)with cholesterol crystals, hair fragments, and activated macro-phages (Figure 2). Within the preparation background, follicu-lar cysts can also contain melanin granules that should be differ-entiated from bacteria. Cyst rupture and immunogenic keratinexposure to the dermis or subcutis can result in foreign bodyreaction with mild-to-marked infiltrates of neutrophils, macro-phages, and multinucleated giant cells. Because, on cytology, acyst cannot be differentiated from a cyst within a neoplasm(usually benign follicular neoplasm), histopathology is requiredto assess architecture; cytologic differentials include trichoepi-thelioma, infundibular keratinizing acanthoma, and piloma-trixoma.

LipomaLipomas, benign neoplastic adipocyte growths, aretypically soft, freely movable masses of varying sizeswithin the subcutis; however, infiltrative lipomasmay be firmer and attached to underlying muscula-

ture. Before staining, lipoma preparations appear greasy and fail

to air-dry. During staining, adipocytes may dissolve in the fixa-tive to produce an acellular cytologic sample. If they do notcompletely dissolve in the fixative, adipocytes appear as largeballoon-like cells arranged in aggregates held together by finefibrovascular stroma (Figure 3). Individual cells are round topolygonal and contain a large, colorless intracytoplasmic vac-uole that peripherally displaces a small, round, condensednucleus. Complete surgical excision, if permitted by owners, canbe curative.

HistiocytomaCanine histiocytomas are benign, self-limiting der-mal growths that usually occur on the ears, face, anddistal extremities of young dogs. Histiocytomas canalso occur in older dogs, but other tumors (eg, other

round cell tumors) should be considered. These lesions are frequently erythematous, alopecic, and dome-shaped with orwithout ulceration. On cytologic examination, many individual-ized, (irregularly) round histiocytes may display minimal ormild anisocytosis and anisokaryosis and contain some lightlybasophilic cytoplasm that often stains paler than the platformbackground and is paler at the periphery as compared with thatevident on perinuclear staining. A few punctate, colorless vac-uoles may be noted. Nuclei are round to ovoid, variably placedin the cell, and display lacy chromatin with absent or occasionalnucleoli (Figure 4). Varying numbers of small lymphocytes maybe dispersed with increased lymphocyte numbers observed inregressing histiocytomas. Because histiocytomas cannot becytologically differentiated from cutaneous or systemic histiocy-tosis, the latter conditions should be considered in dogs withmultiple histiocytoma-like skin lesions.

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Fine-needle aspirate of skin from a dog with vaccine reaction showingmany small lymphocytes and 2 activated macrophages that havephagocytized a purple, granular material (vaccine adjuvant) admixedwith a few lysed lymphocytes. (Diff-Quik stain, 1000× originalmagnification)

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Fine-needle aspirate from a follicular cyst in the skin of a dog showinga few hyalinized, moderately basophilic, angular keratinocytes (whitearrow) and 2 colorless, rectangular-to-rhomboidal cholesterol crystals(black arrows). (Modified Wright stain, 1000× objective field)

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Mast cell tumorCanine mast cell tumors may be solitary or multi-centric in the skin and can appear sequentially orsimultaneously. They tend to be alopecic, erythema-tous, and varied in size. Ulceration may be present

in larger masses, and the mass may change size as histamine issporadically released from the neoplastic cells. On cytologicexamination, these tumors frequently contain several mast cellsand eosinophils with fewer neutrophils, fibroblasts, and collagenstrands. Mast cells are round and individualized and containlightly basophilic cytoplasm with few-to-numerous deeplybasophilic intracytoplasmic granules (Figure 5A). The degree ofgranularity depends on the granule’s staining characteristics,stain type, degree of cellular differentiation, and whether mastcells have recently degranulated in situ.

Often, the granules are so numerous that they physically obscurenuclear features or absorb so much stain that the nucleus stainspale. In contrast, some aqueous-based Wright stains (eg, Diff-Quik) stain poorly or fail to stain the granules (Figure 5B). If

the granules do not stain, diagnosis can be achieved by observ-ing eosinophils admixed with individualized round mast cellsthat display round, centrally placed nuclei and foamy or vacuo-lated cytoplasm. Any mast cell tumor should be consideredpotentially malignant, but greater nuclear pleomorphism (ie,anisocytosis, binucleation) and observation of mitotic figuresshould increase concern for malignancy. Canine mast celltumors should be surgically removed with wide margins (3 cmor 1 fascial plane) and submitted for histopathologic evaluationand grading.

Closing thoughtsMore challenging preparations can be sent to diagnostic labor-atories for pathologist review, but several neoplastic and non-neoplastic lesions, such as those listed here, can be evaluated in-house to facilitate faster diagnosis and treatment. � cb

See Aids & Resources, back page, for references & suggestedreading.

May 2013 • clinician’s brief 17

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Fine-needle aspirate from a lipoma in the skin of a dog showing a large aggregate of balloon-shaped adipocytes held together by a finefibrovascular stroma. (Modified Wright stain, 200× objective field)

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Fine-needle aspirate from a histiocytoma in a dog’s skin showing many round histiocytes with mild anisocytosis and anisokaryosis andcontaining a moderate amount of lightly basophilic cytoplasm. Thenuclei are round with lacy-to-stippled chromatin and 0 or 1 nucleolus.(Modified Wright stain, 1000× objective field)

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Fine-Needle Aspirates of CanineCutaneous Mast Cell Tumors 5

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Poorly stained mast cells (arrows) with few lightly basophilicintracytoplasmic granules. (Diff-Quik stain, 1000× objective field)

Three eosinophils (yellow arrows) and multiple mast cells (blackarrows) with numerous intracytoplasmic, deeply basophilic granulesand round, pale-staining nuclei. (Modified Wright stain, 1000×objective field)