Page 1285 - Small Animal Internal Medicine, 6th Edition

PART TWELVE Oncology

C. Guillermo Couto

VetBooks.ir CHAPTER 74

Cytology

GENERAL CONSIDERATIONS FINE-NEEDLE ASPIRATION

Evaluation of a cytologic specimen obtained by fine-needle In FNA, a single cell suspension is obtained using a small-
aspiration (FNA) in small animals with suspected neoplastic gauge needle (i.e., 23-25 gauge) of the appropriate length for
lesions often yields information that can be used to make a the desired target organ or mass; this needle can be coupled
definitive diagnosis, thereby circumventing the immediate to a 6-, 12-, or 20-mL sterile, dry plastic syringe, but fre-
need to perform a surgical biopsy. At the authors’ hospitals, quently this is not necessary; the size of the syringe is based
almost every mass or enlarged organ is evaluated cytologi- on how comfortable it is for the operator. Although the tech-
cally before a surgical biopsy is performed because the risks nique is still referred to as “FNA,” in most cases no aspiration
and costs associated with FNA are considerably lower than is performed with the syringe (see later). Tissues easily acces-
those associated with surgical biopsy. Frequently, a definitive sible using this technique include the skin and subcutis, deep
cytologic diagnosis allows the clinician to institute a specific and superficial lymph nodes, spleen, liver, kidneys, lungs,
treatment (i.e., multicentric lymphoma treated with chemo- thyroid, prostate, and intracavitary masses (e.g., mediastinal
therapy) and spares the patient the need for a surgical biopsy. mass).
In a study of 269 cytologic specimens from dogs, cats, If the clinician is sampling superficial masses, sterile prep-
horses, and other animal species, the cytologic diagnosis aration of the site is not necessary. However, clipping and
completely agreed with the histopathologic diagnosis in sterile surgical preparation should always be done when
approximately 40% of cases and partially agreed in 18% of aspirating organs or masses within body cavities. Once the
the cases; complete agreement ranged from 33% to 66%, mass or organ has been identified by palpation or radiogra-
depending on the lesion and location, and was highest for phy, it should be manually isolated, if feasible; manual isola-
skin/subcutaneous lesions and for neoplastic lesions (Cohen tion is not necessary when performing ultrasonography-,
et al., 2003). Interestingly, in the author’s experience, the computed tomography (CT)–, or fluoroscopy-guided FNAs.
cytologic and histopathologic diagnoses agree in more than A needle, either by itself or coupled to a syringe, is then
70% of the cases. When a clinician with experience on cytol- introduced into the mass or organ; if the “needle-alone”
ogy evaluates a cytologic specimen, the bias experienced technique is used, the needle is reinserted into the tissue/
after obtaining a history and performing a physical examina- mass several times; this can be referred to as the “wood-
tion is beneficial in the cognitive processing of information. pecker technique” due to the repeated puncturing motion
For example, being fairly certain that a dog has multicentric that mimics a woodpecker at work. This allows the clinician
lymphoma (on the basis of the history and physical examina- to core out small samples, which will be completely con-
tion) makes specimen interpretation easier. tained within the hub of the needle. Once a sample has been
Clinically applicable diagnostic cytologic techniques are obtained, a clean disposable syringe is loaded with air and
summarized in this chapter, with emphasis on sample col- coupled to the needle. The specimen is then gently expelled
lection and the cursory interpretation of the specimens. onto slides, as described later in this chapter. If the needle-
Although some clinicians can obtain sufficient diagnostic syringe technique is used, suction is applied to the syringe
information, a board-certified veterinary clinical pathologist three or four times. If the size of the mass or lesion allows it,
should always evaluate a cytologic specimen before any the needle is then redirected two or three times and the
prognostic or therapeutic decisions are made. procedure is repeated. Before withdrawing the needle and

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