Cytology of The Spleen
Fine needle aspiration (FNA) of the spleen may be helpful in the diagnostic work-up and development of a treatment plan for patients with suspected disease involving the spleen. As sensitive imaging procedures have become more widely available, the use of cytology for evaluation of abdominal organs has increased. Guidance of needle placement by imaging can enhance the probability of collecting a diagnostic sample, especially from smaller masses or those that are difficult to palpate. Although sometimes diagnostic, it is important to remember that cytology is a screening tool that complements other clinical information. Only a small sample of tissue is evaluated, so it is difficult to assess the extent of involvement. Discordance between cytology and other clinical findings should be pursued by histopathology or other diagnostic techniques.
Indications for FNA of the spleen for cytologic evaluation include generalized splenomegaly, presence of a mass or nodular lesions, abnormal echogenicity, or suspicion of neoplasia or inflammation. Potential complications include hemorrhage and seeding of neoplastic cells along the needle tracts, but these are rare. Thrombocytopenia is viewed by some as a contraindication for a FNA of the spleen. Although screening patients for hemostatic abnormalities may minimize the risk of hemorrhage, abnormal test results do not predict excessive bleeding in all patients, and hemorrhage can occur even if screening results are normal. The animal is placed in lateral or dorsal recumbency, the area over the site is surgically prepared, and a 1-1.5 inch 21 or 22 gauge needle is used for sample collection. A non-aspiration technique may result in more cellular samples with less blood contamination, but an aspiration technique with a 6-12-ml syringe also can be used.
Splenic parenchyma comprises red pulp, white pulp, and blood vessels associated with trabeculae that extend from a thick, smooth muscle capsule. The red pulp consists of red blood cells within blood vessels, endothelial lined sinuses, and a reticular meshwork. The white pulp includes lymphatic nodules and periarterial lymphatic sheaths. Macrophages occur throughout, but are prominent in the marginal zone surrounding periarterial lymphatic sheaths. FNA can be minimally to markedly cellular, and often are contaminated with blood. The lymphoid population includes predominantly small lymphocytes with variable numbers of intermediate and large lymphocytes. However, if a lymphoid follicle is aspirated, there may be a focal area in which the majority of the cells are large lymphocytes with moderate amounts of basophilic cytoplasm and nuclei with fine chromatin and prominent nucleoli. These should not be mistaken for neoplastic lymphocytes In lymphoid neoplasia (see below), neoplastic lymphocytes usually are distributed throughout the smear.
The number of macrophages is variable. Macrophages sometimes contain hemosiderin, which appears as bluish green to black pigment. Hemosiderin-laden macrophages often are prominent in aggregates of splenic stromal cells, which appear as dense aggregates of elongated cells amidst a variable amount of eosinophilic extracellular connective tissue. Occasional mast cells and plasma cells can be present in FNA from normal splenic tissue. Neutrophils often are from blood contamination.