Pancreatic cystic lesions
Pathogenesis and description
Cystic lesions are uncommon in veterinary medicine. Although rare, cystic pancreatic neoplasia should be included as a differential diagnosis of cystic intra-abdominal and pancreatic masses (Yoshimura et al., 2013; Torner et al., 2020).
The majority of cystic structures are pseudocysts (VanEnkevort et al., 1999; Charles, 2007). Pseudocysts can be defined as intra- or peripancreatic, fluctuant pockets of pancreatic enzyme secretions, necrotic debris formed by destruction and necrosis of pancreatic tissue, usually caused by pancreatitis or pancreatic trauma. Pseudocysts lack the epithelial lining characteristic of true cysts (Branter et al., 2010). Instead, the wall of a pseudocyst is composed of granulation tissue that can mature into fibrous scar tissue. The lumen contains cellular debris and pancreatic enzymes (Meuten, 2002; Head, 2003; Charles, 2007; Zachary et al., 2013). Pseudocysts are filled with often turbid, sometimes blood-tinged fluid. When a cystic lesion is visualized by ultrasound, the steps necessary to confirm a pseudocyst include a high cystic fluid amylase and/or lipase activity, the exclusion of an abscess by FNA, and histology of both the pancreas and the wall of the lesion. However, no information is available on pseudocystic fluid canine pancreatic lipase (cPL) and feline pancreatic lipase (fPL) concentrations.Additional non-neoplastic cavitary lesions of the pancreas include abscesses, congenital cysts, and retention cysts (Table 8.1) (Coleman et al., 2005; Anderson et al., 2008; Branter & Viviano, 2010). True cysts, like pseudocysts, are variably sized, fluid-filled lesions demarcated by a wall and located in or close to the pancreatic parenchyma (Coleman et al., 2005; Charles, 2007; Branter & Viviano, 2010; Zachary & McGavin, 2013). True cysts can be unilocular or multilocular thin-walled sacs that range from millimeters to several centimeters in diameter and often contain a clear and translucent serous fluid (Bergin et al., 2002; Meuten, 2002; Head, 2003; Coleman et al., 2005).
Histologically, pancreatic cysts are lined by a single layer of low cuboidal to flattened well-differentiated ductal epithelial cells (Head, 2003; Coleman et al., 2005; Charles, 2007). True cysts can be the result of congenital ductular malformations (cystic dilation of pancreatic ducts often in association with polycystic kidney and/or liver diseases) (Meuten, 2002; Head, 2003; Charles, 2007; Zachary & McGavin, 2013) or can be acquired due to obstruction of the duct system and retention of pancreatic secretion (Meuten, 2002; Head, 2003). True congenital pancreatic cysts are extremely rare. Few cases have been reported in cats, and only one in a dog (Healy et al., 2022) They can be defined as a fluid-filled cyst lined with cuboidal-to-columnar epithelium and a cystic amylase concentration of juvenile acinar atrophy; primary (starvation…) or secondary atrophy (inflammation, neoplasia…); multifocal degeneration/necrosis (viruses, toxics, ductal obstruction...); lipofuscinosis; vacuolation (lysosomal storage diseases); acquired retention cyst (due to obstruction of ductular lumen)Ultrasound
True cysts, pseudocysts, abscesses, and cystic neoplasia may be difficult to differentiate on the sole basis of sonographic appearance (Larson, 2016; Griffin, 2020). Round to oval shape, thin and sharply demarcated wall, anechoic content, and strong distal acoustic enhancement are the usual and classical descriptions of cystic sonographic images. However, in the particular case of pancreatic pseudocysts, irregular margins, thick wall, and echoic content with slight acoustic enhancement (fluid rich in enzymes and debris) are possible findings. Obtaining a sample of cyst content under ultrasound guidance can help rule out abscess or cystic neoplasia. Aspiration and drainage of pseudocysts are reported to be safe and should be considered for the evaluation of cystic pancreatic lesions (VanEnkevort et al., 1999).
Cytology
As for any cyst or pseudocyst from any organ or structure, cytology is often of low cellularity, and a clear to slightly hazy fluid is obtained, suggestive of a modified transudate. In contrast with abscesses, cystic samples are composed of an abundant amorphous background and few inflammatory cells, such as few non-degenerated neutrophils and reactive macrophages (VanEnkevort et al., 1999; Bjorneby & Kari, 2002; Borjesson, 2014). One description of a congenital cyst in a dog reports a similar composition with a proteinaceous fluid associated with necrotic cellular material and mild non-septic neutrophilic inflammation with moderate to marked accumulation of uniformly blue-black pigment (Healy et al., 2022). Cytology should be considered as an exclusionary diagnostic tool for abscess.