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Identification of focal liver masses is the easiest task to learn with the POCUS examination. It is important to remember that is not possible to differentiate between benign and malignant processes based on ultrasound alone. Focal lesions may be caused by regenerative nodules or nodular hyperplasia, benign or malignant neoplasia, focal inflammatory disease (e.g., abscesses and granulomas), cysts, and hematomas.

The origin of a large midabdominal mass can sometimes be difficult to determine when the mass contacts multiple origins. In this case, try repositioning the patient from dorsal to lateral recumbency. This will often separate the liver and spleen from one another and help determine the origin of the mass. Generally, the spleen is differentiated from the liver by its hyperechoic capsule (bright white) and its vasculature splitting its capsule unlike the liver.

 Benign hepatic nodular disease is common and nodules associated with nodular hyperplasia may be variable in echogenicity when compared to normal hepatic parenchyma (hyperechoic, hypoechoic, isoechoic or mixed echogenicity). Generally, well‐defined hyperechoic homogeneous nodules are more likely to be benign but cytology or histopathology is necessary for definitive evaluation. Nodular hyperplasia is common in older dogs (Nyland et al. 2002) (Figure 8.5).

 Hepatic cysts consist of round to oval anechoic (fluid‐filled) structures with well‐defined walls. These are generally benign unless a significant amount of hepatic architecture is disrupted and there is evidence of hepatic dysfunction (polycystic disease). Cysts may be parenchymal or biliary in nature, and may be congenital or acquired. Cysts that have irregular margins or contain echogenic material are less likely to be benign and may represent abscesses, inflammation or neoplastic changes (Figure 8.6A–C).

 Other parenchymal masses may have variable appearances and include abscess, hematoma, nodular hyperplasia and neoplasia.

 Hepatic abscesses are uncommon, usually indistinguishable from nodules and mass lesions of other etiology, and may have variable echogenicity, appearance, and margination (Figure 8.7A). Most commonly, abscesses are relatively hypoechoic, thick‐walled lesions which may have cystic or cavitary components or mixed echogenicity. A more definitive finding is the presence of gas (due to gas‐forming bacteria) within the lesion, which appears as focal areas of hyperechogenicity with distal edge shadowing, comet tailing, and reverberation. Fine needle aspiration with cytology and microbial culture and sensitivity is helpful in confirming the diagnosis. However, neoplastic lesions can have necrotic centers that can be mistaken for abscesses and therefore, diagnosis should always be in conjunction with additional clinical findings that support the diagnosis. The Global FAST approach is helpful as a quick screening test for disseminated versus localized disease and the presence or absence of lung nodules (quick pulmonary metastasis check).Figure 8.5. Nodular hyperplasia. (A) Small hyperplastic nodule as demarcated by the caliper marks made by the sonographer, consistent histopathologically with benign nodular hyperplasia. This is a common finding in older dogs. Generally, nodules are seen as discrete, well‐demarcated hyper‐ or hypoechoic homogeneous nodules and can be singular or multiple and poorly demarcated. Nodules can be any size <3 cm (mass defined as >3 cm). Hence, findings are nonspecific and fine needle aspiration and/or biopsy is needed to differentiate these nodules from malignant processes. (B) Nodular hyperplasia with multiple varying sized, homogeneous, hypoechoic nodules with the largest marked with calipers (+). (C) Example of an older dog with multiple hypoechoic nodules confirmed to be benign nodular hyperplasia. (D) In contrast to (B), an example of a severely nodular liver in a 10‐year‐old male castrated Chihuahua with poorly controlled diabetes and pancreatitis. The process is diffuse and the liver margins are severely rounded. The calipers (+) demonstrate a hypoechoic cyst. Notice its distal acoustic enhancement and the multiple, hypoechoic (dark) nodules throughout the parenchyma. Fine needle aspiration results were consistent with a vacuolar degeneration and lipidosis. Benign hyperplasia and neoplasia would be considered additional differentials. A biopsy would be more definitive in this case.

 Hepatic hematomas may be variable in appearance and initially are generally anechoic or hypoechoic to normal hepatic parenchyma. Hematomas may be difficult to distinguish from other mass lesions, including neoplasia (hemangiosarcoma). Frequently, large hematomas usually form with ruptured hepatic masses and can complicate diagnosis (Figure 8.8A,B; see also Figure 8.7B). Fine needle aspiration of hematomas generally yields nonspecific cytological results and the absence of neoplastic cells on cytology does not rule out neoplasia.Figure 8.6. Hepatic cyst and biliary cysts. (A) Hepatic cyst (arrow) with distal acoustic enhancement (seen as a hyperechoic, bright enhancement extending from the far margin of the hepatic cyst and marked with subtle black cursors [> <] on both sides of the hyperechoic track). Hepatic cysts are typically thin‐walled and filled with anechoic fluid. A fluid‐filled structure can be differentiated from a solid nodule by looking in the far‐field for acoustic enhancement artifact (the bright track extending from the far field of the cyst to the text L LIVER). A solid or mineralized structure will conversely have acoustic shadowing artifact in the far‐field (a dark track extending from the far‐field of the solid or mineralized structure). (B) Multiple benign biliary cystadenomas in a cat (labeled liver masses by the sonographer). This can be mistaken for malignant cystic neoplasia. (C) Large hepatic cyst in a dog. Note the large anechoic structure with distal acoustic enhancement artifact extending from the far border of the cyst (labeled > <). Cysts can be complex in architecture and within this cyst is a large, echogenic, ovoid to amorphous structure. Irregularly marginated cysts or those containing echogenic material can be malignant. An additional differential for the structure within the cyst includes fibrinous material or hemorrhage and may represent abscessation, inflammation or neoplastic change. Fine needle aspiration may allude to the diagnosis. Alternatively, excisional biopsy may be needed.

 Neoplastic lesions have a wide range of morphological changes. Again, it should be emphasized that it is not possible to distinguish between benign and malignant lesions based on ultrasonographic appearance alone. There are a variety of primary liver tumors including hepatocellular adenoma and adenocarcinoma as well as mesenchymal tumors (see Figure 8.7B–D). Lymphosarcoma can appear as either focal or multifocal mass lesions or as diffuse parenchymal disease. Hemangiosarcoma generally appears as complex mass lesions with mixed echogenicity and cavitations (see Figures 8.7B and 8.8A,B). Hematomas can have a similar appearance. Metastatic neoplasia is common in the liver (Figure 8.9A,B). Hepatic mass lesions must be characterized by cytology or histopathology for a definitive diagnosis. Ultrasound‐guided fine needle or Tru‐Cut biopsy of mass lesions can usually be accomplished with minimal difficulty and risk, depending on the location of the mass.Figure 8.7. Various liver masses. Benign and malignant liver masses cannot be differentiated based on ultrasound characteristic alone. (A) Semisolid hepatic mass in a cat confirmed to be a hepatic abscess. The gallbladder (GB) is juxtaposed to the mass and may be confused with free fluid on this still image, highlighting the value of a several‐second video when images are submitted to the specialist for interpretation. Although not always present, the hallmark of an abscess regardless of location is the presence of gas within the mass effect as evidenced by comet tails and other air‐related artifacts caused by gas‐producing bacteria (such artifacts are not evident here). Diagnosis was made by liver biopsy. (B) A large, mixed echogenicity mass (MASS). It can be difficult to determine the origin of large masses. The liver, spleen, kidney or other adjacent structures should be considered if a region of organ confluence cannot be identified. The histopathological diagnosis in this case was hepatic hemangiosarcoma. (C) Another example of mixed echogenicity with a hepatic mass adjacent to the diaphragm. Note the abrupt change of echogenicity between the mass and normal hepatic parenchyma. Percutaneous biopsy result was hepatocellular carcinoma. (D) Large hyperechoic, lobulated mass with mild peritoneal effusion within the gallbladder fossa. The lesion was confirmed to be hepatocellular carcinoma. Note the differences between sonographic appearances of the same tumor type in (C) and (D) demonstrating the poor specificity of ultrasonographic findings with regard to histopathological diagnosis. Additional differentials might have included a hepatoma (benign liver mass) or other types of solid parenchymal neoplasia (GB, gallbladder; RLiver, right liver; RK, right kidney).Figure 8.8. Liver and splenic masses. The origin of large masses can be challenging to determine ultrasonographically and is sometimes not possible. (A) and (B) show the same large mass in a dog with mixed echogenicity (unlabeled and labeled). Careful evaluation showed confluence of the tumor with the splenic parenchyma. (C,D) Images of a cat (same patient) with hepatic mast cell tumor. Note that a well‐defined mass is not seen. Rather, the parenchymal changes are subtle and the liver is coarse in echotexture. This case highlights the importance of fine needle aspirate and/or liver biopsy in obtaining the patient’s definitive diagnosis.

 Target lesions consist of alternating layers of different echogenicities, giving the appearance of a target or bull's eye. Single target lesions have a high predictive value for malignancy. Multiple target lesions increase the positive predictive value of malignancy up to 74–81% (Cuccovillo and Lamb 2002) (see Figure 8.9A,B). Regardless, target lesions are nonspecific and a biopsy is required for further characterization. If target lesions are detected, then the authors recommend screening for additional evidence of neoplasia with a complete detailed abdominal ultrasound, thoracic radiographs, and lung ultrasound. However, performing a Global FAST immediately following your POCUS examination is helpful for a quick survey of the remaining abdominal structures, the thorax (TFAST), and lung surface, especially for nodules (Vet BLUE).