Introduction

Diagnostic imaging in oncology has many uses including determining the nature and extent of the disease, searching for metastasis and to monitor progression of the lesion and the response to treatment. It also aids in diagnosing comorbidity which may affect the selection and effectiveness of certain treatment regimes.

The ability of ultrasound to evaluate the internal structure of organs has made it an essential diagnostic tool that has largely replaced abdominal radiography as the first-line choice in evaluating the abdomen.

The nature of scientific research in veterinary ultrasound over the last decade has largely focused on differentiating benign from malignant lesions. Most of these studies have been largely unsuccessful, with only a few neoplastic conditions having classical, pathognomonic lesions on ultrasound. Following in the research footsteps of human ultrasound, several studies are currently underway to try to differentiate benign from malignant lesions with the use of contrast-enhanced ultrasound. This uses a contrast agent that is composed of micro bubbles, whose size is specifically engineered and adapted to permit passage through the lungs and into the parenchymatous organs. The dynamics of the rate of arterial perfusion and venous drainage of the diseased tissue is examined. Certain malignant tumours of the liver, spleen, pancreas and kidney have been shown to have different dynamics when compared to the benign counterparts in human medicine, and the hope is that this can be extrapolated and applied to veterinary medicine. However, the contrast agents have proved costly to date and the data that has been obtained has been largely ambiguous and unrewarding. However, the researchers soldier on.

Doppler techniques can also prove useful in trying to differentiate benign from malignant lesions, with tumour vasculature generally being more tortuous and of higher velocity when compared to benign tissues vasculature.

The ability to perform accurate ultrasound-guided fine-needle aspirates and biopsy is one of the most important advantages of ultrasonography - both in diagnosis of the primary tumour and in the detection of metastasis.^1,2 Fine-needle aspirates and ultrasound-guided Tru-Cut biopsies are cost effective and minimally invasive. The success of the FNA depends largely on the nature of the lesion in terms of exfoliation with lymphoma likely to exfoliate and provide a diagnostic result, compared to sarcomas which exfoliate less readily.²

Hepatic Neoplasia

Several primary and secondary neoplastic conditions can affect the liver in dogs and cats. Hepatomegaly is expected in most types of diffuse neoplasia; however, the extent and distribution of the enlargement can be variable.^1-3 Hepatic neoplasia can manifest as diffuse, focal or multifocal pathology.^1-3

Lymphoma, mastocytosis and histiocytic sarcoma are the most common tumours that lead to diffuse disease.³ However, they can also remain ultrasonographically undetectable.² Lymphoma can cause diffuse hepatic hypoechogenicity, hyperechogenicity or a mixed echogenicity, with or without hypoechoic nodules and masses.^2,3 Mast cell infiltration tends to cause a diffuse liver hyperechogenicity and histiocytic sarcoma a more generalised hypoechogencity.^2,3 Hepatic lymphadenopathy is also common with round cell tumours, and their presence can help to diagnose these neoplasias.^1,2

Primary hepatic neoplasia is uncommon with metastatic hepatic neoplasia 2.5 times more frequent than primary neoplasia.² Hepatic carcinomas tend to be diffuse and involve multiple lobes and the ultrasonographic appearance can be variable depending on the degree of necrosis, inflammation, cavitation and haemorrhage.² A mixed pattern of echogenicity is common with carcinoma.^2,3

Focal hypoechoic lesions of the liver and spleen with a hyperechoic centre are termed target lesions and are more commonly associated with metastatic disease; however, benign nodular hyperplasia can also cause target lesions.⁴ The finding of at least one target lesion in the liver or spleen has a positive predictive value of 74% for malignancy in small animals.⁴

Splenic Neoplasia

Splenic nodules of variable echogenicity and size are a common and nonspecific finding in the spleen and the possible differential diagnosis for such lesions extends from benign lesions, such as nodular hyperplasia, extramedullary haematopoiesis, haematoma and infection, to neoplastic lesions such as mast cell tumours, lymphoma, histiocytic sarcoma and metastatic disease.^1-3,5 According to recent literature, benign splenic mass lesions are more common than malignant mass lesions in dogs.² A spotted echo texture of the spleen with multiple small hypoechoic nodules is highly suggestive of infiltrative lymphoma, but once again, this finding is nonspecific.^2,3,5

The most common solitary splenic tumour is haemangiosarcoma, and these tumours can be a mass of variable size and variable, often mixed echo patterns.² A malignant neoplasia can occasionally disrupt the splenic capsule and invade the surrounding mesentery. Both malignant and the nonmalignant counterparts can haemorrhage, resulting in haemoabdomen.² Hence, although the presence of concurrent haemoabdomen raises the suspicion for malignancy, this is not pathognomonic.^2,3

Pancreatic Neoplasia

Pancreatic exocrine tumours such as adenocarcinoma arise from the acinar cells or ductal epithelium. They are rare and tend to develop from the central portion of the gland. Although rare, they are the most common exocrine pancreatic tumours in small animals. As they grow they can compress the common bile duct, invade the adjacent duodenal and gastric segments and frequently metastasise to the liver. Hence, if a pancreatic tumour is suspected, a thorough abdominal ultrasound examination must be performed to look for secondary signs. These tumours are often variably echogenic nodules or masses.^2,3,6

Pancreatic endocrine tumours are uncommon with insulinomas being the most frequently diagnosed endocrine tumour in the dog. The detection of these tumours can be difficult due to their small size. To further hamper interrogation, overlying duodenal gas may decrease the visibility of these tumours further. Insulinomas are solitary or multiple nodules with an ill-defined area of abnormal echogenicity.²

Most of the pancreatic tumours are malignant and tend to metastasise to the regional lymph nodes and the liver.^2,6 The regional lymph nodes are often enlarged and hypoechoic. Abdominal effusions and metastatic spread to the mesentery is also a feature of such tumours.²

Gastrointestinal Neoplasia

The classic ultrasonographic finding with any gastrointestinal tumour is loss of the normal wall layering. Specific findings of each type of tumour affecting the gastrointestinal tract will be discussed in further detail during the oral presentation. Lymphoma and mast cell infiltration of the GIT tract may be focal or diffuse with hypoechoic thickening and a loss of wall layering.^2,3 Most other GIT tumours (adenocarcinomas, smooth muscle tumours and mast cell tumours) also cause loss of wall layering.⁷ Adenocarcinoma of the stomach has been documented as resulting in "pseudolayering" - the apparent retention of thickened wall layering. On careful inspection, it may be possible to ascertain the layer from which a tumour arises and this together with the size of the mass and the echogenicity of the lesion may help to differentiate the lesion.² The regional lymph nodes should always be interrogated to predict malignancy and ultimately a biopsy is needed to confirm the tumour type.²

Ultrasonographic Findings in Lymph Nodes Associated with Neoplasia

The discrimination between benign and neoplastic lymph nodes is important for staging and prognosis. Although it is not possible to differentiate benign and malignant nodes on ultrasound, certain ultrasonographic features can be used to predict malignancy in the face of lymphadenomegaly.^9,10

Primary malignant and metastatic lymph nodes tend to be more round with a short to long axis ratio greater than 0.5 while normal or reactive lymph nodes are oval.¹⁰ Hypoechogenicity is also associated with malignancy.¹⁰ Doppler flow studies can also be used to predict malignancy with hilar blood flow strongly associated with benign conditions and a more peripheral blood flow pattern associated with malignancy.⁹

Heterogeneous neoplastic lymph nodes have been described with carcinoma metastasis, lymphoma, histiocytic carcinoma and mast cell tumour in canine patients and with lymphoma in feline patients.^2,3

References

1.  Penninck D, d'Anjou MA, eds. Atlas of Small Animal Ultrasonography. 1st ed. Oxford, UK: Blackwell Publishing; 2008.

2.  Barr F, Gaschen L, eds. BSAVA Manual of Canine and Feline Ultrasonography. 1st ed. Gloucester: British Small Animal Veterinary Association; 2011.

3.  Dennis R, Kirberger RM, Barr F, Wrigley RH, eds. Handbook of Small Animal Radiology and Ultrasound: Techniques and Differential Diagnoses. 2nd ed. Edinburgh: Churchill Livingstone Elsevier; 2010.

4.  Cuccovillo A, Lamb CR. Cellular features of sonographic target lesions of the liver and spleen in 21 dogs and a cat. Vet Radiol Ultrasound. 2002;43(4):275–278.

5.  Hanson JA, Pappageorges M, Girard E, Menard M, Hebert P. Ultrasonographic appearance of splenic disease in 101 cats. Vet Radiol Ultrasound. 2001;42(5):441–445.

6.  Hecht S, Penninck D, Keating JH. Imaging findings in pancreatic neoplasia and nodular hyperplasia in 19 cats. Vet Radiol Ultrasound. 2007;48(1):45–50.

7.  Laurenson MP, Skorupski KA, Moore PF, Zwingenberger AL. Ultrasonography of intestinal mast cell tumours in the cat. Vet Radiol Ultrasound. 2011;52(3):330–334.

8.  Paoloni MC, Penninck DG, Moore AS. Ultrasonographic and clinicopathologic findings in 21 dogs with intestinal adenocarcinoma. Vet Radiol Ultrasound. 2002;43(6):562–567.

9.  Prieto S, Gomez-Ochoa P, De Blas I, et al. Pathologic correlation of resistive and pulsatility indices in canine abdominal lymph nodes. Vet Radiol Ultrasound. 2009;50(5):525–529.

10. De Swarte M, Alexander K, Rannou B, D'Anjou M-A, Blond L, Beauchamp G. Comparison of sonographic features of benign and neoplastic deep lymph nodes in dogs. Vet Radiol Ultrasound. 2011;52(4):451–456.