Clínica Veterinaria de Pequeños Animales - Volumen 38 / Nº 1 / Marzo 2018

Introduction

With increasing medical improvements in diagnostics and treatment of diseases there is a tendency for companion animals to reach an older age. Since animals get older, neoplastic diseases are a major cause for presentation to the veterinarian.¹ Diagnostic imaging plays a vital role in the work-up of an oncologic patient for adequate diagnosis and staging. Sensitive detection of lesions is necessary to determine appropriate local or systemic therapy and to monitor therapeutic results. Radiography and ultrasound are routinely used as screening tool in veterinary medicine. However, computed tomography (CT) is the preferred modality during the different stages of the work-up of an oncologic patient, namely in a first diagnostic work up, in staging, regarding radiotherapy or surgical planning and for assessing the outcome in follow up scans.^2,3 CT as a cross sectional radiographic modality overcomes limitations of conventional radiography such as summation images and offers an increased spatial resolution. Multiplanar reconstruction allows optimally localizing a lesion, describing the effects to the surrounding tissue, and giving aid in surgical questions regarding resectability. In order to better biologically characterize a lesion, iodinated contrast media gives information about vascularization and thus allows improved demarcation from the surrounding tissue.⁴ However, CT is not available at every hospital, is more cost intensive and requires anaesthesia. So, the benefits need to be weighted against the expenses.

This review describes the main indications for CT in the oncologic patient using examples to illustrate the application for several specific disease processes.

Head and neck

One main indication for CT is the assessment of nasal tumours. In cats lymphoma is the most common nasal neoplasia,^5,6 whereas in the dog carcinomas are most frequent.⁷ CT features associated with nasal neoplasia include soft tissue masses within the nasal cavity with heterogeneous contrast uptake, destruction of the turbinates, lysis of the adjacent bones and intracranial extension.^5,6 CT is the modality of choice in displaying bone changes, which showed to best assess lysis of the turbinates and cribiforme plate (Fig 1).⁸ Nevertheless, a recent study showed that CT and magnetic resonance imaging (MRI) had a high level of agreement concerning bone involvement of nasal cavity and head. In addition, MRI showed a higher likelihood of identifying meningeal enhancement, which can be due to tumoral extension, peritumoral meningitis or oedema. MRI has a better soft tissue detail, however it is less available, more expensive and CT might be needed anyhow for radiation therapy planning.⁹

CT is also commonly used in characterizing skull tumours due to the complexity of this region. Although tissue sampling is paramount to obtaining an accurate diagnosis, some tumours have specific CT imaging characteristics, such as multilobular osteochondrosarcoma showing a fine to coarse granular, nonhomogeneous bone mass with well defined margins usually in the occipital region or at the level of the zygomatic arch (Fig. 2).¹⁰ In particular to assess for oral masses, CT is an important modality for preoperative purposes that gives valuable information about extent and a possible infiltrative nature.¹¹ Besides, CT has been shown to more confidentially identify nasopharyngeal masses as polyps despite their typical location by a characteristic CT feature. A recent study reported the described rim enhancement of nasopharyngeal polyps in cats to represent a zone of inflammation.¹²

Due to the close relationship of anatomical structures of the neck, cross sectional imaging provides important information of origin and extension of neoplasia in this region. Thyroid gland tumour is the most common neoplasia in the neck and can occur from the base of the tongue to the base of the heart. Although ultrasound is a good screening tool, CT or MRI are indicated if surgical resection is considered. The masses are usually highly vascularized, heterogeneous and can contain regions of dystrophic mineralization. CT findings of carotid body and thyroid tumours are different based on anatomical location. Carotid body tumours have a dorsolateral location to the larynx at the level of the bifurcation of the common carotid artery (Fig. 3A). Thyroid masses are located more caudally (Fig. 3B).^13,14

Thorax

Pulmonary masses can be assessed by CT in order to better delineate margins and tumour extension for surgical planning along with imaging for concurrent pulmonary nodules and regional lymph nodes involvement. Carcinomas are the most frequently primary lung tumours in dogs and cats. Carcinomas are most commonly localized in the caudal lung lobes and described as solitary, well circumscribed, bronchocentric masses with internal air bronchograms, which can cause various degree of bronchial compression (Fig. 4). Other imaging features include contrast enhancement and internal mineralization. Cats in contrast to dogs tend to have more irregular margins and can show intratumoral cavitations and pleural effusion.^15,16 Canine pulmonary histiocytic sarcoma has been reported to present predilection for right middle or left cranial lung lobes (Fig. 5). Carcinomas generally metastasize to tracheobronchial lymph nodes but dogs with histiocytic sarcoma also show involvement of craniomediastinal and sternal lymph nodes.¹⁷

Common mediastinal tumors in dogs and cats include thymoma, heart base tumors, and lymphoma. Other tumor types in this region include thyroid carcinoma, carcinomas of other origin, sarcomas, and mesothelioma. CT evaluation of these tumors is generally nonspecific; however, thymomas are often quite large causing significant mass effect over the adjacent lung lobes, trachea and mediastinal vascular structures, and can extend caudally mainly along the left hemithorax. They can show cystic areas and a solid component with a heterogeneous contrast uptake pattern (Fig. 6).¹⁸ An important reason for imaging cranial mediastinal neoplasms is to determine the presence and extent of vascular invasion, which can determine operability and prognosis.¹⁹

Abdomen

Liver and splenic masses are the more common detected abdominal masses, which can represent a diversity of malignant and non-malignant origins. Ultrasound is routinely used to assess these masses.²⁰ Benign and malignant masses can present with similar imaging features, and thus cytologic or histologic sampling is an important diagnostic tool. Moreover, vascularization pattern is crucial in defining masses. Dual- or triple phase CT imaging in dogs has been used in differentiation among hepatocellular carcinoma, benign nodular hyperplasia, and hepatic metastasis.^21,22

Angiographic CT techniques are also used to evaluate pancreatic insulinomas.²³ Ultrasound is a good screening modality to evaluate the pancreas, however insulinomas, mainly represented by small hypoechoic nodules, have been described challenging to image. Ultrasound is highly dependent on imaging skills of the ultrasonographer and accessibility of the pancreas, however is less cost intensive than CT and does not requires anesthesia.²⁴ The big advantage of CT is that offers multiplanar reconstruction, which is especially helpful in a small and anatomically challenging organ, and that angiographic CT techniques maximize the conspicuity of small pancreatic messes. A report of three dogs with insulinoma found that these masses showed a strong contrast enhancement that was present during the arterial phase (Fig. 7).²⁴ However, it has lately been reported to show variable attenuation patterns ranging from being hyperattenuated in the arterial phase to also being hypoattenuated in some cases.²³

Primary neoplasia of the canine adrenal gland includes adenoma, carcinoma and pheocromocitoma. CT imaging of adrenal masses is especially useful to detect adjacent vascular invasion to assess the resectability of the mass but it is not capable of delineating tumor type. Adrenal masses can contain mineralizations and cystic regions, and can show peripheral rim enhancement representing fibrous encapsulation, or a heterogenous contrast enhancement in delayed postcontrast CT images associated with haemorrhage or infarction. The proximity of the adrenal gland to the caudal vena cava predisposes to vascular invasion of the phrenic and caudal vena cava veins and the formation of thrombus (Fig. 8).²⁵

Another important indication for CT are intrapelvic masses of various origin, namely rectal, neural, vascular, glandular, adipose tissue, which are not accessible to transcutaneous ultrasound and are difficult to assess with radiography due to summation, but are better imaged with CT due to multiplanar reconstruction to localize the origin of intrapelvic masses and evaluate for bone or adjacent structures involvement.²⁶

Skeletal system

Primary bone tumors, of which the osteosarcoma is the most common in dogs, are usually assessed with radiographs, showing varying degree of osteolysis, periosteal reaction, cortical destruction and transition zone length (Fig. 9).²⁷

In terms of treatment options, limb amputation is commonly performed, although may not be chosen due to concomitant orthopaedic disease, owner compliance or neurologic disease. In case of limb-sparing surgery, determining lesion margins is essential. Various modalities including radiographs, non-contrast CT, MRI and scintigraphy have been reported with a tendency to overestimate lesion length.²⁸ A more recent study described multidetector CT with acquisition of submillimeter thick images to be a feasible modality. It has been shown that assessing with CT intramedullary/endosteal abnormalities represents the best predictor of tumor length.²⁹

The role of CT in the staging process

The purpose of diagnostic staging is to assess the extension of local disease and to evaluate for regional and distant metastasis to evaluate treatment options.¹ To assess for regional metastasis, the corresponding sentinel lymph node (SLN), which is the first drainage area of a tumour, needs to be always included in the study and evaluated carefully. It has been suggested that the medial retropharyngeal lymph nodes, which are the sentinel lymph nodes for the head and cervical masses, are more likely to be metastatic than reactive if they present with an asymmetric size or a loss of a detectable hilus of the lymph node in CT; however cytologic or histopathologic examination remains vital (Fig. 10A).⁵

The lung is a common site of distant metastases among various neoplasms.³⁰ CT is the most sensitive modality for detection and assessment of pulmonary lesions in human and veterinary medicine.^31-33 CT as a cross sectional modality and due to increased spatial resolution is superior to radiography in detecting pulmonary nodules (Fig. 10B). In conventional radiography, pulmonary nodules need to be at least 3 to 5 mm to be detected, whereas in CT pulmonary nodules of 1 mm are visualized. However, CT is not able to replace thoracic radiography for pulmonary metastasis monitoring, especially in light of recent concerns about cumulative diagnostic radiation dose from CT.^34-37

In case of primary neoplasia, especially with high metastatic rate, emerging whole body CT has been recommended as the most efficient tumor staging technique.^38-39 In a retrospective study about whole body CT, lymphadenomegaly was the finding most commonly reported. Depending on the type of tumor the spread to regional and distant lymph nodes may vary.³⁹ Tracheobronchial lymph node enlargement has been described to be associated especially with neoplasms including lymphoma, histiocytic sarcoma of the lung and metastatic adenocarcinoma.⁴⁰ In that retrospective study it has been further reported that the most common finding in the abdomen was splenomegaly, which may be attributed to the use of anesthetic drugs or neoplasia including hemangiosarcoma, lymphoma, or mast cell tumors.³⁹ Concerning staging canine appendicular osteosarcoma, whole body CT for evaluation for bone metastases has been assessed in comparison to the current recommended method including thoracic radiographs and whole body bone scintigraphy. It has been concluded that whole body CT is useful assessing potential metastatic lesions on bone scintigraphy, however it seems inferior as a screening tool alone compared to a combination of modalities.⁴¹ In cases in which CT scans have been performed for diagnostics, staging and/or radiation therapy planning for neoplasms such as oral/neck neoplasms or thoracic wall masses, a study focused on describing synchronous findings in dogs. They found 5 % tumor-associated abnormalities, including synchronous primary neoplasms, metastases of the primary tumor mainly to the lungs or both.⁴² Another study aimed at describing rarer findings in a late disease course such as skeletal and cardiac muscular metastatic neoplasia in dogs and cats. Multiple nodules were detected in the majority of the cases and most commonly affected epaxial/paraspinal muscles of the cervical, thoracic, and lumbar spine and superficial muscles of the thoracic wall.³⁸

Thus, on the one hand whole body CT is a time-efficient modality, which shows an increased spatial resolution to commonly used modalities for staging such as digital radiography and abdominal ultrasonography. However, on the other hand it is more expensive as a single modality and requires anesthesia.³⁹ So advantages and disadvantages need to be discussed with the owner and decided individually according to the patient.

Conclusion

CT is increasingly attractive to clinicians by offering increased spatial resolution and multiplanar reconstruction in order to describe localization, lesion margins and texture of neoplasms. CT is used to assess nearly all anatomical regions of the body with a varying degree; however, it might be used in combination or be replaced by other imaging modalities depending on availability, costs and patient.