Page 137 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

116 PART II Diagnostic Procedures for the Cancer Patient

VetBooks.ir

A B
• Fig. 6.5 Ultrasound images from a 10-year-old female spayed mixed breed dog. (A) A soft tissue mass is
originating from the left adrenal gland (not visible) and invading into the caudal vena cava. (B) Color Doppler
evaluation of the caudal vena cava with evidence of blood flow around the mass, but approximately 60%
occlusion of the vessel by the mass and associated thrombus.

noise and partial volume averaging artifacts. Partial volume aver-
aging can result in blurry margins, false attenuation measurements
(Hounsfield units), and pseudolesions. Thin-slice images will have
increased noise that can be offset by increasing mAs setting. This
will improve image quality, but will also increase the radiation
dose to the patient. Thin-slice imaging is more challenging on
older scanners, as the heat load on the x-ray tube can slow the scan
time, but newer generation CT scanners, particularly multislice
helical scanners, have largely overcome the challenges, allowing
large areas to be scanned quickly. The scan field of view (SFOV) is
the area from which the image can be reconstructed. Because the
reconstruction matrix size is constant, keeping the SFOV only as
large as the anatomy to be imaged will improve spatial resolution.
This is not critical for the majority of body parts, but a large SFOV
used with whole-body CT can result in decreased resolution.
Although CT has decreased spatial resolution compared with
radiographs, the reduced anatomic superimposition and superior
contrast resolution result in CT being more sensitive to detect
lesions throughout the body. For assessment of noncardiac tho- • Fig. 6.6 A 3D reconstruction of a dog with a skull tumor can aid in
racic disease, CT provides more information on the presence, assessment of tumor extension and surgical or radiation planning.
location, and extent of disease in the majority of patients com-
pared with survey radiographs. 6,50 Accuracy for detection of tra- the treatment plan than necessary. 55,56 Detection of local tumor
cheobronchial lymphadenopathy is 93% for CT and 57% for invasion is an important finding on CT and is evident by loss of
radiographs, with a sensitivity of 83% and 0% respectively (using adjacent fascial planes and vascular filling defects. Large masses
56
size, shape, and contrast enhancement on CT). CT is also being can cause compression of adjacent structures and, in these cases,
4
used more frequently to evaluate the abdomen for primary and it can be difficult to differentiate compression from invasion.
57
metastatic disease. The ability to image pelvic canal contents and Although imaging characteristics are rarely pathognomonic for a
visualize anatomy deep to gas-filled structures improves the sen- specific tumor type, the CT appearance of fatty masses is highly
51
sitivity for detection of lesions. For example, CT evaluation of predictive of lipoma, infiltrative lipoma, and liposarcoma. Three-
58
insulinomas is superior to ultrasound, detection of sacral lymph dimensional reconstructions are also very valuable for assessing
node (LN) metastasis within the pelvic canal in dogs with apo- skeletal structures and surgical planning (Fig. 6.6).
crine gland anal sac adenocarcinomas is superior with CT com- CT is more sensitive than radiographs in the detection of pul-
pared with ultrasound, and the margins of soft tissue tumors, monary metastasis and allows for detection of nodules as small
such as sarcomas and lipomas, are better delineated with contrast as 1 mm in diameter compared with 7 to 9 mm for radiogra-
enhancement. 51–55 Tumors of the nasal cavity and head and neck phy. 7,8,50 In people, the high sensitivity of CT also results in
have superior delineation on CT compared with radiographs or detection of more benign lesions (up to 80%), with nodules <5
ultrasound. 53 mm infrequently developing into metastasis. 59,60 The incidence
CT is also superior to palpation in determining tumor extent, of benign pulmonary nodules has not been fully investigated in
54
as has been shown in cats with injection-site sarcomas, and this veterinary medicine, but all nodules detected on CT developed
is important implications in defining the aggressiveness of treat- into radiographically visible metastasis in one study of osteosar-
ment with surgery and/or radiation therapy (RT). However, it is coma in dogs. 50
not possible to differentiate between peritumoral inflammation Ground-glass pulmonary lesions, also referred to as subsolid
and local invasion, so more normal tissue may be included in nodules, are characterized by an increased density that does not