Page 134 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

PART II Diagnostic Procedures for the Cancer Patient

VetBooks.ir 6

Diagnostic Imaging in Oncology

STEPHANIE NYKAMP AND ELISSA RANDALL

Diagnostic imaging is essential in the diagnosis, clinical staging, the sensitivity of radiographs is lower than that of CT for the
and evaluation of response to therapy of cancer patients. Radiog- detection of pulmonary metastasis, they will likely continue to
raphy, ultrasound, computed tomography (CT), magnetic reso- remain the initial screening test because of the low cost and high
nance imaging (MRI), positron emission tomography (PET), availability (Fig. 6.1). 6–8
and nuclear scintigraphy can all be used to evaluate the cancer Radiography is also the primary method for the diagnosis and
patient. Advanced functional imaging (e.g., PET/CT, PET/ monitoring of dogs and cats with tumors of the appendicular
9
MR) may also prove valuable in predicting response to therapy. skeleton (Fig. 6.2). Detection of an aggressive bone lesion on
The choice of modality is dependent on availability and desired radiographs is not definitive for neoplasia, but signalment, his-
outcome, with each modality having advantages and disadvan- tory, clinical signs, and travel history can help differentiate neo-
9
tages in regard to cost, sensitivity, and specificity. When evalu- plasia from infectious causes. Although radiographs remain the
ating the primary tumor, accurate detection of tumor margins primary diagnostic tool for appendicular bone tumors, CT and
is paramount. When evaluating for metastasis, the ideal modal- MRI are more accurate for determining tumor margins, which
ity would be both highly sensitive and specific so that all lesions is necessary to optimize outcomes when using advanced treat-
are detected accurately. If such a modality is not available, initial ment techniques such as limb-sparing surgery or stereotactic
imaging should be done with a modality that is highly sensitive radiosurgery. 10
and followed with one that is more specific. Limitations of radiography include superimposition of over-
8
The use of advanced imaging and improved techniques for lying structures and the relatively limited contrast resolution.
functional imaging are improving the accuracy and precision of Radiology software exists that can suppress overlying bony
cancer imaging. It should be noted that this can also result in structures using advanced processing and pattern recognition
1
stage migration. The effect of this additional information is not algorithms, which significantly increase sensitivity for detec-
11
fully understood for many and should be used with caution when tion of lung nodules ; however, they are of limited availability
2
predicting outcome. This chapter outlines the principles of each in veterinary medicine, and other imaging modalities have been
modality and their utility in assessing the veterinary cancer patient. used to overcome this limitation. When the preferred imaging
modality is not immediately available, radiographs can be useful
Imaging Modalities in detecting masses in other body regions (e.g., abdomen) as well.
Radiographs may provide evidence that a mass is present such
Radiography that abdominal ultrasound, CT, or MRI can then be performed
to detect the origin of the mass.
Conventional radiography has historically been the primary
modality for assessment of cancer patients because it is readily Ultrasonography
available at a low cost, but it is gradually being replaced with
other modalities that are more sensitive, specific, and becoming Ultrasound is widely available and relatively inexpensive, result-
more readily available. ing in widespread usage as a first-line diagnostic modality for a
With few exceptions, radiography is a screening test rather variety of diseases, including cancer diagnosis and staging and
than providing a definitive diagnosis. The most common appli- for restaging or evaluating response to treatment. The ability to
cation for radiographs is screening for pulmonary metastasis. In evaluate the internal structure of organs and to better evaluate
dogs and cats, thoracic radiographs are obtained with the patient body cavities in the presence of effusion has resulted in ultra-
in a recumbent position, and this results in atelectasis of the sound replacing survey and contrast radiographs as the primary
dependent lung. Three views (left and right lateral and dorso- method of abdominal imaging.
ventral or ventrodorsal) are recommended because this position- Ultrasound is very good for identifying and localizing lesions,
dependent atelectasis can reduce lesion conspicuity. The diagnosis but the sonographic appearance may not be specific to the nature
would change in 12% to 15% of patients when only two views of the lesion. 12–21 The lack of specificity is more apparent when
are obtained, with left lateral and ventrodorsal views being less assessing multicentric diseases, such as lymphoma and mast cell
sensitive than right lateral and ventrodorsal views. 3–5 Although tumor, where the sonographic appearances are variable and a

113113