Page 138 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

CHAPTER 6 Diagnostic Imaging in Oncology 117

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• Fig. 6.7 Transverse computed tomography of the caudal thorax shows
a focal ground-glass nodule (arrow) in the right caudal lung lobe. Ground-
glass pulmonary lesions, also referred to as subsolid nodules, are char-
acterized by an increased density that does not obscure the underlying
pulmonary structures.
• Fig. 6.8 A 3D volume rendering of computed tomography angiography
of a thyroid mass shows the feeding blood vessels and highly vascular
obscure the underlying pulmonary structures (Fig. 6.7). These nature of the mass, consistent with a thyroid carcinoma.
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nodules may be pure or mixed (containing some solid compo-
nents). These lesions are frequently detected in people being
staged for pulmonary metastasis, with up to 70% being caused mediastinal and thyroid masses, which may assist in the determin-
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by transient inflammation or hemorrhage. Pure ground-glass ing the preferred treatment options for these patients. 71,72
lesions >8 mm have a high probability of malignancy, whereas CT-guided biopsy is useful for the sampling of intracavitary
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those <4 mm are considered benign. Lesions >22 mm, mixed lesions, such as pulmonary nodules, not readily identified with
lesions (partially ground-glass and partially solid), mixed lesions ultrasound. Manual CT-guided biopsy is performed using the
where the solid component grows to >2 mm in maximal diameter, internal laser of the scanner to orient to the transverse plane that
and pure ground-glass lesions progressing to mixed lesions have includes the lesion of interest. Needle placement is assisted by
a higher risk of being malignant. 61–63 Ground-glass lesions have radiopaque markers in bands in the sagittal or parasagittal plane.
not been fully investigated in veterinary medicine, but 75% of Barium can be used to create radiopaque bands, but commercially
dogs with ground-glass lesions went on to develop radiographi- available opaque grids are also available that can be adhered to the
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cally visible metastasis in one pilot study. To clearly discern the skin (Fig. 6.9). 73
characteristics of pulmonary nodules, thin-slice techniques are Although MRI provides superior contrast resolution, CT is
required to avoid partial volume averaging, as this artifact can used for RT planning because it provides a map of electron density
result in pure ground-glass nodules appearing to have a solid cen- information that is used by most planning computers to calculate
tral component. 64 dose distribution. Fusion of CT and MR images can aid in RT
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Because CT has higher sensitivity to lesion detection than planning by maximizing both the spatial resolution of CT and the
ultrasonography and radiography, there is increasing use of whole- contrast resolution of MR. In human cancer therapy, CT com-
body CT for cancer staging. In tumors with high metastatic bined with PET (PET/CT) is also used for RT planning, particu-
potential, CT may detect muscular metastasis that may have been larly with pulmonary lesions as PET/CT can differentiate between
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otherwise missed. Whole-body CT for dogs with osteosarcoma tumor and atelectic lung, which is not always possible with CT
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has not been shown to be superior to radiography or nuclear scin- alone.
tigraphy for the detection bone metastasis, but concurrent neo-
plasia has been detected, and this may alter treatment options and Magnetic Resonance Imaging
prognosis. 50,66 Detection of metastatic and concurrent disease
may be better with newer scanners and higher resolution refor- MRI provides superior soft tissue resolution to CT and is highly
matted images. sensitive to detection of pathology. MRI is generally considered
CT angiography (CTA) is being performed routinely to assess superior to CT for neurologic and soft tissue imaging, but a study
tumor vascularity, perfusion, and vascular invasion (Fig. 6.8). of feline injection-site sarcomas showed no difference in the evalu-
Multiphase CTA can improve detection of small tumors, such as ation of peritumoral lesions between CT and MRI. MRI is also
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insulinomas, and may be helpful in differentiating between benign excellent for detecting infiltrative diseases of the musculoskeletal
and malignant lesions. 67,68 CTA is also advantageous in evaluating system, including accurate determination of the local extent of
patients with pericardial effusion and thus enabling the detection appendicular osteosarcoma lesions. 10,76
of cardiac masses. CTA may assist in predicting malignancy of With MRI, numerous imaging sequences are used to provide
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hepatic and splenic masses and nodules, although more research is complementary information. T1-weighted images provide good
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needed. CTA also provides more accurate assessment of cranial spatial resolution to assess anatomy and are used with contrast