Page 139 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

118 PART II Diagnostic Procedures for the Cancer Patient

compared with tissues that do not contain normal hepatocytes,
allowing for the differentiation between hyperplastic and malig-
87
nant nodules.
VetBooks.ir Nuclear Scintigraphy

Nuclear medicine uses radiopharmaceuticals that accumulate in
areas of interest based on physiologic processes. The low spatial
resolution of these images precludes anatomic detail, but the
functional information provided is advantageous in differentiat-
ing between occult and active disease processes. Scintigraphy lacks
specificity, so it cannot differentiate between benign inflammatory
lesions and malignant lesions. The two studies most commonly
performed in cancer patents are thyroid and bone scintigraphy.
Bone scintigraphy provides physiologic information by detect-
ing areas of increased bone activity (Fig. 6.11). Scintigraphy is
highly sensitive for increased metabolic activity, but is not specific,
• Fig. 6.9 Computed tomography (CT)-guided biopsy of the lung mass was and this can lead to false-positive diagnoses. 88,89 To reduce the
facilitated by placement of a hypodermic needle into the body wall along the risk of false-positive diagnoses, the use of supplementary imaging
laser line that corresponded with the location of the mass. After placement modalities, such as radiography, of suspected bone lesions is rec-
of the marker needle, CT images were obtained to determine the best place- ommended. Radiographs require at least a 30% to 50% change
50
ment of the biopsy needle relative to the marker needle. This image shows in mineral density to be detected, which means that early or small
the biopsy needle being placed ventral to the marker needle and perpendic- lesions may be missed on radiographs but detected with more sen-
ular to the body wall. The tip of the needle is visible in the pulmonary mass.
sitive CT scans. 50,90
Thyroid scintigraphy is used to evaluate patients with ventral
to show vascular enhancement of tissues. T2-weighted images cervical masses. Thyroid scintigraphy can be used for the diagnosis
are water-weighted sequences that provide high-contrast images of adenomatous hyperplasia in cats and can raise suspicion for
of water-dense pathology. On both T1- and T2-fast spin echo thyroid carcinoma in cats and dogs. In cats, thyroid carcinoma
sequences, fat has a high signal intensity (bright) that can mask typically has a large amount of hyperfunctional tissue, and ectopic
the margins of the pathology. The use of fat suppression tech- hyperfunctional tissue may be present extending into the thorax
91
niques on T1 postcontrast and T2 images (e.g., Short-tau Inver- (Fig. 6.12). Thyroid carcinomas in dogs and cats are typically
sion Recovery [STIR]) is important in cancer imaging to allow for heterogeneous with irregular margins. 91,92 Imaging is typically
−
clear assessment of the extent of pathology (Fig. 6.10). Diffusion- performed with 99m-technetium pertechnetate ( 99m TcO There
4
weighted imaging measures the random Brownian motion of is a superscript negative sign beside the O and over the 4.) because
water molecules in a voxel of tissue. Highly cellular tissues and of its availability and cost effectiveness.
swollen cells have restricted motion of water that can be detected Renal scintigraphy can be performed when nephrectomy is
77
with diffusion-weighted imaging. Although this has been evalu- being considered for cats and dogs with a renal tumor or in cases
ated in humans, there is limited information on the effectiveness in which adrenal tumors have invaded the renal vessels or compro-
in veterinary medicine. Veterinary studies have found diffusion- mised the adjacent kidney. A nuclear medicine glomerular filtra-
weighted images and apparent diffusion coefficient maps helpful tion rate (GFR) study provides an assessment of global GFR, as
in diagnosing acute infarcts, which is hyperintense on diffusion- well and right and left kidney GFR, so that the function of the
weighted images and hypointense on apparent diffusion coeffi- single remaining kidney can be predicted. 93
cient maps. 78,79 However, in one study of 37 dogs, there was a PET/CT or PET/MR is often preferred to scintigraphy to
wide range of apparent diffusion coefficient distribution within overcome some of the spatial limitations of nuclear scintigraphy.
disease groups and significant overlap between groups when evalu-
ating multiple tumor types, inflammatory disease, and infarcts. 80 Positron Emission Tomography/Computed
Because of the high contrast resolution, whole-body MRI has
also been investigated in cancer patients. Whole-body diffusion- Tomography (PET/CT) and MRI (PET/MR)
weighted MRI provides similar results to PET/CT in people with PET/CT and/or PET/MR combines the functional imaging of
diffuse B-cell lymphoma. In veterinary medicine, a protocol a nuclear medicine study and the high spatial resolution of CT
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using large overlapping imaging fields and focused high-contrast and MR. The most commonly used radiopharmaceutical with
18
sequences showed whole-body MRI can be performed in a reason- PET imaging is the glucose analog 2-deoxy-2- F-fluorodeoxy-
able time with sufficient quality to identify known lesions, but glucose (FDG) bound to the positron emitter fluorine-18 (F-18).
images were suboptimal for the skeletal system. Further inves- As a glucose analog, FDG is transported into hypermetabolic
82
tigation to optimize protocols for veterinary patients is necessary. cells, where it becomes trapped after phosphorylation by hexoki-
94
Recently hepatocyte-specific MR contrast agents, such as nase, as it is not a suitable substrate for glucose-6-phosphatase.
gadoxetate disodium (Gd-EOB-DTPA), have been evaluated. 83,84 The F-18 portion of the radiopharmaceutical allows for it to be
These agents accumulate in normal hepatocytes, potentially allow- imaged, as the positrons emitted create two annihilation pho-
ing for the differentiation between benign liver nodules and meta- tons that travel 180 degrees from each other. These photons are
static lesions, and aid in identification of primary liver tumors detected by the PET detector ring. Both tumors and inflamma-
94
on delayed imaging. 84–86 These agents cause shortening of the T1 tion result in increased glucose metabolism; therefore detection of
and T2 relaxation times, resulting in increased T1 signal intensity a hypermetabolic lesion is not definitively indicative of neoplasia.