Sherman is a 10-year-old neutered male terrier mixed-breed dog who first came to see me last fall. The primary complaint was a multilobular soft tissue mass arising from the left chest wall.
The reason for referral was to determine if Sherman's mass could be excised and, if not, what alternatives could be offered, such as radiation therapy.
Diagnostic imaging remains one the most useful tools in the clinical staging and re-evaluation of animals with cancer. The sensitivity and specificity of the various modalities, such as survey radiography, ultrasonography (US) and X-ray computed tomography (CT) vary, and therefore, their abilities to distinguish tumor size, margins, and invasiveness differ.
The value of various diagnostic imaging techniques in cancer detection is determined by four properties: spatial resolution, contrast discrimination, temporal resolution and distortion. These properties differ for each of the imaging techniques and Sherman is a classic example to illustrate the differences.
Sherman presented with a freely movable left sided mid-chest wall mass. The mass was soft, non-ulcerated, not painful, and not appearing to be fixed to underlying costal muscles or rib.
There was a complete lack of appreciable resolution of the mass by survey radiography. Survey radiography has the advantages of being readily available, inexpensive and non-invasive.
Although the spatial resolution of survey radiography, compared with the other modalities, is excellent, the contrast discrimination between normal and neoplastic soft tissue is poor. Structures lying in contact with one another cannot be distinguished as separate entities if they have the same physical density. Invasiveness and tissue architecture cannot be assessed.
Ultrasound evaluations of small soft tissue masses occurring along the extremities and body wall are also problematic. Sometimes paraffin offsets can be used to obtain greater definition of margins.
Measurements of Sherman's mass could not be performed in real time or by the developed image because of poor temporal and spatial resolution. Medical gray-scale ultrasonography is a non-invasive, nonionizing imaging technique that uses high-frequency, low-intensity sound.
Different tissues or matter can be recognized ultrasonographically by different reflective interfaces.
Ultrasonographic imaging provides information on soft tissue architecture and neoplastic size that survey radiography usually does not provide; however it does not define spatial resolution as well as survey radiography does.
Tissue orientation relies predominantly on the skill of the ultrasonographer. Artifacts (echo distortion) are caused by ultrasound-matter interactions, reverberations and acoustic shadowing through refractive and reflective zones; therefore, complete determination of neoplastic contour, invasiveness, and architecture is prohibited.
Solid structures with a composite of tissue types tend to be hyperechoic, resulting in poor definition of the far margins.
When Sherman's tumor was imaged by CT it became apparent that considerable tissue invasion along and between the fascial planes had developed. X-ray computed tomography provides excellent contrast discrimination among various tissue types.
With this technique, a narrow beam of X-rays is directed at the patient by a source that rotates slowly about the patient in synchrony with detectors.
The data from the anatomic slice is reconstructed into a two-dimensional picture. Pictures may be reconstructed to provide dorsal, sagittal and transverse planes for accurate tumor measurements.
Furthermore, tumor physical density may be compared with that of normal tissues during imaging (without contrast enhancement) to provide further information on neoplasm size and/or degree of invasiveness.
As seen in this example, neoplasm size, contour, degree of invasiveness and tissue density can be accurately evaluated by comparing subtle differences in soft tissue density and architecture.
What to Use
Unlike US, the unique cross-sectional imaging ability of CT makes it possible to detect soft tissues independent of overlying bony structures. However, spatial resolution is poor because CT, like US, examines only one slice of tissue at a time. Furthermore, to decrease artifact production by patient movement, anesthesia is required.
While physical findings suggested otherwise, CT demonstrated that Sherman's tumor was well beyond the possibility of excision with clean margins.
So, should every soft tissue mass be examined by CT before removal?
Possibly. If surgical completeness of a body wall, facial, or appendicular mass is under question, CT offers the assurance (or insurance) of avoiding unnecessary procedures or unrealistic prognostic expectations.
Kevin A. Hahn, DVM, Ph.D., Dipl. ACVIM (Oncology), is director of Oncology Services at Gulf Coast Veterinary Specialists, Houston (www.gcvs.com/oncology), and is the oncology consultant for YourNetVet (www.yournetvet.com).