Practical Matters: Increase your ability to detect pulmonary metastases

Thoracic radiography is the cornerstone of staging disease in a cancer patient. Clinicians differ on whether to perform two-view or three-view thoracic radiography for evaluating the pulmonary parenchyma for metastasis. Most veterinary oncologists prefer three thoracic radiographic views (a right lateral, a left lateral, and a ventrodorsal or dorsoventral) for cancer staging. This method is important for accurate imaging and radiographic interpretation of pulmonary lesions, especially for solitary nodules. For instance, if a solitary mass is present in the dependent portions of the right caudal lung lobe, then it may be difficult to see this nodule or mass on the right lateral view. Two factors make abnormalities in the dependent lung difficult to see: the proximity of a lesion to the diaphragm and the poor inflation of the dependent lung parenchyma. When doubt exists, obtain a contralateral view to provide optimal lung inflation.

Other problems encountered in thoracic radiographic interpretation can often be resolved by evaluating three views. Some of these problems include extrapulmonary shadows of nipples (less common in cats) and subcutaneous nodules or masses and summation shadows, or superimposition, of structures such as ribs or major blood vessels. Other rule-outs for pulmonary nodules should include pulmonary osteomas, metastasis, granulomatous disease, abscesses, cysts, hematomas, and lung consolidation secondary to infection or pneumonia.

In a few cases it is still difficult to identify pulmonary metastases because they can appear as well-defined interstitial nodules, ill-defined interstitial nodules, or a diffuse pulmonary pattern. Correct exposure resulting in high-quality radiographs is also essential to be able to identify pulmonary nodules, particularly small ones.

The increased availability of computed tomography for veterinary patients allows more thorough evaluation for metastasis or other pulmonary parenchymal abnormalities. Computed tomography can detect small pulmonary parenchymal nodules or subtle changes at an early stage. A lesion must be a minimum of 8 mm to be imaged radiographically, but computed tomography will consistently image 2-mm to 3-mm lesions. In certain cases and if clinically indicated, consider a CT evaluation of the thorax.