Three radiographic views of the thorax should be made in all cancer patients. ^2,3 The reason for this is twofold. The primary objective of this examination is detection of pulmonary metastasis, disease extension into the mediastinal or hilar lymph nodes, or some other extension of the neoplastic process into the thorax. The secondary objective is to detect any concurrent disease process that would have a negative impact on treatment of the cancer. Examples of the types of diseases that might be detected are heart disease, pulmonary diseases, mediastinal and pleural disease, or diseases of the skeleton. Advanced disease of any of these types could either alter or delay treatment or could obviate it altogether. For example, treatment of a patient with an enlarged heart using doxorubicin may be ill advised or a protracted course of radiation therapy would have little therapeutic value in a patient with advanced pulmonary metastatic disease.

It is essential that the radiographic examination of the thorax consist of both the right and left lateral recumbent views as well as a ventrodorsal (VD) view. ^2,3 Because of the recumbent nature of the radiographs generally taken in dogs and cats, there is rapid migration of fluid within the lung to the down side, as well as compression of the down lung lobes by the overlying heart and upper lung. There is also reduced motion of the chest wall and diaphragm on the dependent side, owing to the compressive effects of the animal’s weight. The down side of the diaphragm is also displaced forward by the weight of the abdominal viscera. All of this results in a marked increase in the density of the dependent lung, which can hide not only metastatic lesions but also other lung pathology ( Figure 9-1 ). These effects take place rather quickly and are increased as the animal spends more time on its side before the radiograph is made. Sedation or anesthesia advances the rate and severity of these changes. Radiographs of the thorax taken with an animal under anesthesia should be made with the thorax in a state of forced inhalation by inflating the lungs with a ventilation device. Radiographing the lung field from both sides with a short period of accommodation between allows examination of both sides of the lung field in a relatively aerated state. This facilitates detection of pulmonary lesions, particularly those in the periphery.

The orthogonal view should generally be a VD view rather than a dorsoventral view, in order to allow more of the lung field to be seen. The same type of density changes that occur for the lateral positions also occur for the VD view but are generally minimized by the fact that animals are not usually held on their backs for extended periods. Again, anesthesia or sedation will exacerbate the effects. Dorsoventral views are acceptable as orthogonal views and may be preferable if the lateral views detect suspect lesions in the dorsal part of the lung field.

Typical metastatic lesions and other small opacities within the lung are frequently more difficult to detect on the VD view than on the lateral views. This is due to the larger amount of tissue through which the x-ray beam must pass on its way to the film or detector. The heart occupies a relatively larger proportion of the lung field in the VD view, and mediastinal structures obscure the portions of the lung field that cross it. However, the VD view is important for demonstrating that a lesion seen on the lateral view is not located on or in the chest wall (see Figure 9-1 ). A lesion seen over the lung field on orthogonal views is quite likely to actually be within the lung field.

Radiographic detection of metastatic lesions on chest radiographs has a substantial negative impact on the prognosis of a cancer patient. Owners may opt not to proceed with treatment if pulmonary nodules are identified. For this reason it is very important that suspected metastatic nodules be confirmed. Metastatic lesions in the lung field share the general characteristics of any soft tissue nodular lesion in the lung field. Namely, they are single or multiple, round to ovoid, poorly to moderately circumscribed, and of a wide range of sizes and soft tissue in opacity. These characteristics are also those of cysts, abscesses, granulomas and primary neoplasms, and there is essentially no way to specifically differentiate one from the other on the basis of plain radiographs (see Figure 9-1 ). A diagnosis of metastasis should be confirmed by aspiration or at least by demonstrating growth on sequential films. However, the presence of large numbers of nodules of uneven size coupled with the known metastatic potential of the primary neoplasm allows a tentative diagnosis of metastatic disease with a high level of confidence. Likewise, development of new lung nodules in a cancer patient is highly suggestive of neoplasia. However, a diagnosis of metastasis should not be made when the nodules are calcified. ⁴ Pulmonary metastatic disease, even that arising from bone tumors, essentially never calcifies in dogs and cats, to the extent that it appears calcified on radiographs. On occasion, primary lung tumors may faintly calcify, but even this is rare. Calcified nodules in the lung field are typically calcified granulomas or benign osteomas (see Figure 9-1 ). Thus, any calcified lung lesion must be biopsied to confirm a tentative diagnosis.

Radiographs are extremely useful in the diagnosis of osseous neoplasia. It would seem that every oncologist has obtained biopsies of osseous lesions thought to be neoplasia, only to receive a biopsy diagnosis of reactive or inflammatory bone despite a typical clinical presentation. Biopsy only yields a small part of the lesion for histological examination, and bone tumors vary widely in the phenotype of the neoplastic cells from one side of the lesion to the other. There is also usually an inflammatory component in advanced tumors that may obscure the underlying disease. Radiographs depict the entire lesion, and some radiographic lesions very reliably indicate the lesion in question is a neoplasm ( Figure 9-2 ). It has been shown in human oncology that radiographic diagnosis of bone tumors is up to 20% more specific than biopsy. ⁵ Although improvement in biopsy techniques has closed this gap somewhat, the reliability of a radiographic diagnosis of bone neoplasia by a radiologist in the face of a negative biopsy indicates that the biopsy should be repeated.