Diagnostic Imaging: Skin mass or pulmonary nodule

Thoracic radiographs for metastatic disease are part of every day practice. A diagnosis of pulmonary nodules has an important effect on treatment decisions, and some radiographs are difficult to interpret. One scenario is a dog with one or several masses on the thoracic body wall and an anal-sac carcinoma. How do you decide whether the soft-tissue opacities you can see on the radiographs are on the skin or in the lung parenchyma? There are a few ways you can try to distinguish them.

Photo 1: On the dorsoventral radiograph, the skin mass is superimposed over the right crus of the diaphragm. The margin is very sharp, and can be followed for about 90 degrees of the circumference.

Sharpness of the margin

We can see pulmonary nodules or skin masses as separate from lung or skin because they are surrounded by air, creating a soft-tissue/air interface that has very high contrast. A mass on the thoracic wall is surrounded by air only. Nodules in the lung are surrounded by air, small vessels and pulmonary tissue, and the thoracic wall. This means that skin masses have much sharper margination than lung nodules. Compare the sharpness of the body wall mass on the caudoventral right thorax to the pulmonary nodules in the images. This dog has multiple pulmonary metastases from a large liver mass as well as a thoracic wall mass. Digital radiographs such as these will make the margination of external masses even sharper than on film because of the increased contrast of the images.

Completeness of the margin

Because body-wall masses are attached to the skin, the visible margin is usually incomplete. The air surrounds the mass on two or three sides, but not circumferentially. Lung nodules have margins that are visible for 360 degrees unless they are silhouetting with another structure. Check multiple nodules for completeness of the margins.

Photo 2: The mass is outlined by air ventral to the sternum, and is outside the thorax.

Use a marker

If you still can't tell if the soft-tissue structure you are looking at is a skin mass or a pulmonary nodule, then place a marker on the skin mass and repeat the radiograph. A dab of liquid barium or metallic adhesive markers both work well. This technique helps you to document which masses are accounted for externally.

Location

Since radiographs are a two-dimensional representation of a three-dimensional body, we need two views to pinpoint the location of a nodule. For example, the pulmonary nodule between ribs 4 and 5 on the left lateral projection is visible in the same rib space on the dorsoventral projection.

On both radiographs, this nodule is superimposed on lung tissue, which means it's located in the lung. The skin mass is superimposed over lung on the dorsoventral projection but is ventral to the lung margin on the left lateral projection. This triangulation proves that it is outside the lung.

Photo 3: On the right lateral projection, the mass is superimposed on the ventral thorax and diaphragm and could be misinterpreted as being located in the left caudal lung lobe. Approximately half the margin is visible. There also is a pulmonary nodule between ribs 8 and 9. It has a 360-degree visible margin.

Practice

Next time you look at radiographs, compare some of these features using structures outside and within the body. Notice the sharpness and incomplete margin of nipples compared to end-on vessels near the heart base, as well as their position on two projections. With practice, these small but useful details become part of your radiographic interpretation skill set.

Dr. Zwingenberger is a veterinary radiologist at the University of California-Davis.