One characteristic of most highly malignant tumors, regardless of their point of origin, is that they invariably spread to and end up in the lungs if not detected and treated soon enough. Some cancers spread, or metastasize, more readily to the lungs than others.
For instance, malignant melanoma of the skin might be present in the lungs even before the actual skin tumor becomes noticeable. The clinical signs exhibited by pets afflicted with metastatic lung disease can be similar to those seen with pneumonia.
Unfortunately, the prognosis for dogs and cats harboring such tumors in their lungs is grave, even with specific cancer treatment.