Although primary pulmonary neoplasms are rare findings in dogs and cats, the reported incidence may be increasing.1-6 This potential rise may reflect multiple factors, including increased exposure to environmental carcinogens, the availability of more sensitive diagnostic tools, a greater number of necropsies being performed, and improved animal healthcare services resulting in the increased longevity of companion animals.1,2,4,7
Through the growing appreciation for the human-animal bond, many pet owners now support the practice of preventive or early-detection medicine in companion animals for several disease processes, including cancer. The benefit of annual senior pet examinations, which include survey thoracic radiographs, may allow for the early diagnosis of lung cancer in asymptomatic animals. Although the prognosis is generally better for patients with small, well-differentiated, peripherally located lung tumors,2,3,8 additional prognostic factors should be considered for predicting survival outcomes in dogs and cats with pulmonary neoplasms.
Illustration by Bonnie Hofkin
Pneumonectomy remains the cornerstone of therapy for primary lung tumors, but novel adjunctive therapies (e.g. combination chemotherapy) are being developed and evaluated for improving local disease control. In addition to advances in detecting and treating primary lung tumor growth, new systemic therapies have been investigated for prolonging the disease-free interval, as well as palliating secondary bony metastasis.9-11
The goal of this article is to describe the common histologic variants, clinical signs, biologic behavior, and newest options for early diagnosis and effective treatment of primary lung tumors in dogs and cats.
PREDILECTION AND INCIDENCE
Primary lung tumors represent about 1% of newly diagnosed tumors in dogs and cats, although the overall incidence is much lower in cats (Table 1).4,6,7,12-14 Some studies suggest that these tumors may be more common. In a life span study in a closed beagle colony, the incidence of primary lung tumors correlated with increasing age.15 For all beagles, an overall incidence of primary lung tumor formation was 8.8%; the incidence reached as high as 25% in geriatric dogs at necropsy.15 The factors that may have contributed to the high incidence of primary lung tumors in this beagle colony were not identified but may have included genetic, dietary, and unique environmental factors specific to the closed colony.
Table 1: Comparison of Lung Cancer in People, Dogs, and Cats
Primary lung tumors occur most commonly in larger, older animals, with a mean age of 10 or 11 and 12 years in dogs and cats, respectively.2,4,5,8,12-14,16 Although no definitive breed or sex predilection has been identified, boxers represented 37% of all lung tumor patients in one study,12 and in a feline investigation, older females were more often affected than males.10,14
Similar to people in which chemical carcinogenesis plays a prominent role in the genesis of primary lung tumors, a causal link is suspected to exist between smoking households and primary lung tumors in dogs, especially in mesocephalic and brachycephalic breeds.17 Moreover, dogs living in urban environments more commonly have lung tumors than do dogs in rural settings.4 This apparent environmental effect could reflect the greater pollutants within an urban environment or may simply be due to the greater population density of animals receiving routine veterinary care.
In dogs, the right caudal lung lobe is the most common anatomical location of primary pulmonary neoplasia.5,6,8,10,13,16,18 This observed anatomical preference is thought to be a result of greater lung tissue mass, thereby increasing the likelihood for spontaneous mutations within the right caudal lung fields.12 In cats, the caudal lobes are also more commonly affected, but they tend to have an equal occurrence in the right and left lung fields.14