HISTOLOGIC CLASSIFICATION
Carcinomas are the most frequently diagnosed lung tumors, but consensus for the most common histologic subtype is lacking.
This lack of agreement is partially attributed to researchers using differing classification schemes, precluding direct comparisons
between historical and recent studies.
In veterinary medicine, the simplest classification scheme is by general cellular morphology, which includes adenocarcinoma,
squamous cell carcinoma, and anaplastic carcinoma.6,19 Primary lung carcinomas can also be organized by topographical location, inclusive of bronchial, bronchoalveolar, or alveolar
carcinoma.6,10 Confusingly, some studies categorize bronchoalveolar carcinoma into a separate cellular morphologic subtype altogether.5,8,14 Although organizing primary lung tumors by topographical location is useful in people, veterinary pathologists do not recommend
this classification scheme in companion animals because of the advanced stage of the disease at the time of diagnosis, which
makes determining the exact topographical origin of lung tumors extremely difficult.10,19 Instead, in veterinary studies, pulmonary carcinomas are generally divided into four subgroups—adenocarcinoma, squamous
cell carcinoma, bronchial gland carcinoma, and alveolar cell carcinoma.4,10
Adenocarcinoma, regardless of tissue cell origin, is the most common histologic type in dogs and cats.2,4,10,12-14,16,19 In two studies, adenocarcinoma made up 74% to 77% of all primary lung tumors histologically examined.4,10 Other reports suggest adenocarcinoma is more common in people and cats and bronchoalveolar carcinoma is more common in dogs.5 Squamous cell carcinoma, which is the most common pulmonary neoplasm in people, is less common in companion animals, representing
6% and 4% of all of canine and feline lung tumors, respectively. Anaplastic carcinoma is the rarest morphologic subtype in
dogs and cats,10 but it is still more common than primary mesenchymal lung tumors, which are extremely rare in dogs and cats.4,10,20
In a case report of four dogs with mesenchymal or mixed-cell-origin lung tumors, the age range was 4 to 15 years, with spayed
females and castrated males in equal proportion.20 All tumors were in the right lung fields—two were hamartomas, one was a pulmonary chondroblastic osteosarcoma, and one was
a biphasic pulmonary blastoma. Only the patient with the chondroblastic osteosarcoma had pulmonary metastasis visible on thoracic
radiography. Both patients with hamartomas were alive at least one year post pneumonectomy.20 Other reports of canine sarcomas include an osteosarcoma, a chondrosarcoma, and a Spirocerca lupi-associated fibrosarcoma, with two of the three patients presenting with hypertrophic osteopathy.21-23
Pulmonary lymphomatoid granulomatosis is an extremely rare, diffuse disease of the canine lung that has a yet undetermined
cause and may be immune-mediated or preneoplastic in origin.24-26 Fine-needle aspiration provides poor diagnostic yield with this disease process, and definitive diagnosis must be based
on histologic examination. However, because of the historically rapid response to chemotherapy, empirical treatment may be
used to aid in diagnosis.24 Of the primary neoplastic diseases of the lungs, pulmonary lymphomatoid granulomatosis carries the best prognosis.24,25
1. A lateral thoracic radiograph of a 10-year-old castrated male German shepherd presented for evaluation of an intermittent,
nonproductive cough of six months' duration. A soft tissue mass effect is identified in the left caudal lung field. The histologic
diagnosis was pulmonary adenocarcinoma. (Photograph courtesy of Dr. Laura Garrett.)
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The anatomical location and histologic subtype of primary pulmonary tumors in dogs tend to share similar distribution patterns
observed in people. Adenocarcinomas usually present as peripheral, solitary, well-defined masses (Figure 1), while bronchoalveolar carcinomas often manifest as peripheral multifocal densities (Figure 2).5,12,14,18 And primary lung squamous cell carcinomas tend to grow as solitary, hilar masses.10,12,14,18
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