Page 529 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

CHAPTER 24 Tumors of the Respiratory System 507

Comparative Aspects Pathology and Natural Behavior

VetBooks.ir Laryngeal cancer in humans is primarily related to smoking and Pulmonary tumors can arise from any tissue in the lung, but most
commonly they originate from the epithelium of the airways or
alcohol consumption. The vast majority are SCCs. Initial man-
alveolar parenchyma. Tumors derived from epithelium of large
agement strategies were aimed at early laryngectomies with wide
surgical margins for locoregional control, but this resulted in airways are typically located near the hilus, whereas parenchymally
aphonia. The pendulum then swung toward chemoradiation to derived tumors tend to be peripherally located. However, in the
preserve the larynx and laryngeal function, but the quality of life most recent World Health Organization (WHO) guidelines on
and functional outcome did not correlate with an anatomically classification of pulmonary neoplasms, lung tumors of domestic
preserved larynx. Currently, the preferred treatment strategy has animals are largely classified by histologic pattern and not by site
evolved into conservative laryngeal resections with preservation of of origin. 264
laryngeal function, including good swallowing function without Approximately 85% of canine primary epithelial lung
aspiration and speech production. 253 Early stage laryngeal cancer tumors are bronchoalveolar in origin, whereas adenocarci-
has overall survival rates of over 90%, but this falls to 27% to 39% noma, adenosquamous carcinoma, and SCC collectively com-
in advanced cases treated with chemoradiotherapy. 254 prise the remaining 13% to 15% of primary epithelial lung
Intraluminal stenting of tracheal masses has been described tumors. 261,262 Primary pulmonary histiocytic sarcoma (HS)
in humans and is being performed increasingly commonly, 255 may also represent a significant percentage of diagnosed lung
although primary tracheal cancer (unrelated to bronchial or lung tumors, particularly in predisposed breeds such as miniature
cancer) is very uncommon. schnauzers. 276–278 Adenocarcinoma represents 60% to 70% of
feline lung tumors, whereas bronchoalveolar carcinoma, SCC,
and adenosquamous carcinoma are less common. 264,265 Small
SECTION D: PULMONARY NEOPLASIA cell carcinoma represents approximately 25% of human pul-
monary neoplasms, but rarely occurs in the dog or cat.
Lung tumors can spread by local invasion or via hematogenous
WILLIAM T.N. CULP AND and lymphatic routes, resulting in locoregional spread to other
ROBERT B. REBHUN areas of the lung or LNs, or distant metastasis. Intrapulmonary
metastases are believed to occur through vascular and lymphatic
Incidence and Risk Factors invasion or intraairway seeding. Local vascular or lymphatic inva-
sion was present in 71% of canine pulmonary malignant tumors
Lung cancer is the leading cause of cancer-related human deaths in one study, and 23% had distant metastasis beyond the tracheo-
worldwide, but primary lung cancer remains relatively uncom- bronchial LNs. 262 SCC and anaplastic carcinomas have meta-
mon in pet dogs and cats. The incidence of primary lung cancer in static rates exceeding 50% and 90%, respectively, and are more
dogs and cats presenting for necropsy is less than 1%. 256–258 Inci- likely to metastasize than adenocarcinoma or bronchoalveolar
dence rates in dogs range from 4.2 per 10,000 dogs per year in the carcinoma. 258
United States to 15 per 100,000 dogs per year in the United King- Metastasis is common in the cat with a reported metastatic rate
dom. 259,260 In contrast, the incidence of pulmonary neoplasia was of 76%. 265 The size of the largest mass has also been associated
8.8% in a closed colony of beagles, with a high incidence of pulmo- with metastatic potential in the cat. 266 Metastasis to bone or the
nary tumors in dogs dying after the median lifespan of 13.6 years. 261 nervous system is not uncommon in dogs or cats. Metastasis to
The average age of dogs diagnosed with primary lung tumors the digits, otherwise known as acrometastasis or lung-digit syn-
is approximately 11 years, 256,262 with the exception of anaplastic drome, is a common and well-described clinical phenomenon in
carcinomas that occur at an average age of 7.5 years. 263 The Boxer, cats. 279,280
Doberman, Australian shepherd, Irish setter, and Bernese moun-
tain dog breeds are possibly overrepresented. 256,262,264 The average
age of cats with pulmonary tumors is 12 to 13 years. 258,263,265 Clinical Signs and Physical Examination
Persian cats have been reported to be overrepresented. 266 Findings
In people, the risk of developing primary lung cancer is strongly
associated with environmental tobacco exposure, but no defini- Dogs and cats are often diagnosed with a primary pulmonary
tive risk factors have been identified in dogs and cats. Urban liv- tumor incidentally during a routine geriatric screen. 256,261,281
ing and second-hand smoke exposure have both been implicated Up to 30% of cases of primary pulmonary tumors will be diag-
as potential causes of lung cancer in dogs but are yet to be clearly nosed without the presence of clinical signs. 281 The most com-
demonstrated. 267,268 An increased risk of lung cancer was found in mon clinical sign reported in dogs with pulmonary neoplasia is
dogs with increased amounts of anthracosis, suggesting an associa- coughing, which is noted in 52% to 93% of dogs. 256,281–284 Other
tion between inhalation of polluted air and lung cancer. 269 Anthra- clinical signs include dyspnea (6%–24%), lethargy (12%–18%),
cosis has also been correlated to high EGFR expression in canine hyporexia (13%), weight loss (7%–12%), hemoptysis (3%–9%),
primary lung tumors. 270 Cytologic analysis of bronchoalveolar and lameness, likely secondary to hypertrophic osteopathy (HO,
lavage fluid also revealed increased anthracosis in dogs exposed to 4%). 281–283
passive tobacco smoke compared with dogs without a history of The clinical signs in cats are similar to dogs; however, the
exposure. 271 In the experimental setting, laboratory dogs trained to occurrence of these signs is variable and gastrointestinal signs may
smoke cigarettes through a tracheostoma (in the presence or absence be noted as well. 285,286 As in dogs, signs referable to the respiratory
of asbestos exposure) did develop lung cancer at a higher rate than tract are common with dyspnea (20%–65%), cough (29%–53%),
control dogs. 272,273 Experimentally induced exposure to radiation, tachypnea (9%–14%), and hemoptysis (10%) being noted regu-
such as plutonium, also significantly increases the occurrence of larly. 285–289 Recently, a study found that 5 of 35 cats with sponta-
lung cancer when inhaled as an aerosol in research dogs. 274,275 neous pneumothorax had intrathoracic neoplasia and at least three