Pathology and Natural Behavior

The incidence of primary lung tumors in the dog and cat is low. The majority of tumors are malignant, and the most frequently reported tumor type is adenocarcinoma in the dog and cat. The average age at presentation is 9 to 10 years in dogs and 11 to 12 years in cats. Primary lung tumor has no breed or sex predisposition and no proven cause. Clinical signs tend to occur late in disease and vary at presentation. Often a chronic nonproductive cough is present; however, lung tumors may be diagnosed incidentally during radiography for another indication. Other signs include lethargy, dyspnea, weight loss, and tachypnea. Lameness can be seen in cats with musculoskeletal metastasis (lung-digit syndrome seen with pulmonary adenocarcinoma, bronchial carcinoma, and squamous cell carcinoma), and in dogs with hypertrophic osteopathy (rare in the cat).

Diagnostic Approach

Thoracic radiographs are the most important diagnostic test. Common findings are soft tissue density mass(es) (discrete or ill defined); lobar consolidation; and diffuse interstitial, alveolar, peribronchial, or mixed patterns. The appearance of pulmonary neoplasia in cats can vary dramatically, ranging from solitary, possibly cavitated lesions to diffuse mixed patterns. Differential diagnoses for discrete soft tissue opacity masses are abscess, hematoma, cyst, and granuloma; for other patterns they are pneumonia, metastasis, hemorrhage, edema, and fibrosis. Thoracic ultrasonography and cytologic examination of needle aspirates, pleural fluid, or bronchoalveolar lavage washes can aid in diagnosis. However, the sensitivity of detecting neoplasia in pleural effusions is as low as 60% in the dog and cat. Biopsy may be required for definitive diagnosis (Hirschberger et al, 1999). Computed tomography (CT) may be used to better determine the extent of disease before surgery if this information would alter the owner’s decision regarding treatment (Paoloni et al, 2006).

Treatment and Prognosis

Surgical removal is the preferred treatment for primary lung tumors. Reported median survival times after lung lobectomy are in the 10- to 13-month range. Positive prognostic indicators are absence of clinical signs at presentation, solitary nodule, peripheral location, and size of less than 5 cm in diameter. Negative prognostic indicators are lymph node involvement, more than one nodule, central location, size greater than 5 cm in diameter, and high histologic grade (Hahn and McEntee, 1998: McNiel et al, 1997). Small case series of chemotherapy with vindesine/cisplatin or vinorelbine have been reported in dogs, with antitumor activity observed with both protocols (Mehlhaff et al, 1984; Poirier et al, 2004). Radiation therapy for solitary lung masses has not been reported in veterinary medicine. Hypertrophic osteopathy in dogs, if present, usually resolves after removal of the primary mass. No information regarding the efficacy of postoperative chemotherapy for dogs or cats with lung tumors is available, although this treatment may be considered in patients with “high-risk” disease.

Pathology and Natural Behavior

Primary cancer of the pleura is called mesothelioma. Exposure to asbestos, especially amphibole fibers (long and thin), is considered a risk factor in humans. The mechanism by which it causes malignant transformation is unknown. Mesothelioma is highly metastatic and can invade the diaphragm and implant on abdominal structures. Neoplastic differentials include metastatic tumors of the pleural space, which in dogs and cats most commonly occur from carcinomas. The most common clinical sign is respiratory distress caused by pleural effusion (pericardial effusion also can occur with mesothelioma of the pericardium). Reactive mesothelial cells may be interpreted as malignant mesotheliomas based on cytology. However, a true mesothelioma in veterinary medicine is rare.

Diagnostic Approach

Fluid collected from the pleural space can range in appearance from hemorrhagic to chylous. Fine-needle aspiration can be obtained with ultrasound guidance, but cytologic diagnosis of pleural disease is problematic. Reactive mesothelial cells can appear malignant; thus cytology of fluid usually does not give a definitive diagnosis. Diagnosis usually requires a biopsy of the pleura via thoracotomy or thoracoscopy. Differentials include tumor seeding from cardiac hemangiosarcoma (HSA), chemodectoma, or metastatic carcinoma. Imaging of the thoracic and abdominal cavities is indicated to rule out primary tumors.

Treatment and Prognosis

Prognosis is poor for primary mesothelioma or for disease metastatic to the pleura. Because no successful definitive treatments have been reported, the goal of treatment is palliation of clinical signs. Intracavitary chemotherapy for palliation of pleural effusions is discussed in Chapter 75. Systemic chemotherapy has not been effective in either dogs or cats. Cytoreductive surgery may be used for larger masses before intracavitary therapy. Similar therapeutic approaches may be considered for animals with pleural carcinomatosis.

Pathology and Natural Behavior

The incidence of mediastinal masses is low in dogs and cats. The two most common differentials for a mediastinal mass are thymoma and lymphoma. Branchial cyst, ectopic thyroid carcinoma, and chemodectoma also can occur but are less common. The mean age at presentation is 9 years in dogs and 10 years in cats. Benign mediastinal cysts also are recognized in cats and carry a good prognosis with transthoracic needle drainage.

Thymomas originate from thymic epithelium and are infiltrated with lymphocytes (making differentiation from lymphoma difficult). The epithelial portion of the thymic tissue is considered neoplastic. Benign thymomas are noninvasive and well encapsulated, whereas malignant thymomas are locally invasive and aggressive. Thymomas rarely metastasize.

Lymphoma is considered a systemic disease in dogs and cats. Cats presenting with mediastinal lymphoma usually are young (mean age of 2 years) and positive for feline leukemia virus (FeLV). Clinical signs for mediastinal masses include coughing, tachypnea, dyspnea, caval syndrome (dependent edema of the head and neck, and/or forelimbs, owing to restricted lymphatic flow), muscle weakness, or megaesophagus caused by myasthenia gravis (a paraneoplastic syndrome associated with thymoma).