Primary pulmonary neoplasia

Terminology

Primary pulmonary neoplasia in dogs and cats mainly refer to tumors originating within the lung parenchyma or airway epithelium. Epithelial tumors are the most frequently encountered type, with bronchial origin linked to central neoplasms and bronchiolar or alveolar origin linked to peripheral neoplasms ¹. Pulmomary carcinomas have subtypes according to their tissue of origin, such as bronchial, alveolar or bronchoalveolar.

Epidemiology

Primary pulmonary tumors in dogs and cats are rare compared to other types of neoplasia.

• Incidence: Less than 1% in dogs and cats presented for necropsy. Specifically, the incidence is 4.2 to 15 per 100,000 dogs per year and higher in older animals ¹.

• Age and Breed Predilection: Average age for dogs is 11 years, but anaplastic carcinomas occur around 7.5 years. Boxer, Doberman, Australian Shepherd, Irish Setter, and Bernese Mountain Dog breeds may be overrepresented. Cats average 12 to 13 years, with Persian cats being overrepresented ¹.

• 87% of tumors were carcinomas (majority adenocarcinomas), 7.6% histiocytic sarcomas, 1.5% neuroendocrine tumors ³.

• Bernese Mountain Dogs and miniature Schnauzers are highly predisposed to histiocytic sarcoma.

Risk factors

• No definitive risk factors identified. There’s some evidence linking anthropogenic factors like urban living and second-hand smoke. Increased anthracosis and experimental radiation exposure have associations with higher incidences of lung cancer ².

Clinical presentation

Common symptoms in dogs include coughing, dyspnea, lethargy, hyporexia, weight loss, hemoptysis, and lameness (likely secondary to hypertrophic osteopathy). 37% of cases were asymptomatic in one study ³. Cats show similar signs, often including gastrointestinal symptoms. Approximately 30% of cases are diagnosed incidentally during routine screenings ².

Pathology

Primary pulmonary neoplasms most often originate from the epithelial tissues of the bronchi, bronchioles, or alveoli ¹. Carcinomas are the most common type of canine lung tumor, up to 87% ³. Common subtypes include adenocarcinoma, which can be further subclassified. Papillary and bronchoalveolar subtypes of adenocarcinoma are most common ³.

Non-epithelial tumors include histiocytic sarcoma, sarcoma, and adenoma. Neuroendocrine tumors are rare.

Location

Pulmonary neoplasia can occur in any lung lobe. Higher probability of presence of adenocarcinoma in the left caudal lung lobe, and of histiocytic sarcoma in the left cranial or right middle lung lobe have been described ⁴. In one study, 19/20 histiocytic sarcoma were in the right hemithorax, 1/20 in the left hemithorax (left cranial lobe) ³. Another study had tumor distribution primarily in the caudal lung lobes (56.8%), followed by the cranial lung lobes (27.7%), right middle lung lobe (11.2%), and accessory lung lobe (5.3%) ³.

Classification

Pulmonary tumors are classified by histologic pattern. Major canine types include adenocarcinoma, adenosquamous carcinoma, and squamous cell carcinoma. In cats, adenocarcinoma predominates ².

Macroscopic appearance

Common presentations include solitary well-circumscribed masses in the lung, which can vary from firm and pale to multinodular, occasionally with eosinophilic or mucus secretion within the tumor ¹.

Microscopic appearance

Histologic patterns range from acinar, papillary, to solid. Tumors are typically composed of epithelial cells with varying degrees of differentiation. In cats, patterns like lepidic, papillary, and adenosquamous are common ¹.

Immunophenotype

For primary epithelial neoplasms, most express TTF-1, especially adenocarcinomas. Pan-cytokeratins (AE1/AE3) and vimentin can also co-express in less differentiated tumors ¹.
Histiocytic sarcomas express CD18.

Genetics

Breeds with predisposition to histiocytic sarcoma include Bernese Mountain Dogs, Golden Retrievers, Labrador Retrievers, Flat-coated Retrievers, and Rottweilers.

Radiographic features

Primary pulmonary tumors in dogs and cats are predominantly malignant and often involve the caudal lung lobes. Tumors typically present as solitary, well-circumscribed masses, and CT imaging is preferred for a more definitive diagnosis due to its superior sensitivity over radiography.

Radiography

On radiographs, primary pulmonary neoplasms may appear as single or multiple masses, often involving the caudal lung lobes. They are usually well-defined but can vary in appearance. Solitary solid masses are usually primary pulmonary neoplasia. Multiple nodules may also represent pulmonary neoplasia with metastasis (17% of dogs). Cavitary nodules may be larger tumors with necrotic centers ⁷.

Pulmonary nodules are generally referred to as < 2 cm and masses > 2 cm. The lower limit of detection for pulmonary nodules on radiographs is 7-9 mm ⁷. Conspicuity is also affected by the position in the lung lobe where it may be superimposed on a more dense structure such as the rib or the heart, or obscured by the mediastinum. Multiple projections help to optimize the visibility of these lesions.

Cats have a higher metastatic rate of up to 76%. Metastasis can occur to the digits in this species (acrometastasis) ². Distant metastasis is uncommon in dogs, <1% ³.

Histiocytic sarcoma has some differentiating imaging characteristics. The masses tend to be large, peripheral or lobar, and contain air bronchograms. The left cranial and particularly the right middle lung lobes have the highest frequency of involvement ⁴. In one study, 19/20 histiocytic sarcoma were in the right hemithorax, 1/20 in the left hemithorax (left cranial lobe) ³.

Fluoroscopy

Fluoroscopy can be used to confirm if a nodule is intrapulmonary based on movement during respiration ⁷.

Ultrasound

Ultrasound is limited to imaging lesions at the visceral pleural surface where the aerated lung does not block ultrasound transmission. Pulmonary neoplasms in peripheral lung tissues appear as solid, homogeneous masses with echogenic margins. Cavitation and internal echotexture variations may be present ⁵. Positioning can aid in compression of the lung to decrease intervening aerated lung. Fine needle aspirates of lung masses were diagnostic for neoplasia in 75.6% of samples ³.

CT

CT imaging is more sensitive than radiography for detecting primary pulmonary tumors. Tumors appear as solitary, well-circumscribed masses that may show mild to moderate heterogeneous contrast enhancement. Associated findings can include tracheobronchial lymphadenopathy, pulmonary nodules, pleural effusion, and pulmonary metastases ⁶.

Larger masses can contain gas or fluid, and the center can be non-enhancing if necrotic. Primary bronchial neoplasms tend to be more central with bronchial invasion. In cats, bronchial neoplasia may be more diffuse and include pulmonary consolidation, bronchial thickening, and mineralization ⁸.

Pulmonary metastasis occurred in 17.6% of dogs in one study ³. Distant metastasis was present in 0.9%.

CT identified lymphadenopathy in 28.7%, with half of those biopsied having metastatic disease. Conversely, 30% of the normal appearing lymph nodes that were biopsied had evidence of metastasis ³.

Radiology report

A radiology report on primary pulmonary tumors should include:

• Detailed description of the mass or lesions

• Tumor margins (well-circumscribed or irregular)

• Enhancement patterns on CT

• Evidence of metastasis (pulmonary nodules, lymphadenopathy)

• Additional findings (pleural effusion, bronchial involvement)

Morphology

• Solid tissue mass, centrally or peripherally located, potential cavitation (gas or fluid-filled)

Relationship to/involvement of

• Deviation and compression of bronchovascular structures

Extension

• Involvement of adjacent lung lobes, body wall, lymph nodes, or distant metastasis

Treatment and prognosis

Surgical resection is the treatment of choice. One year survival was 46% and two year survival was 20.9% in one study, with median survival time 242 days. Adjuvant chemotherapy does not appear to increase median survival time ³. Minimally invasive surgery such as thoracoscopy can be used to approach some tumors.

Factors that adversely affected prognosis included primary tumor size > 5 cm, pleural effusion, increasing mititic count, distant metastasis, and lymph node status N1 or N2 ³.

Complications

Complications include metastasis to distant organs and the development of hypertrophic osteopathy, particularly in dogs ¹.

History and etymology

Pulmonary neoplasms are named according to their histological pattern and cell of origin (e.g., adenocarcinoma from glandular epithelium) ¹.

Differential diagnosis

• Pulmonary metastasis from distant primary tumor

• Inflammatory disease - granuloma, cyst, abscess

Practical points

• Most primary pulmonary neoplasms in dogs are carcinomas

• Histiocytic sarcomas have specific distribution, appearance, and breed predisposition

• Pulmonary metastasis is frequent (18%), distant metastasis is rare (1%)

• Lymph node metastasis is approximately 45% and CT is not sensitive or specific

• Routine screening in older animals can aid early detection.

• Immunohistochemistry is valuable for differentiating primary from metastatic lung tumors.

• CT scanning provides better sensitivity for small nodules compared to radiographs.

• Be cautious of secondary health conditions, such as hypertrophic osteopathy or metastasis to uncommon sites like digits in cats.