An update on diagnosing and treating primary lung tumors - Veterinary Medicine
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An update on diagnosing and treating primary lung tumors
The incidence of this rare cancer in dogs and cats may be on the rise. Find out how to spot a primary lung tumor and what new forms of therapy may soon be at your disposal.



Primary pulmonary neoplasms are usually an incidental finding in animals presented for general health checks or evaluation of nonspecific signs. Thoracic radiography is often the initial step in evaluating patients with respiratory disease, but radiographic patterns are not specific for a single disease process so further diagnostic tests are necessary.

Thoracic radiography

The appearance of primary lung tumors varies from a solitary mass (Figure 1) to involvement of an entire lung lobe or multiple lobes (Figure 2), indicating metastatic disease or multicentric pulmonary tumors. Radiographic assessment may be confounded by atelectasis secondary to pneumothorax or pleural effusion. Pleural effusion may result from regional lymph node metastasis, extension of the tumor into the pleura, or concurrent disease such as congestive heart failure or other nonneoplastic processes such as hypertrophic cardiomyopathy, feline infectious peritonitis, and chylothorax.1,14 But keep in mind that pleural effusion devoid of neoplastic cells on cytologic examination does not necessarily rule out primary pulmonary neoplasia.14,19,28

In most patients without pleural effusion or severe concurrent thoracic disease, survey thoracic radiographs are considered a high-yield diagnostic test, often revealing a pulmonary mass when present.16 Differential diagnoses for pulmonary masses include primary lung tumors (77% in one study), granulomas, cysts, abscesses, infarcts, diaphragmatic hernias, and metastatic neoplasia.4,29


3. A photomicrograph of a cytologic sample revealing primary lung adenocarcinoma in a geriatric dog. A cohesive cell population is identified with cytologic criteria of malignancy including anisocytosis, anisokaryosis, multiple nucleoli, nuclear molding, and foamy cytoplasm (Wright's stain, 1,000X). (Photograph courtesy of Dr. Laura Garrett.)
Although the most accurate way to definitively diagnose a pulmonary mass is through surgery and histologic examination, other less-invasive diagnostic techniques, such as cytology, may be attempted first. Fine-needle aspiration appears to be the safest and most convenient method of harvesting cytologic samples for initial diagnosis (Figure 3). A recent study comparing cytologic and histologic diagnosis of pulmonary neoplasia revealed that cytologic samples retrieved with 25- to 27-ga needles showed agreement with histologic diagnosis in 82% of cases, with 100% specificity and 77% sensitivity.30 As would be expected, cytologic accuracy for diagnosing neoplasia is higher with ultrasound guidance vs. blind aspiration, as visualization of the suspect primary lung tumors is necessary to maximize appropriate sample collection (Figure 4).30 Fluoroscopy also facilitates the accurate and efficient retrieval of cellular samples from suspect pulmonary tumors.31

4. A thoracic ultrasonogram in a dog with a peripherally located primary lung tumor demonstrates a hypoechoic lesion immediately adjacent to the thoracic wall (arrows). Ultrasound-guided fine-needle aspiration improves the chances of a diagnostic sample. (Photograph courtesy of Dr. Laura Garrett.)
Although fine-needle cytology with small-gauge needles (25 to 27 ga) appears safe and relatively accurate, studies using large-gauge needles (18 to 21 ga) demonstrate even greater correlation between cytologic and histologic results collected from suspect lung lesions.32 However, despite a higher cellular yield, the complication rate using 18- to 21-ga needles is reported to be 46%, with a fatality rate of 15%.32 In contrast, no substantial complications have been reported with using 22- to 25-ga needles in companion animals for retrieving cytologic samples from lung parenchymal lesions.33 Furthermore, in a recent human study no significant difference was identified in diagnostic sample retrieval using 18-, 22-, or 25-ga needles.34 These studies suggest that 22-ga needles should be sufficient to offer good diagnostic yield with a low complication rate.

In addition to needle size, the technique used for sampling is important, as fenestration (sampling without the use of negative pressure) has proved to be more effective than aspiration (using a syringe to create negative pressure). Although sample volume is decreased with fenestration, cellularity is improved because of less hemodilution.35

The limitations of fine-needle aspiration cytology include poor cell recovery, poor exfoliation of certain cell types (i.e. mesenchymal cells), failure to obtain a representative sample, and the inability to provide architectural information (i.e. vascular lesions or fibrosis).30 Other methods of obtaining cytology samples such as transtracheal wash and bronchoalveolar lavage offer poor diagnostic yield with primary pulmonary tumors when compared with fine-needle aspiration.4,16,30,36-38


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