Imaging of exogenous lipoid pneumonia simulating lung malignancy in a dog

Abstract

This report presents peculiar radiographic and computed tomographic features of a lung mass in a dog, later diagnosed by histology as exogenous lipoid pneumonia. Although lipoid pneumonia is a well-known pathological entity in pets, this is the first imaging description of a single infiltrative lesion mimicking a neoplasm.

A small-sized 10-year-old male cross-breed dog was admitted to a private veterinary clinic for fatigue and persistent cough. The dog had spent 10 mo in a dog pound and had been adopted by the owner 1 mo before presentation to the clinic. Due to recurrent episodes of constipation, the staff of the dog pound had administered 5 mL of mineral oil orally by means of a syringe twice a day.

Case description

On thoracic auscultation, there were crackles on the right ventral thoracic side that were suggestive of lower respiratory tract disease. Hematological and clinical chemistry parameters were within normal limits.

Survey laterolateral and ventrodorsal thoracic radiographs revealed a soft-tissue opacity affecting the right middle, right caudal, and accessory lung lobes. In the laterolateral projection a caudal silhouette sign with the heart and in the ventrodorsal projection a right lobar sign were evident (Figure 1). As no apparent mediastinal shifts were observed, an infiltrative/ neoplastic disease was considered as the most likely diagnosis whereas a lobar collapse and the presence of a mass were less likely. A conventional computed tomography (CT) scan (contiguous 5-mm-thick slices) of the thorax showed areas of lung consolidation with air bronchograms in the ventral part of the right middle, caudal, and accessory lobes along with an interstitial nodular pattern in the dorsal areas of the lobes (Figure 2). Density of the consolidated lung averaged between −60 and −50 Hounsfield units (HU). Post-contrast scan revealed a mild diffuse enhancement of the affected area (−30/−20 HU). Cranial mediastinal and tracheobronchial lymph nodes were within normal limits.

Figure 1.

Latero-lateral (A) and ventro-dorsal (B) radiographic projections of the thorax of a 10-year-old cross-breed dog showing an increased soft tissue opacity in the right caudal and accessory lung lobes. There is no mediastinal shift. Multiple round, hyperdense opacities are occasional findings consistent with shotgun bullets.

Figure 2.

Transverse 5-mm, post-contrast, conventional CT image of the thorax showing consolidated areas of the lung in the dependent portions of the right caudal and accessory lobes.

Ultrasound-guided aspiration cytology of the lung lesion revealed the presence of large epithelioid cells characterized by microvacuolated hyperchromatic cytoplasm and nuclear pleomorphism, features suggesting a neoplastic process. In light of the clinical findings the dog underwent surgical excision of the involved lobes (Figure 3) through a right lateral thoracotomy. Lung samples were fixed in 10% buffered formalin, routinely processed and stained with hematoxylin and eosin. Histological examination showed multifocal granulomatous peribronchial pneumonia, characterized by interstitial fibrosis, accumulations of macrophages and small numbers of neutrophils around large lipid vacuoles and plasmacellular perivascular cuffing. In particular, alveolar spaces were filled with foamy macrophages and multinucleated giant cells, suggesting a foreign body reaction. Sudan IV histochemical stain revealed the presence of lipid material in the cytoplasm of foamy macrophages and multi-nucleated cells (Figure 4). A diagnosis of lipoid granulomatous pneumonia was made on the basis of these histopathological findings. The dog recovered completely a few days after surgery. Follow-up performed 7 mo later confirmed complete remission of symptoms and survey radiographs of the thorax did not show any abnormality.

Figure 3.

Surgically excised, formalin fixed, affected right pulmonary lobes of the dog. The cut section reveals a discolored area of consolidation in the caudal lobe. Bronchi within the consolidated area are still recognizable. Bar = 1 cm.

Figure 4.

Pyogranulomatous peribronchial infiltration around lipid vacuoles in the lung of the dog. Hematoxylin and eosin. Bar = 100 μm. Inset: detail of a multinucleated giant cell (foreign body reaction) containing lipid vacuoles coloured red by Sudan IV staining. Bar = 100 μm.

Discussion

Lipoid pneumonia has occasionally been reported in cats (1), horses (2), cows (3), and dogs (4–6). Based on the source of the lipid, lipoid pneumonia can be classified as endogenous or exogenous. The endogenous form occurs when lipids produced by the body, frequently cholesterol, are deposited into the alveoli after breakdown of pulmonary cell walls, due to obstructive bronchopulmonary disease or modified lipid metabolism. Endogenous lipoid pneumonia is characterized by subpleural and perivascular lesions with the presence of foamy macrophages within alveoli. Exogenous lipoid pneumonia results from aspiration or inhalation of mineral, vegetable, or animal oils. Exogenous lipids may reach the airways following forced administration of food, anatomic defects (palatoschisis), anesthesia, gastrointestinal disorders (megaoesophagus, gas-troesophageal reflux), or neurological deficiencies. Mineral oil easily gains access to the tracheobronchial tree, inhibiting the motility of the mucociliary apparatus. In the alveolar spaces lipids become emulsified and are engulfed by alveolar macrophages. Macrophages which are unable to metabolize the lipid material, degenerate and release the oil back into the alveoli. The lipid triggers a local cell-mediated inflammatory response, leading to fibrosis (7).

In humans, exogenous lipoid pneumonia is often distributed in the dependent portions of the lower lobes or in the right middle lobe, and may be multifocal and bilateral (8). Most chest radiographs of humans affected by exogenous lipoid pneumonia show airspace consolidation with 3 common patterns: reticular, mixed alveolar/interstitial, and nodular lesions (9). As reported in most human cases, the characteristic radiological finding for lipoid pneumonia consists of nodular lesions (10–12).

In the present case, the main localization to the ventral part of the right caudal and accessory lobes, the peculiar histological findings, the severity of lesions, and the history of oral administration of mineral oil, pointed to a diagnosis of exogenous lipoid pneumonia despite the fact that the condensed/infiltrated radiographic and CT scan pattern was highly consistent with a neoplastic lesion. A diagnosis of lipoid pneumonia needs to be considered in cases of focal lung lesions showing a negative (HU) density with tumor-like radiographic and CT scan features, with a history of forced oral administration of mineral oil.

To the authors’ knowledge this is the first report describing peculiar radiographic and CT images of exogenous lipoid pneumonia accompanying gross and microscopic investigation in veterinary medicine. CVJ