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A five-year-old 28-kg (61-lb) neutered male American pit bull terrier was presented for coughing and progressive dyspnea. Approximately six months prior to presentation, the dog was presented to an emergency clinic for hemoptysis. The patient was treated for pneumonia with a course of antibiotics of unknown drug, dose, and duration. The dog responded well and returned to normal however, excitement periodically elicited coughing. Two weeks prior to presentation to this institution, signs had progressed to persistent coughing, and three days prior to lethargy and dyspnea. The primary care veterinarian administered flow-by oxygen, performed thoracic radiographs (Fig. 1) and transferred the dog on the same day to the emergency service of the College of Veterinary Medicine of the University of Georgia.

 

rDVM radiographs performed the day of admission. Bilateral pneumothorax is present with near-maximal expansion of the chest with flattening of the diaphragm and only moderate loss of pulmonary volume. Associated with the right middle lung lobe, there is a moderate-size, sharply margined multi-cavitary mass. The tracheal bifurcation is filled with soft tissue opaque material.

Physical examination revealed bilaterally quiet lung sounds in the caudodorsal lung fields with severe inspiratory dyspnea, which resolved after thoracocentesis of approximately three liters of air.
A thoracoscotomy tube was placed and between 800 milliliters and three liters of air were aspirated every four to six hours.
Computed tomography was performed with the patient sedated and spontaneously breathing.  Postcontrast images of the thorax were acquired in arterial phase and approximatively 3 minutes after contrast injection.

 

Thoracic CT before contrast (left) and in arterial phase (right), transverse plane

Thoracic CT in lung window, transverse (left) and dorsal (right) planes

Thoracic CT in bone window (post-contrast)

Thoracic CT, delayed phase, soft tissue window, transverse plane

Findings

Despite the presence of a chest tube in the left hemithorax, a moderate amount of pleural gas is still present bilaterally, as well as a moderate amount of pleural fluid (28HU). In the right middle lung lobe is an approximately 8 x 6cm heterogenous mass composed of coalescing, poorly enhancing nodules, moderately enhancing septations, and dilation of peripheral airways with formation of large pneumatoceles.

The right principal bronchus is distended by a finger-like extension of the mass 1.6cm in diameter and at least 3.5cm long, partially filling the tracheal bifurcation and opening of the left principal bronchus. There is extensive filling of the right cranial, caudal and accessory bronchial lumen by these tissues.

The right cranial lung lobe is hyperinflated and hypoattenuating, a phenomenon observed to a lesser degree in the right caudal lung lobe. Bronchi to this lobe are bronchiectatic. The accessory lung lobe is collapsed. The middle trachoebronchial lymph node is mildly enlarged, but remaining intrathoracic nodes and left lung lobes are unremarkable.

Conclusions:

  1. Right middle lung lobe mass, large intrabronchial component
    Primary lung tumor was prioritized, such as bronchogenic carcinoma or adenocarcinoma. The clustered hypoenhancing portions were interpreted as necrosis, and less likely as granuloma or abscess formation, or accumulation of mucus or hemorrhage. Lesion cavitation and right cranial pulmonary overinflation were consistent with one-way valve effect of the intrabronchial portion of the mass. Pneumothorax was attributed to rupture of pneumatoceles. In this context, mild middle tracheobronchial lymph node enlargement was considered more likely to represent metastasis than reactive changes.

 

Outcome

US-guided FNAs were non-diagnostic. Thoracotomy was performed. The mass extended incorporated the hilus and made identification of individual vessels and bronchi difficult. A right middle bronchotomy to extract the intraluminal tumor tissue was unsuccessful due to the friability and adherence of the tumor tissue within the bronchus. Right cranial and middle lung lobectomies were performed after individual stapling, ensuring not to incorporate the hili of the right caudal and accessory lobes. A tracheobronchial lymph node was removed. After anesthetic recovery the patient continued to have a persistent inspiratory dyspnea, which remained unresponsive to medical management. Euthanasia was elected the next day.

Histology

Routine histology revealed a neoplasm comprised of two primary cell types; 1) a primitive mesenchyme, which replaced much of the interstitium and 2) an epithelial component, which lined immature alveoli and larger airways. The primitive mesenchyme was multifocally separated by immature alveolar structures, lined by cuboidal cells and larger, irregular airways, which were lined by pseudostratified columnar epithelium with cilia. Mitoses were rare with only one mitotic figure noted per 2.37 mm2. Neoplastic elements extended to the cut surgical margins.

The tracheobronchial lymph node displayed chronic follicular hyperplasia.

Immunohistochemistry of the tumor tissue showed uniform immunoreactivity to vimentin by the primitive mesenchyme. Conversely, epithelial cells lining immature alveoli and larger airways had strong intracytoplasmic immunoreactivity for cytokeratin, and in epithelial cells lining immature alveoli, thyroid transcription factor-1 immunoreactivity was observed. This constellation of findings is consistent with pulmonary blastoma.

 

Discussion

Pulmonary blastoma is a rare malignant neoplasm of the lungs, which histologically represent fetal pulmonary tissue, comprised of a primitive mesenchymal interstitium and alveoli lined by epithelium. These biphasic malignant neoplasms often occur in adulthood with a poor long-term prognosis. Mesenchymal cells are blastemal, characterized by large, round to ovoid nuclei with coarsely stippled chromatin and one prominent nucleolus. This tumor type has been described in rats, cattle, and horses with few case reports in dogs as well.1 Comparatively, pulmonary blastomas in cattle are described as smooth, off-white, soft masses which occupy entire lung lobes with involvement of the mediastinal and superficial cervical lymph nodes, and parietal pleura.

Extensive endobronchial growth, the most remarkable imaging finding in this case, has been reported in people with this condition2,3. On post-contrast CT, the bulk of the mass appeared predominantly composed of spherical non-enhancing zones, similar to a cluster of grapes, matching its macroscopic appearance, representing loci of necrosis. Pulmonary hyperinflation secondary to partial bronchial obstruction is suspected to have mitigated lung lobe collapse: the radiographically documented large thoracic volume and flattened diaphragm suggested tension pneumothorax, with contradictory relative preservation of lung volume.

In people, pulmonary blastomas frequently appear on imaging as large and well demarcated masses,3,4 similar to the case presented here, and the presence of necrosis is said to raise the suspicion for this condition5.

The single abnormal tracheobronchial lymph node in this dog was evaluated histologically and showed no evidence of metastasis; in humans nodal metastasis is also reported to be curiously rare with this condition3. Calcification, occasionally encountered with bronchial carcinoma5, is considered to be very rare in pulmonary blastoma3 and was absent here. Prognosis for this condition is considered poor, and as size is understood as a predictive factor by some4, rapid surgical resection remains the intervention of choice.

References

  1. Meuten D. Tumors in Domestic Animals. 5th ed. Ames, IA: John Wiley and Sons, Inc., 2017.
  2. Wang J, et al. Pulmonary blastoma with endobronchial growth. J Thorac Oncol2009;4:543-544.
  3. Lee HJ, et al. Pulmonary blastoma: radiologic findings in five patients. Clin Imaging2004;28:113-118.
  4. Robert J, et al. Pulmonary blastoma: report of five cases and identification of clinical features suggestive of the disease. Eur J Cardiothorac Surg2002;22:708-711.
  5. Marolf AJ, Gibbons DS, Podell BK et al. Computed tomographic appearance of primary lung tumors in dogs. Vet Radiol Ultrasound 2011; 52 (2): 168-172.

Right middle lung lobe following surgical resection. Grossly, the lesion had solid areas of pallor and multifocally necrotic areas with cavitation, which occupied the whole lobe. When sectioned the mass lacked normal large and small airways.

Histologic and immunohistochemical analysis of a canine pulmonary blastoma. B: the primitive mesenchyme is strongly immunoreactive to vimentin. C: cytokeratin highlights the epithelial component lining alveoli and larger airways. D: intranuclear immunoreactivity of respiratory epithelium to TTF-1.