Acute Necrotizing PCP with Pneumatoceles/Cysts

In acute disease, focal invasion of organisms into lung parenchyma may cause tissue necrosis and pneumatocele/cyst formation. The cause of the tissue invasion is unknown, but other agents, such as cytomegalovirus, may play a role. Intraparenchymal pneumatoceles/cysts may dissect to the pleura to cause pneumothorax [1,2].

Figure 1. Pneumatocele Formation in PCP

Frontal chest radiograph shows bilateral ground glass opacity, with a cystic lesion developing within the ground-glass opacity in the left mid-lung.

 

Pathologists refer to radiographic pneumatoceles as cysts. These cysts arise in foci of necrosis and have necrotic tissue or, later, fibrosis in their walls.

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Figure 2. Diffuse Disease with Focal Intraparenchymal Cysts

This slice of upper lobe from another patient shows multiple, rounded, pale lesions becoming confluent. In the midst of this diffuse disease are irregularly-shaped cysts. Some have thick, necrotic walls and others have thin walls from which necrotic material has been cleared. Note the edematous interlobular septa near the pleura.

Find the large central cystic lesion with a thick, necrotic wall.

Find 2 thin-walled cysts.

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Figure 3. Invasive Pneumocystosis

This section shows foamy material in both the alveolar spaces and alveolar walls. The septal involvement predisposes to septal lysis (necrosis) underlying cyst formation.

Find the foam in the alveolar wall.

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Figure 4. GMS Stain

A GMS stain of Figure 3 shows organisms in the alveolar exudate as well as in the alveolar wall.

Find the alveolar wall with organisms.

Find the alveolar exudate with organisms.

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Figure 5. Invasion by Pneumocystis of Vascular Wall

Another image shows foamy exudate in alveoli, alveolar walls, and in the wall of a blood vessel. Organisms may gain access to the blood stream as a result of vessel wall invasion.

Find the vessel with mural invasion by foamy material.

 

Figure 6. Necrotizing Pneumocystosis

Septal lysis may cause pools of exudate, followed by cyst formation, if the exudate is cleared. If a cyst does not occur, organization and fibrosis of the exudate may occur.

Necrotizing Lesion

Septal invasion and necrosis have destroyed alveolar architecture and left diffuse exudate with organisms in a central area. Inflammatory cells are sparse. Clearing of the exudate leaves a cyst. Such cystic lesions may predispose to interstitial emphysema, or they may collapse and heal with time.

pool

 

Note the typical alveolar exudate and lack of fibrosis around the pooled exudate.

Find the central pool of exudate.

 

References

1. Murry C, Schmidt R. Tissue invasion by Pneumocystis carinii: a possible cause of cavitary pneumonia and pneumothorax. Hum Pathol 1992; 23:1380-1387. Abstract

2. Watts J, Chandler F. Evolving concepts of infection by Pneumocystis carinii. Pathol Annu Part 1, 1991; 26:93-138.

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