Differential Diagnosis of Homogeneous Pink Staining

Protein-rich edema fluid or inspissated mucus in bronchioloalveolar carcinoma

The homogeneous, pink, mucinous alveolar exudate of bronchioloalveolar carcinoma is also PASD-positive. It is not foamy as in pneumocystis pneumonia or granular as in alveolar proteinosis. It stains with alcian blue, whereas the others do not. In this photo, not all of the alveoli are lined by tumor. The tumor cells (arrows) should not be mistaken for hyperplastic type II cells. Protein-rich pulmonary edema fluid can have a similar appearance but is PASD-negative.


Pulmonary alveolar proteinosis--PAS stain

Granular, pink, PAS+ alveolar exudate fills alveoli. Denser pink clumps, acicular clefts, and a few disintegrating macrophages are present in the exudate. Electron microscopy shows myelin figures. Lavage fluid appears milky. Alveolar walls are only slightly thickened.

Silicoproteinosis is an alveolar proteinosis-like reaction to silica. Silicotic nodules may or may not be present.


Amyloid occurs as interstitial, extracellular deposits in bronchial, alveolar, or blood vessel walls. This parenchymal lesion presented as a solitary pulmonary nodule. There may be giant cells, calcification, or ossification as a reaction to it.

Congo red stain for amyloid

The left panel shows the Congo red stain. Positive red staining is present around the large central artery and a smaller vessel to its upper right. The right panel shows the green birefringence that is diagnostic of amyloid when the Congo red stain is viewed with polarized light. All amyloids (AA, AL, transthyretin) have a fibrillar ultrastructure that gives this reaction.

See also Case 31