Amyloid in cytologic specimens. Differential diagnosis and diagnostic pitfalls

To describe and illustrate the characteristic features of amyloid in cytologic preparations and point out its diagnostic pitfalls.

Five fine needle aspirates and one bronchial washing that contained amyloid were retrospectively reviewed. The aspirates were obtained from each of the five following sites: lung, occipital lymph node, thyroid gland, proximal humerus and subcutaneous soft tissue. Smears of all of the aspirates were stained with Papanicolaou stain, and in two cases they were also stained with Diff-Quik. Cell block sections were stained with hematoxylin and eosin. Congo red, CD45 and CD20 were used on selected cases.

Amyloid appears as either flocculent material or irregularly shaped fragments with scalloped and pointed edges. The amorphous fragments are acellular and frequently associated with connective tissue cells. They stain eosinophilic to cyanophilic with Papanicolaou stain and deep blue with Diff-Quik. In two cases an exuberant giant cell reaction almost obscured the amyloid. In the thyroid aspirate, the amyloid was misinterpreted as colloid. In bronchial washings and lung aspirates, amyloid has to be distinguished from mucus, alveolar proteinosis, chondroid material and corpora amylacea. When circumferentially surrounded by lymphocytes or plasma cells, flocculent amyloid deposits may simulate adenoid cystic carcinoma.

Amyloid can be easily overlooked or mistaken for other entities with similar staining qualities. Congo red staining can help to confirm the diagnosis.