This middle-age man slowly developed a large nodule on his lower back. Amyloidosis occurs in a variety of forms. Primary amyloidosis of the skin can present as macular amyloidosis, lichen amyloidosis (keratin amyloid, or AK), or nodular amyloidosis. In the nodular type, there are often conspicuous plasma cells and the amyloid is of the AL (light chain) type. This was an unusual case for the skin. It formed a large mass, or amyloidoma, with associated osteoma cutis. The patient is currently being evaluated for systemic disease but so far the work-up has been negative. Amyloid can be detected by Congo Red stains conventionally. One can also detect its autofluoresence (orange color) or enhance it with a thioflavin T stain. Immunohistochemical stains for amyloid P, SAA, light chain, or keratin may also be of help. Our case stained for amyloid P and SAA.
Examination under polarized light reveals the classic apple green birefringence. [Of note, cotton fibers and sometimes elastic tissue will also polarize]. The backgroud collagen has an orange tint.