People seem to like unknown cases! So I’ll continue adding to our little collection. You can find other cases here, here, here, here, here and here. In case you’re just joining us, these cases are kind of like the unknown cases you’d see in a pathology residency (or as a medical student rotating through pathology). Only minus the anxiety! Take a look at the photo and read the question…then scroll down for the answer.

What are the red structures scattered throughout this biopsy of a cerebellar mass in a 13 year old male? (Bonus points if you can figure out the diagnosis in this case.)

A. Blood vessels
B. Fungal organisms
C. Lewy bodies
D. Rosenthal fibers
E. Negri bodies

(Keep scrolling down for the answer…)

And the answer is…D, Rosenthal fibers. Rosenthal fibers (or Rosenthal bodies) are eosinophilic structures present in great numbers in pilocytic astrocytomas. Pilocytic astrocytomas are indolent tumors that usually occur in childhood and often affect the cerebellum. Composed of long, thin, pilocytic (“hair-like”) astrocytes, these tumors tend to grow very slowly, even if a higher-grade histologic appearance is present.

Rosenthal fibers are present in pilocytic astrocytoma – but they are not specific for that tumor. They are also seen in areas of long-standing gliosis and in Alexander disease. They are weird little structures that contain two heat-shock proteins and ubiquitin (a protein that seems to be present everywhere! Hmm..ubiquitous…I guess that’s why they named it ubiquitin.).

As for the other answers: A is wrong; blood vessels should have red cells in them. These structures are more of a pinkish red, and they are solid, opaque, and stubby. B is incorrect because the structures don’t look like a typical fungus – they are rod-shaped, yes, but organisms with hyphae tend to have long, continuous hyphae (sometimes with septae), not short, stubby, disconnected ones. C is incorrect: Lewy bodies are light-pink, round inclusions that you see within neurons in Parkinson disease.  And E is incorrect: Negri bodies are pink-purple round inclusions present in neurons in rabies.