What is the likely diagnosis in this 8 year old child?
What comment should you make about the fact that the mass does not appear to enhance very much?
The post contrast images have been obtained early (probably within a minute of injection). Enhancement on CT is due to a combination of capillary density (how much contrast is in vessels) and vascular leakiness (how much contrast leaks into the extravascular / extracellular space). Most enhancement seen on routine post contrast CT brain is due to the latter, with images obtained after at least 5 minutes of injection.
CT of the brain with and without contrast demonstrates a large partially cystic mass in the right cerebellar hemisphere. The solid component only minimally enhances with a focal region demonstrating more prominent ring enhancement. The fourth ventricle is effaced, and the lateral ventricles are enlarged. There is also marked effect on the brainstem with the prepontine and perimesencephalic cisterns obliterated.