What is the likely diagnosis? Why?
The most likely diagnosis is primary CNS lymphoma, on account of periventricular distribution and hyperdensity.
What is the main differential? Why is this less likely?
Glioblastoma is the other entity to be considered. Lack of oedema and necrosis make it less likely.
Non-contrast CT of the brain demonstrates a lobulated periventricular lesion which is hyperdense compared to normal cortex. It crosses the corpus callosum. There is no significant associated vasogenic oedema and no evidence of central necrosis.