Most likely diagnosis? Why?
Primary CNS lymphoma, given the homogeneous vivid enhancement, location and restricted diffusion.
Second most likely entity?
The main, but far less likely differential is that of high grade glial tumour (WHO III or IV astrocytoma).
A bifrontal mass with abnormal signal crossing the corpus callosum associated with moderate frontal mass effect, extensive vasogenic oedema and bulging of the genu of the corpus callosum.
There are large rounded areas iso to hypointense T2 signal that display dense contrast enhancement on either side of the midline within both frontal lobes, with no necrosis.
Mild generalised diffusion restriction is present within the areas that display enhancement suggesting increased cellularity.
No additional enhancing lesions are seen elsewhere.