What is the most likely diagnosis?
This lesion most likely represents a low grade glioma.
What other entities can be briefly entertained but can be safely discarded from the differential?
Middle cerebral artery infarction and HSV encephalitis can both involve the insular cortex, but have different presentations. The the case of MCA infarction one would expect other parts of the MCA territory to be involved without so much mass effect (at least not in the short term). HSV encephalitis usually involves the mesial temporal lobe, has less mass effect and is often bilateral. Patients are usually unwell. Metastases would be expected to enhance, and are unusual in this age group.
Pre and post contrast CT of the brain demonstrates are relatively well circumscribed region of low attenuation centred immediately deep to the anterior part of the right insular cortex, displacing the lentiform nucleus posteromedially. There is no evidence of haemorrhage and no convincing contrast enhancement. Despite its large size it exerts little mass effect. The rest of the brain is unremarkable.