What are some of the differentials you should entertain?
Ependymoma, central neurocytoma, exophytic high grade glioma and possibly metastasis.
Does this represent an intraventricular lesion or a parenchymal lesion with an exophytic component into the ventricle?
Although it can be difficult to be certain, best seen on sagittal T2 images, tumour can be seen to involve the thalamus, caudothalamic groove and wrap around the lateral and superior aspect of the lateral ventricle. The overall morphology favours an exophytic parenchymal lesion.
What diagnosis(es) do you now favor? Why?
Central neurocytomas are rarely parenchymal and leptomeningeal spread is very rare, as such this diagnosis can be essentially discounted. Metastases are rarely periventricular. Lymphoma (in the absence of immunocompromise/treatment) rarely undergo necrosis and usually enhance more vividly. As such, the differential is mostly between a high-grade primary tumour (e.g. glioblastoma) and an ependymoma.
The large mass is comprised of a haemorrhagic component (yellow dotted line) and a solid enhancing cellular component (blue dotted line).
Extensive leptomeningeal disease is also present (green arrows).