The patient had a pacemaker which precluded an MRI. They went on to have a craniotomy and debulking of the tumour.
MICROSCOPIC DESCRIPTION: The sections show features of a moderately cellular glial tumour. Some of the tumour cells (fibrillary astrocytes) show slightly elongated, angulated and hyperchromatic nuclei. Others have perinuclear haloes and rounder nuclei, representing oligodendroglial component.
Mitoses are seen, up to 6 per 10 high-power fields. There is mild endothelial cell hyperplasia. Necrosis is absent. IDH-1 immunostain is negative. The Ki-67 index is about 20%. The features are those of anaplastic oligoastrocytoma.
FINAL DIAGNOSIS: Anaplastic oligoastrocytoma (NOS) (WHO Grade III).
This case illustrates that no matter how important your pre-test probability is (i.e patient has a known malignancy and now brain lesion), you need to carefully look at the images. A metastasis does not usually extend into the corpus callosum.
NOTE: This case predates the 2016 WHO classification of CNS tumour revision. As no 1p19q co-deletion status is available a formal diagnosis cannot be reached and the NOS is therefore used (not otherwise specified) - which is recognised in the current classification for cases where molecular information is unavailable. It should also be noted, that under the new classification both an astrocytic and oligodendroglial component needs to be identified, each with its own molecular markers. True oligoastrocytomas are therefore going to be rare, and this case would most likely be classified as either an astrocytoma or an oligodendroglioma.