GBM
Meaning of GBM: Three letters standing for a diagnosis that now depends as much on molecular testing as on what the tumor looks like.
Definition of GBM
GBM is the common abbreviation for glioblastoma, the most aggressive of the primary brain tumors. It grows from glial cells, the supporting cells that surround and maintain neurons, rather than from neurons themselves. It arises mainly in the cerebral hemispheres of adults and sends threads of tumor cells outward into normal brain, so its edge is never sharp.
What does the abbreviation actually stand for?
The M came from glioblastoma multiforme, a name given because the tumor looked so varied under the microscope. That name has been retired, though the abbreviation outlived it. More importantly, what counts as a glioblastoma has narrowed. Classification of a brain tumor now combines its appearance with its molecular profile, and the name glioblastoma is reserved for grade 4 diffuse astrocytic tumors that carry no mutation in the IDH gene. Tumors that look identical but do carry that mutation are named separately, arise in younger adults, and follow a slower course. Grades are now written in Arabic numerals, so grade 4 rather than grade IV.
What does it look like?
| Seen by | Characteristic findings |
|---|---|
| Microscope | Densely packed, wildly varied cells; abnormal new blood vessels; dead patches ringed by crowded cells |
| MRI | An irregular bright ring around a dark dying core, with swelling reaching far beyond it |
| Molecular testing | No IDH mutation, and changes such as TERT promoter mutation, EGFR amplification and gain of chromosome 7 with loss of chromosome 10 |
One further marker guides treatment: when the MGMT gene has been switched off by methylation, tumor cells lose a repair enzyme and respond better to the chemotherapy used alongside radiation therapy.
Why can surgery not remove it all?
Because there is nothing to remove it from. The tumor has no capsule and no true border; individual cells have already migrated along nerve fiber tracts and around blood vessels into brain that looks normal on a scan and normal to the surgeon. Taking out everything visible therefore leaves cells behind by definition, and taking out more means taking out working brain. Surgery aims instead at the largest removal that can be made safely, which relieves pressure and supplies tissue for the molecular testing above, and is followed by radiation to the surgical bed plus a margin, with drug treatment directed at what remains. Spread outside the nervous system is rare; the problem is local persistence rather than distant travel.
Also known as
Used in a sentence
Imaging findings were consistent with a GBM involving the frontal lobe.
Good to know
GBM is a grade 4 tumor, the highest grade in its category, and current classification reserves the name glioblastoma for tumors carrying no IDH mutation, separating them from IDH-mutant grade 4 astrocytomas that behave differently.