Why Brain Tumor Surgeons Don’t Always Remove the Entire Tumor
The difference between removing the most tumor and performing the best operation
One of the most understandable questions I hear from patients and families after brain tumor surgery is:
“Why is there still tumor on the MRI?”
It seems intuitive. If a tumor is dangerous, shouldn't the goal of surgery simply be to remove all of it?
For many brain tumors—particularly gliomas—how much tumor we remove matters. Greater extent of resection has repeatedly been associated with better outcomes when it can be accomplished safely.
But there's an important word in that sentence:
Safely.
The goal of modern brain tumor surgery isn't simply maximum resection. It's maximal safe resection.
And understanding the difference explains much of how brain tumor surgeons think in the operating room.
A brain tumor doesn't have to respect the brain's boundaries
Some tumors elsewhere in the body grow more like a ball: tumor here, normal tissue around it. Gliomas are different.
They arise from cells within the brain itself and can infiltrate into the surrounding tissue. Under the microscope, tumor cells may extend beyond what we can clearly see on an MRI.
That creates an immediate problem because there isn't always a clean line that says: TUMOR ENDS HERE! or NORMAL BRAIN HERE!
Instead, the edge can be a transition zone where tumor and functioning brain coexist. That functioning brain may contain the networks responsible for speaking, moving, seeing, reading, remembering, paying attention, or simply being able to live independently.
So the question during surgery isn't: Can I remove more?
It's: Can I remove more without taking away something that matters?
The last 5% may be very different from the first 95%
Imagine a tumor that can largely be removed without encountering an essential brain network. The first portion may come out relatively safely. But as surgery progresses, the anatomy changes. The remaining tumor might approach (see previous stacks for more info on these regions):
the corticospinal tract, which carries movement signals
the arcuate fasciculus and other language pathways
the optic radiations, which carry visual information
the internal capsule, where enormous amounts of motor and sensory wiring are concentrated
deep blood vessels that supply normal brain
networks involved in attention, cognition, memory, or behavior
Suddenly, another few millimeters of tumor removal can carry a very different risk. That's one of the most important concepts for patients to understand:
Every additional percentage of tumor removed does not carry the same benefit-to-risk ratio.
The last 5% may be the most dangerous 5% and it can cause a huge percentage of the problems post-operatively.
What does “maximal safe resection” actually mean?
Maximal safe resection means removing as much tumor as possible while maintaining an acceptable risk to neurological function and quality of life. That balance is different for every patient.
A tumor near the motor pathways of someone who walks for a living raises one set of considerations. A tumor near language networks in a teacher raises another. A tumor affecting visual pathways in someone who already has limited vision may change the calculation again. This is one reason I think brain tumor surgery increasingly needs to move beyond anatomy alone.
We aren't operating on an MRI. We're operating on a person.
Gross total, subtotal, supratotal: what do these terms mean?
Patients frequently encounter these terms in operative reports and MRI results.
Gross-total resection
Generally, this means that all of the tumor visible within the imaging target being used for that particular tumor has been removed. Importantly, gross total does not necessarily mean every tumor cell is gone. Glioma cells can extend microscopically beyond what conventional MRI can see.
Subtotal resection
Some visible tumor remains. Sometimes this is unavoidable. Sometimes it is intentional.
If tumor extends into a critical functional pathway or surrounds important blood vessels, leaving some tumor behind may provide the best balance between tumor control and neurological function.
That isn't necessarily an incomplete operation. Sometimes it is exactly the operation that was planned.
Supratotal or supramaximal resection
This concept is particularly interesting in selected gliomas.
Sometimes the visible MRI abnormality does not represent the true biological boundary of the tumor. If functional mapping shows that surrounding tissue can be safely removed, surgeons may sometimes extend the resection beyond the conventional radiographic tumor margin.
But the principle remains the same. The goal isn't: “Remove more at all costs.” It is: “Remove more when the functional anatomy allows it.”
Biopsy
Sometimes the safest and most useful operation is not a resection at all. A biopsy obtains enough tissue to determine exactly what the tumor is and any molecular information that may determine chemotherapy, radiation, targeted therapy, or clinical-trial options.
For tumors embedded within particularly high-risk structures, obtaining the diagnosis without causing a neurological deficit may be the best surgical result. Sometimes this can be combined with Laser Ablation for a cytoreductive option when carefully planed.
When leaving tumor behind is a decision.
This distinction between a decision and a complication matters. Imagine that during surgery we are approaching the pathway responsible for moving someone's hand. The patient may still be moving normally. The monitoring may still look normal. But stimulation tells us that the motor pathway is now only a few millimeters away. We have learned something extremely valuable. We have found the functional boundary of the operation.
Continuing simply because tumor remains on the MRI could transform a successful tumor operation into a permanent disability. Stopping can be the more sophisticated decision.
But stopping too early matters, too
There is an equally important other side to this discussion. We should not use the importance of neurological function as an excuse for unnecessarily conservative surgery. For many gliomas, extent of resection matters. Leaving safely removable tumor behind may compromise the oncologic benefit of surgery. That means excellent brain tumor surgery requires navigating between two errors: Going too far. and Not going far enough.
The goal is to find the boundary between them.
So how does a surgeon know when to stop?
This is where modern brain mapping becomes so important. Before surgery, we may combine structural MRI with tools such as:
functional MRI
diffusion imaging and tractography
individualized connectomic analysis
neuropsychological testing
detailed neurological examination
These give us a hypothesis about where important networks may be. But during surgery, we can sometimes test those predictions directly.
Using cortical and subcortical stimulation, we can temporarily interrupt tiny regions or pathways and observe what happens. Does speech stop? Does naming become difficult? Does a hand move? Does the patient make a consistent error?
As surgery moves deeper, mapping can help identify how close we are getting to critical white-matter pathways. This is one of the fundamental differences between simply seeing anatomy and understanding functional anatomy. MRI tells us where the tumor is. Mapping helps tell us where the functional boundary is.
Why connectomics changes the conversation
For much of neurosurgical history, the brain was described as a collection of individual regions. Language was “here.” Movement was “there.” Vision was somewhere else. We now understand that the brain works much more like a network. Language depends on multiple regions connected by white-matter pathways. Movement requires communication between cortex, deep nuclei, cerebellum, brainstem, and spinal cord. Attention, decision-making, personality, and memory are even more distributed. That means preserving one small patch of cortex isn't always enough.
We need to think about the connections between regions. This is the idea behind connectome-informed brain tumor surgery: understanding not only where the tumor sits, but which networks pass through and around it.
Why two patients with similar MRIs may have different operations
This is another question families sometimes struggle with. They may find someone online with a tumor that looks remarkably similar and wonder: “Why did that surgeon remove everything, while mine says some tumor should remain?”
The answer is that the MRI is only part of the story. Two tumors that appear similar can differ in:
molecular biology
infiltration pattern
vascular relationships
hemisphere dominance
white-matter anatomy
functional organization
neuroplasticity
previous treatments
patient priorities
The safest boundary may therefore be completely different. There is no universal percentage of tumor that should be removed from every patient. There is an individualized functional boundary.
What should patients ask before brain tumor surgery?
Instead of asking only: “Can you remove the whole thing?”
I think there are better questions:
What is the realistic goal of my surgery?
Which brain networks are closest to my tumor?
What function is most at risk?
How will you determine the safe boundary during surgery?
Will you use awake mapping, subcortical stimulation, neurophysiological monitoring, tractography, or other techniques?
If tumor is intentionally left behind, where will it most likely be and why?
Those questions tell you much more about the surgical strategy than a promised percentage ever could.
The operation is only one part of the treatment
Surgery is enormously important in glioma care. It provides tissue for diagnosis and molecular testing. It can relieve pressure. It can improve symptoms. It can reduce the amount of tumor that subsequent treatments need to control. And, in appropriately selected patients, extent of resection can influence outcomes.
But surgery does not exist in isolation.
Modern brain tumor care may also involve radiation oncology, neuro-oncology, neuropathology, neuroradiology, rehabilitation, genetics, clinical trials, and increasingly molecularly targeted treatments.
The best surgical decision therefore isn't necessarily the operation that produces the cleanest-looking postoperative MRI. It is the operation that puts the patient in the best position for everything that comes next.
The bigger idea: good surgery is not measured only in cubic centimeters
For decades, one of the easiest ways to judge brain tumor surgery was to look at the postoperative MRI.
That's important, but it isn't enough. We should also ask:
Can the patient speak?
Can they move?
Can they see?
Can they think clearly?
Can they return home?
Can they work?
Can they interact with the people they love?
And did we accomplish enough oncologically to meaningfully change the trajectory of the disease?
Those outcomes belong in the same equation.
Good brain tumor surgery is aggressive about tumor and conservative about irreplaceable function.
That balance is the real goal of maximal safe resection.
Frequently Asked Questions
Is residual tumor after brain surgery always bad?
Not necessarily. Residual tumor may be intentionally left when removing it would create an unacceptable neurological risk. The significance also depends on tumor type, molecular biology, location, and subsequent treatment options.
Does gross-total resection mean the tumor is cured?
No. Particularly with infiltrating gliomas, microscopic tumor cells can remain beyond what is visible on MRI. Surgery is often one component of a larger treatment strategy.
Why can't surgeons simply see the difference between tumor and normal brain?
Sometimes they can. But infiltrating gliomas often blend into surrounding brain. Technologies such as fluorescence, ultrasound, imaging, mapping, and pathology can help, but none creates a perfect biological boundary.
Is more tumor removal better?
Generally, greater safe extent of resection is desirable in appropriately selected gliomas. The critical qualifier is safe. A major permanent neurological deficit can profoundly change quality of life and the ability to receive additional treatment.
How do surgeons decide when to stop removing a brain tumor?
They integrate anatomy, imaging, tumor characteristics, neurophysiological monitoring, functional mapping, surgical judgment, and the patient's individual priorities. In selected cases, direct brain mapping can identify functional boundaries in real time.
Bottom line
The question in brain tumor surgery isn't simply:
“How much tumor can we remove?”
It is:
“How much tumor can we remove while preserving the networks this person needs to live their life?”
That distinction is the foundation of modern maximal safe resection. As our ability to map individual brain networks improves, the definition of what is safe and what is possible continues to become more personalized.
