The Table on the Wall Was Never Meant to Be a Wall
Most of us learned the Timmerman tables the same way: as a wall of numbers to check a plan against. Green if you're under it, red if you're not. Somewhere along the way, that's what "the table on the wall" became for our profession — a pass/fail checklist bolted to the side of a treatment planning workflow.
That's not how Dr. Robert Timmerman built it.
In a recent conversation, Timmerman — the physician whose name is now shorthand for SBRT organ-at-risk constraints — made a point that should unsettle anyone who's ever treated his tables as a hard stop. His original goal wasn't to draw a line no one should cross. It was to find the Maximum Tolerated Dose — a research question, not a regulatory ceiling. He never intended for those numbers to function as a line you simply cannot cross. And yet that's how many in the field use them today.
The checklist trap
Here's the pattern many planners fall into: run a plan quality checklist, get green or red marks against a set of constraints, and push Organ-At-Risk (OAR) sparing as hard as possible until the plan stops improving — then it’s ‘clinically viable.’ It's reproducible, and it's genuinely useful for catching gross errors. But it's not the same thing as thinking about the ‘why?’
Timmerman's own framing of radiosurgery captures why this matters. Beams are getting tighter and tighter — more like a knife than a wash of dose. And a knife implies something checklists don't capture well: we are wounding tissue in order to treat it. The real clinical question was never "did we clear the constraint," it was "will that wound heal?" A checklist can tell you whether a number is under a threshold. It cannot tell you whether the tissue you spared, or the tissue you didn't, will recover.
We must wound in order to treat. But will that wound heal?
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Timmerman's actual priority order
It's worth sitting with the hierarchy Timmerman himself puts in place, because it's not the order most plan-review checklists implicitly enforce:
Obey the spinal cord constraint.
Treat the tumor — cover it fully.
Have very steep dose gradients.
Respect normal tissue constraints.
Notice where tumor coverage sits relative to normal tissue constraints. In a lot of clinical practice, physicists treat OAR constraints as something close to equal footing with coverage — sometimes we'll fight harder for OAR sparing than we fight for the last few percent of PTV coverage. Timmerman's ordering suggests that's backwards, at least in his framing: cord constraint aside, cover the tumor, get your gradients steep, then worry about the rest of the normal tissue.
That ordering comes from a specific vantage point, though — and understanding whose vantage point it is matters as much as the list itself.
Two different jobs in the same room
A physician looks at a patient with curative and problem-solving intent — the patient walks in with a problem, and the physician is the one tasked with solving it. That's a fundamentally different lens than the one we bring to the planning station.
Our job is to be as objective as the situation allows. We carry a mechanical, scientific approach to a conflict the physician doesn't have to resolve numerically: tumor coverage versus OAR sparing. Tumor coverage cures — or in most cases, extends life from what is otherwise terminal disease. OAR toxicity damages healthy tissue across a spectrum, from low-grade and transient to high-grade and permanent. Those are two different kinds of harm, measured on two different axes, and it's our job to hold both at once rather than defaulting to whichever one has a red checkmark in front of us.
That means OAR toxicity isn't a single variable — it's really two: severity, and duration. A grade 2 toxicity that resolves in six weeks and a grade 2 toxicity that never resolves are not the same clinical event, even if a checklist scores them identically.
The variable the checklist can't see
There's a piece of information Timmerman's framing doesn't explicitly address, but it's essential to doing this work objectively: what does a perfect treatment actually buy the patient?
Imagine the best-case scenario plays out exactly as hoped — full tumor coverage, zero OAR toxicity. What have we actually won? In some cases, that's a cure. In many, it's palliative. And in others, it's somewhere in between — we're not curing anything, we're mitigating recurrence, and the data on staging tells us we're most likely extending life by some bounded window. Five to ten years. Sometimes only six to eighteen months.
Those nuanced cases are where the real tension shows up, because the length of life you're extending can directly compete with the quality of the life you're extending it into.
Take a palliative GBM case. A well-executed plan might extend a patient's life from six months to two years. But if the OAR dose overlaps functional tissue — brainstem, hippocampus — and carries a 20% chance of damaging it, you're not just weighing "constraint met or not met." You're weighing two years of life against a one-in-five chance that a meaningful fraction of that extended time comes with a functional deficit the patient didn't have before.
A checklist has no field for that trade-off. A physician and a physicist, talking to each other, do.
The length of life you're extending can directly compete with the quality of the life you're extending it into.
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Where this leaves physics
None of this is an argument to discard the tables, or to treat constraints as optional. It's an argument that the tables were built as evidence for a research question, not as a substitute for judgment — and that judgment is exactly what our training equips us to bring to the cases that are genuinely grey.
If a physician, in conversation with colleagues, decides the risk to the brainstem isn't worth it, that decision might reshape the target volume itself — dropping a 20% probability of harm down to 3–5% by contouring differently, before optimization ever starts. That's a decision physics can inform, but only if we're bringing more to the table than a pass/fail readout.
We work as a multidisciplinary team. When a case sits in that grey zone — when outcome versus toxicity, or coverage versus sparing, are genuinely in competition — that's exactly the moment a physicist should be having a conversation with the treating physician, not silently enforcing a constraint table as if it were law. Timmerman didn't build his numbers to end that conversation. He built them to start it.
