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Radiation Therapy · Radiobiology & Plan Literacy
What Is a DVH? — The Dose Volume Histogram, Explained
Medically reviewed by Dr. Kirti Ranjan Mohanty, Radiation Oncologist, MBBS · MD (Radiation Oncology), Senior Consultant · Last reviewed August 2026
If you have asked to see your radiation plan, the dose volume histogram — the DVH — is the one page worth understanding. It is a single graph on which every structure outlined on your planning scan appears as its own curve: the tumour target, and each healthy organ near it. Your radiation oncologist reads it curve by curve before signing your plan off. This page explains what the axes mean, how a plan is judged on that chart, and the questions worth asking about your own.
- One chart, your entire plan — every outlined structure gets its own curve, so the target and each organ at risk can be read side by side instead of one report at a time.
- Dose across the bottom, volume up the side — every point on a curve answers one question: how much of this structure receives at least this much dose?
- It is what the plan is checked against — target coverage and every organ dose limit are verified on this graph before a radiation oncologist approves your plan.
- The most useful page to carry for a second opinion — a DVH tells another radiation oncologist far more about a plan than the machine name on the brochure does.
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The short answer
What does a dose-volume histogram show?
A DVH is a single graph summarising your whole radiation plan. Dose runs along the bottom, volume runs up the side, and each outlined structure — the tumour target and every organ at risk — gets its own curve. One glance tells your radiation oncologist how much of each structure receives how much dose.
The planning software produces it automatically once a plan has been calculated. It is not an image of you and it is not a scan. It is a summary chart, and it exists because a radiation plan involves too many structures to check one page at a time.
The version used in almost every clinic is the cumulative DVH. That word matters, because it tells you how to read a point on a curve. Pick any dose along the bottom axis, go straight up to a structure’s curve, then read across to the side axis. The number you land on is the share of that structure receiving at least that dose — not exactly that dose. Every curve therefore starts at 100 per cent on the left and only ever falls as you move right.
Because all the curves share one chart, the DVH is the closest thing radiotherapy has to a single-page summary of a plan. It is also why it travels well: a radiation oncologist who has never met you can look at your DVH and understand, in a couple of minutes, what your plan was built to achieve and what it had to give up. That is a far more useful thing to carry into a second opinion than the name of the machine.
Your radiotherapy is delivered at an NABH-accredited partner centre; CION Cancer Clinics coordinates your treatment plan, your oncology team and your care throughout, so the same plan and the same chart follow you from your planning scan to your final session.
Did you know?
The DVH is why modern plan reports rarely quote a single “maximum dose” any more. ICRU Report 83, published in 2010 for intensity-modulated radiotherapy and still the reference used in ASTRO- and NCCN-aligned reporting, recommends describing a structure using near-minimum and near-maximum values read off the histogram — because a single hottest calculation point can shift with the software settings, while a value covering a defined slice of the volume does not.
Reading the chart
How do I read the axes on my DVH?
Six things to know before you look at one. Together they cover almost everything a patient needs in order to follow the conversation.
Horizontal axis
Dose, left to right
Radiation dose, usually in gray (Gy) or as a percentage of the prescribed dose. Moving right means more dose.
Vertical axis
Volume, bottom to top
How much of the structure, almost always as a percentage of that structure’s own volume rather than in cubic centimetres.
One curve each
Every outlined structure appears
If it was contoured on your planning scan, it has a curve. Structures that were never outlined simply do not appear on the chart.
Target curves
They stay high, then drop late
A target is meant to receive the prescribed dose almost everywhere, so its curve holds near the top before falling steeply.
Organ-at-risk curves
They fall away early
A healthy organ is meant to receive as little as the anatomy allows, so its curve is expected to drop while still on the left.
The colour key
Match every curve to a name
The legend beside the graph maps each colour to a contoured structure. Without it the chart cannot be read at all.
Curve shape is a description, not a verdict. Anatomy decides how much separation between the target and an organ is achievable in the first place, so two plans with different-looking curves may both be entirely reasonable for two different people. Only a radiation oncologist reviewing your own scan can judge your plan.
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Understand your plan before you consent to it
A radiation oncologist can open your dose-volume histogram with you and explain what each curve means for your own anatomy — on a first plan or a second opinion.
The short answer
How is a radiation plan judged using the DVH?
Coverage of the target is checked first. Every organ-at-risk curve is then checked against the dose limit set for that structure. Hot spots inside the target are reviewed next, competing plans are compared on the same chart, and any limit that had to be relaxed is recorded before a radiation oncologist signs the plan off.
Target coverage
Does enough of the target volume receive the prescribed dose? This is read as a coverage value off the target’s own curve, and it is the first thing checked because a plan that spares everything but misses the target is not a plan.
Each organ against its own limit
Every contoured organ at risk carries a dose constraint set for your treatment site. The team reads that organ’s curve at the relevant point and checks it sits under the limit, one structure at a time.
Hot spots and uniformity inside the target
A plan can satisfy every organ limit and still concentrate a small pocket of unusually high dose. The near-maximum value read off the target curve is checked so that the dose is not just adequate but evenly delivered.
Rival plans compared on one chart
Where more than one approach is possible, both plans are calculated and their curves overlaid. The comparison is made on coverage and organ sparing for your anatomy — not on the equipment name.
Trade-offs recorded, not hidden
If a limit cannot be met without under-treating the target, that is a documented clinical decision with a reason attached. It should be explained to you before you consent, and you may ask to see it.
Sign-off, then physics quality assurance
Only after the radiation oncologist approves the plan does it go for measurement on the machine. Daily imaging then confirms you are positioned as the plan assumes before each session.
Decoding the notation
What do D95, V20 and Dmax mean on a plan report?
Every one of these is simply a single point read off a curve. The letter says what is being measured; the number is the threshold.
| Notation | Reads as | What it is used for |
|---|---|---|
| Dmax | The highest dose anywhere in the structure | Structures where a single damaged point matters, such as the spinal cord or the optic pathway |
| Dmean | The average dose across the whole structure | Organs that work as many independent units, such as the lungs or the salivary glands |
| D95% | The dose received by at least 95% of that structure’s volume | Target coverage — the standard way of saying “most of the target got the dose” |
| D2% | The dose received by the hottest 2% of the volume | A stable near-maximum, used instead of a single hottest calculation point |
| V20 | The share of the volume receiving 20 Gy or more | Volume-type constraints, where how much of the organ is exposed matters more than the peak |
| D2cc | The highest dose reaching the hottest 2 cubic centimetres | Small critical volumes, commonly used for hollow organs in brachytherapy reporting |
The notation above is standard and internationally used. The values written next to it on any individual plan are set by the treating team for that person, that treatment site and that prescription — this table explains how to read the labels, not what any number should be. Nothing here is a target to compare your own report against.
The honest limits
What can a DVH not tell you?
It has no spatial information in it. A DVH says how much of a structure receives how much dose, but never which part. Two plans can produce almost identical curves while placing the dose in completely different places inside the same organ, which is why the chart is never read on its own.
Alongside the histogram, your radiation oncologist looks at the dose distribution drawn directly onto the planning scan slices. That view shows where every hot region actually sits — whether a peak inside a lung is at the edge or beside a major airway, for example. The two views answer different questions and are read together.
Three further limits are worth knowing. The chart assumes the outlines were drawn correctly, so it cannot flag a contouring mistake — if a structure was missed, it simply has no curve. It describes one snapshot of your anatomy taken on the planning scan, so significant weight change or swelling during treatment is a reason for your team to re-image and, if needed, re-plan. And it is entirely a physics description: it says nothing about how the tumour biology will respond, which is governed by factors the graph never sees.
None of that makes the DVH less valuable. It makes it a specific tool with a specific job — and knowing what that job is, is exactly what lets you ask a useful question instead of a vague one.
The short answer
What should I ask about my DVH?
Six ordinary questions. Every one of them has an answer your radiation oncology team already knows, and asking them is normal rather than confrontational.
Question 1
“Can I see my DVH?”
It exists for every calculated plan. Asking for it is the fastest way to move from being told about your plan to understanding it.
Question 2
“Which curve is the target?”
Identify the target first. Every other curve on the chart only makes sense once you know which line the treatment is aimed at.
Question 3
“Which organ was hardest to protect?”
This single question usually surfaces the whole story of your plan — the anatomy that was tight and the compromise it forced.
Question 4
“Was any dose limit relaxed?”
If one was, ask which, roughly by how much, and why the alternative was worse. That decision should already be documented.
Question 5
“Was another approach compared?”
If a different technique was considered, ask how the two compared on these curves for your anatomy rather than on the equipment name.
Question 6
“What will be monitored because of this?”
Trade-offs on a plan usually come with a follow-up plan attached. Ask what will be watched, how often, and for how long afterwards.
If you are travelling for treatment or coordinating from another city for a relative, ask for the DVH and the constraint table as a PDF. Those two pages let a radiation oncologist anywhere give a meaningful opinion without the full imaging set having to move first.
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Turn your plan from a printout into something you actually understand
Whether you are reviewing a first plan or weighing a second opinion, a radiation oncologist can read your dose-volume histogram with you and answer the questions on this page about your own plan.
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Common questions
Dose-volume histograms — your questions answered
What does a dose-volume histogram show?
A DVH is a single graph that summarises your whole radiation plan. Dose runs along the bottom of the chart, volume runs up the side, and every structure outlined on your planning scan gets its own curve. That includes the tumour target and each healthy organ near it. Reading across a curve answers one question at every point: how much of this structure receives at least this much dose? Because all the curves sit on one chart, a radiation oncologist can see the target and every organ at risk side by side rather than one report at a time.
How is a radiation plan judged using the DVH?
Target coverage is checked first: the team looks at whether enough of the target volume receives the prescribed dose. Each organ-at-risk curve is then checked against the dose limit set for that structure. Hot spots inside the target are reviewed next, because a plan can meet every limit and still concentrate dose somewhere unwanted. Where two plans are being compared, they are compared on this same chart. Any limit that had to be relaxed is recorded as a deliberate, documented decision. Only then does a radiation oncologist approve the plan, after which it goes through physics quality assurance before your first session.
What do D95, V20 and Dmax mean on a DVH?
The letter tells you what is being measured and the number is the threshold. D stands for dose, so D95% is the dose received by at least 95 per cent of that structure’s volume, and D2% is the dose received by its hottest 2 per cent. V stands for volume, so V20 is the share of a structure’s volume receiving 20 Gy or more. Dmax is the highest dose anywhere in the structure and Dmean is the average across it. D2cc is the highest dose reaching the hottest 2 cubic centimetres. The notation is standard; the actual values used on any individual plan are set by the treating team for that treatment site.
What should I ask my radiation oncologist about my DVH?
Start by asking to see it, then ask which curve is the tumour target so you have a reference point. Ask which organ was hardest to protect on your anatomy, and whether any dose limit had to be relaxed to cover the target. If an alternative plan or technique was considered, ask how the two compared on this chart rather than on the machine name. Finally ask what will be monitored during and after treatment as a result of the trade-offs on your plan. These are ordinary questions and your radiation oncology team should be able to answer all of them.
Can a DVH tell me where a hot spot is inside an organ?
No. A DVH has no spatial information in it. It tells you how much of a structure receives how much dose, but not which part of that structure. Two plans can produce nearly identical curves while placing the dose in completely different places inside the same organ. That is why a DVH is never read on its own. Your radiation oncologist looks at it alongside the dose distribution drawn on the planning scan slices, where the location of every hot spot is visible. The chart also assumes the outlines were drawn correctly, so it cannot flag a contouring error.
Where is my radiation plan made and delivered?
Your radiotherapy is delivered at an NABH-accredited partner centre; CION Cancer Clinics coordinates your treatment plan, your oncology team and your care throughout. The planning scan, the contouring, the plan optimisation, the dose-volume histogram review and the physics quality checks all sit inside that coordinated pathway, with your radiation oncologist reviewing and signing off the plan before your first session. If you want your own DVH explained in plain language, or a second opinion on a plan made elsewhere, you can ask for that at any point.
This page explains general radiotherapy planning concepts for education. It is not a substitute for guidance from your own radiation oncology team about your specific diagnosis, anatomy and treatment plan, and it makes no recommendation about any particular plan, dose or technique.