What this guide is, and what it is not
Two machines can deliver radiotherapy to the same tumour: a linear accelerator producing high-energy X-rays, called photons, and a cyclotron or synchrotron producing protons. Both are radiotherapy, given in fractions over days or weeks and planned by a radiation oncologist with a physics team. What differs is where the dose goes.
This page is background reading, not medical advice. It cannot tell you what is right for your tumour; only your own clinicians, with your scans and history, can.
Two beams, two different journeys through the body
A photon beam does not stop. It enters the body, deposits dose along the whole path, peaks a centimetre or two below the skin, and continues out the far side. Techniques such as IMRT and VMAT shape it using several angles, so the tumour receives a high dose and nearby tissue less — but the dose still travels through you.
A proton beam is different: protons are relatively heavy charged particles. They travel a set distance, release most of their energy at a depth set by how fast they were accelerated, and stop. Tuning the energy sets that depth; combining energies and angles covers a tumour with thickness and an irregular shape.
The biology is the same either way: both beams are fractionated to let normal tissue recover between doses. What changes is the geography of the dose.
The Bragg peak, in plain terms
The spike of energy a proton releases near the end of its path is the Bragg peak, named after William Henry Bragg. In practice, a proton beam can deposit its dose inside the target and almost nothing beyond.
One analogy: a photon beam is like a torch shone through a slice of fruit, lighting the skin, the flesh and the far side. A proton beam is closer to a charge timed to go off at a chosen depth.
Why exit dose matters
With photons, whatever sits behind the tumour receives some dose: the brainstem behind a skull-base tumour, the far side of the brain in a child, the rectum and bladder around the prostate.
Modern photon planning keeps that scattered dose within accepted tolerance for most patients, but it is not zero. Less dose to growing tissue, and to organs already treated, is the main reason protons attract interest. Whether that advantage becomes something a patient can feel — fewer side effects, better function, longer survival — varies by tumour type and is often not yet known.
Side by side
| What to compare | Photon (X-ray) therapy | Proton and heavy-ion therapy |
|---|---|---|
| Dose deposition | Dose is laid along the whole beam path, including beyond the tumour (exit dose). | Most dose is released at a chosen depth and the beam stops. Essentially no exit dose. |
| Sessions | Commonly 5 to 33 outpatient visits, depending on site and intent. Shorter courses are increasingly standard. | Fraction numbers are often similar, but each visit takes longer, often half an hour or more. |
| Cost | The least expensive external radiotherapy; prices vary enormously by country. | The most expensive. Reported US prices often reach six figures in dollars; Chinese centres quote less, though still above photons. |
| Availability | Linear accelerators exist in nearly every radiotherapy department, including most provincial hospitals in China. | A few hundred centres worldwide, a few dozen in China; most screen referrals, so not every case is accepted. |
| Evidence base | Decades of randomised trials across nearly every cancer type. | Strong dosimetric data and growing registries. Randomised evidence covers few sites, and usually shows similar tumour control. |
| Typical indications | Most solid tumours, including common breast, lung and bowel cancers; palliative care; spinal cord compression. | Paediatric, skull-base and spinal tumours, selected head-and-neck and prostate cancers, eye tumours, re-irradiation. |
| Practical burden | Usually deliverable near home, which matters over weeks. | Often means weeks away from home. |
Where protons are genuinely useful
Professional consensus supports protons in a limited set of situations, usually because a critical structure sits next to the target.
- Paediatric tumours. Children's tissues are still growing and they have decades in which a second cancer could appear. Reducing the volume of normal tissue irradiated is the strongest argument for protons.
- Skull-base and spinal tumours. Chordomas, chondrosarcomas and some meningiomas sit against the brainstem, optic nerves or spinal cord, where nearby tolerance caps the dose photons can safely deliver.
- Selected head-and-neck cancers. Where salivary glands, swallowing structures or the optic apparatus are close, proton plans often spare them better. Whether that means fewer long-term symptoms is still being measured.
- Some prostate cancers. Randomised comparisons exist in this setting. Reported differences concern bowel and urinary side effects rather than cancer control.
- Re-irradiation. When a second course is needed in an area already treated, protons may make it feasible at all. This remains under study.
Where protons are not proven better
For most common cancers, including the majority of breast, lung and gastrointestinal tumours, modern photon techniques are considered equivalent in tumour control and often in side effects too. Stereotactic treatment, image guidance and adaptive planning have improved photon radiotherapy substantially, so comparisons should be against those methods, not against the radiotherapy of twenty years ago.
In some cases a proton plan does reduce dose to a nearby organ on paper: the heart in some left-sided breast cancers, the lungs in some chest tumours, the bowel in some pelvic tumours. Whether that produces fewer heart attacks or better survival over ten or twenty years is, in most of these settings, not yet established. Choosing protons for that reason can be reasonable, but with the uncertainty in view.
Why "newer" does not mean "better"
New technology has a strong pull. It is also true that in radiotherapy the quality of the plan, the accuracy of daily set-up and the experience of the team often matter more than the beam type. A well-run photon department that treats your tumour type every week may serve you better than a new proton centre that has treated a handful of cases like yours.
"Proton therapy" is also not one thing: planning skill, image guidance and the ability to adapt a plan vary between centres. Heavy-ion (carbon) therapy is newer still, available in very few places, with evidence concentrated in a few tumour types. Newer equipment is a fact about a building, not about your outcome.
Proton and heavy-ion therapy in China: capacity and access
China's particle therapy capacity has grown quickly. The first combined proton and heavy-ion centre opened in Shanghai in the mid-2010s, and others have followed in cities including Guangzhou, Hefei, Zibo, Wuwei, Hangzhou and Beijing. Regulators have approved a few dozen hospitals to install such systems, though far fewer are treating patients.
Two consequences follow. Waiting times at Chinese centres are often reported to be shorter than in parts of Europe and North America, where referral may be screened by national panels or slowed by insurance authorisation. At the same time, capacity is limited and most centres accept only a minority of referred cases. A centre that declines your case may simply be saying that photons would serve you just as well.
Cost is a further consideration: particle therapy is generally self-pay for international patients. If you use a coordination service, ask for the hospital's itemised quotation and any coordination fees to be quoted separately.
Questions to put to any centre that offers protons
- In my tumour type and stage, do protons change the outcome or mainly the side-effect profile?
- What would you plan if protons were unavailable, and can I see a dose comparison on my own scans?
- Is there a trial or registry covering someone like me?
- How many patients with my diagnosis has this centre treated, and how many last year?
- Who will supervise the whole course, and will it be the same oncologist throughout?
- What are the main short-term and long-term risks for someone with my history?
- How many visits, over how many weeks, and can the course be shortened?
- What is the total cost, what does it exclude, and what if I need extra days?
- What happens if the plan must change mid-course?
- What follow-up will you provide once I am home, and who takes over there?
A clinical decision, not a purchase
It is understandable to want the best machine. But there is no such category in the abstract: only the plan that best fits a particular tumour in a particular body, delivered by a team that does this work often. For many patients that plan will use photons, and being told so is not a denial of care.
Ask for the comparison, ask about the evidence, and take a second opinion if you want one. Then decide with a radiation oncologist who has seen your scans — not with a brochure, a price list or a search engine.
Our role, where patients use our service, is logistics: records, appointments, interpretation, and keeping the hospital's bill separate from our coordination service fees. We do not choose the beam.
References
- PTCOG — particle therapy facilities, patient statistics and indications
- IAEA — DIRAC, the Directory of Radiotherapy Centres
- World Health Organization — Cancer fact sheet
- NHS — Radiotherapy: overview
- NICE — published guidance and interventional procedures assessments
- National Health Commission of China — health statistics and service planning
- PubMed Central — free full text of trials and reviews