Photons vs Protons, What´s the difference?

Stereotactic Radiation; GammaKnife, CyberKnife vs Particle therapy; ProtonBeam.
What´s the difference?

This is intended as information, an attempt to describe in a simple way, and to differentiate between the various forms of radiations; x-ray/gamma/electrons vs proton/carbon and not to describe radiotherapy in details!

Each AVM is special and unique and which method of treatment that is best suited will depend on the individual patient.
Treatment options may be surgery, embolization or/and radiation or a combination of these. It will depend on different factors such as AVM size, location, feeding arteries, associated aneurysms, venous drainage and sometimes age, as well as what type of radiation treatment that is available in your area.

The concept of radiation is based on a targeted irradiation of the AVM such that cells of vessel wall damage. Vessel wall swell up and this may eventually lead to the closure of the malformation.

What is Stereotactic Radiation?
Stereotactic treatments can be confusing because many hospitals call the treatments by the name of the manufacturer rather than calling it SRS and SBRT.
Brand names for stereotactic radiation treatments include Axesse, Cyberknife, Gamma Knife, IMRT, Linac, Novalis, Primatom, Synergy, X-Knife, TomoTherapy and Trilogy...
This list changes as equipment manufacturers continue to develop new, specialized technologies to treat cancers/AVM.

Stereotactic radiation is using photons, which is X-ray/gamma and electrons
and is a specialized type of external beam radiation therapy. It uses focused radiation beams targeting a well-defined AVM, using extremely detailed imaging scans.
There are two types of stereotactic radiation. Stereotactic radiosurgery (SRS) is when doctors use a single or several stereotactic radiation treatments of the brain or spine. Stereotactic body radiation therapy (SBRT) refers to one or several stereotactic radiation treatments with the body, such as the lungs.

The most common forms of sterostatic radiotherapy are gamma knife (GK), linear accelerator (LINAC) and Cyber Knife (CK).
While the Gamma Knife is based on many sources of radiation that hits in a defined area, Cyber Knife is based on a radiation source that is placed on a robot arm and the radiation doses shot from many different angles to achieve a similar effect. There are lots of different varieties on the marked.

In stereotactic radiation therapy the radiation pass through the body/head, but the treatment is given with extreme accuracy, the maximum effect (high-dose radiation) being at the defined area/central point of all the sources, which should limit the effect of the radiation on healthy tissues (low-dose radiation). One problem with stereotactic treatments is that they are only suitable for certain small tumors/AVM. Gamma Knife can therefore usually only be used on small AVM < 3cm in diameter for best effect and obliteration, but sometimes fraction-radiation is done, e.g in larger AVM´s with only one or few feeders in the same area.

What is Proton Therapy?
In particle therapy (Proton therapy), energetic ionizing particles; protons or carbon ions, are directed at the target (AVM).

Proton beam therapy utilizes protons (charged particles found in the nucleus of an atom) compared to radiotherapy uses x-ray beams.

Physicists separate positively charged protons from hydrogen atoms by stripping off the negatively charged electrons. A machine called cyclotron or a synchrotron generate and accelerate protons. The protons leave the machine and are steered by magnets toward the tumor/AVM. Other pieces of equipment are used to modify the range of the protons, shape of the beam and to compensate for organ location.

The dose increases while the particle penetrates the tissue, up to a maximum (the Bragg peak) that occurs near the end of the particle's range, and it then drops to (almost) zero. The advantage of this energy deposition profile is that less energy is deposited into the healthy tissue surrounding the target tissue.
The unique aspect of proton beams is that they have a defined scope that can be calculated to the millimeter and can radiate very close to sensitive vital locations. Proton therapy can therefore be used for AVM near sensitive eloquent cortical regions on small and larger malformations/tumors

ProtonBeam an option when Gamma or CyberKnife can not be used?
Conventional stereostactic radiotherapy (X-rays and electron rays) can not avoid that also other tissues than the AVM receives a low-dose of radiation, because radiation pass through the body/head. Which means that even healthy tissue is affected to some degree. Often that is not a problem because is only low doses of radiation. In other cases it may be the reason that the AVM can not be treated with this type of radiation. This is because sufficiently high dose of radiation from conventional stereostactic radiotherapy can not be given to large AVM´s without a significant risk of permanent injuries to healthy sensitive tissues in eloquent brain areas.

Protons interact in a completely different way with the matter than traditional radiation. The energy of a proton beam can be varied so that the radiation dose almost completely lands on a predetermined depth. Therefore, the radiation can be almost entirely concentrated in the AVM with proton therapy, which means that the radiation dose can be increased with much less risk of side effects.
Proton therapy can be an option for most AVM, but is especially considered for patients that is not a candidate to stereostatic radiation and whose AVM´s is close to critical vital areas or with multiple feeders. Furthermore, proton therapy particularly appropriate for children, as traditional radiation provides a degree of risk for tumors later in life.

Again, this is just an attempt to simplify the difference between photons and protons, because I have the impression that there is a lot of confusion around this issue. After writing all this, I do not know if I actually managed the task? lol

Thanks Hanne, this is really helpful - even though I just had my "X-Knife" SRS treatment, I now know what has been done.

It's great for you to have shared your knowledge and given everyone a pretty clear picture of what really is going on with the physics of the procedure. I had been wondering if the beams carried on through my head or not - now I know that they Did!!

Thaks again,


Philip: Thank you :)

The discussion was a result from my own inquisitiveness and questions from other members in here about the topic :)

I am glad the GN went well for you and that you could combine it with your seizures!

Hope you are healing well!

Best wishes and Positive thoughts, from your friend Hanne xxx

i left a long message here but im not sure it posted. lol. this site gives me trouble. it breezes everywhere else. but like i said im 35. everything was fine a few months ago but since ive lost everything i could imagine. my truck my job my life as i knew it. Ive had screws placed. ive had the angiogram and last week they did 65% of the embolization. I need to get the rest done in the next 8 weeks and then i start the proton therapy with dr thornton. my drs have alll been helpful. ive had alot of trouble with my insurance company but that seems like we all deal with that. they wilkl lose. i was given 2 years to live without treatment. The proton info has been asuring. no matter how often i hear it it asuring. they are going to fix me and i will be amazing. thank you so much for your time. my grandparents are here to c mee and i must go take care

Why wouldn’t people just choose proton radiation over photon? I’m considering proton radiation for my son but I have not found enough data to feel 100% sure. Anyone out there had proton radiation?

thanks for taking the time to put that out there

It should be noted that brand names are not the only difference between the Stereostatic Radiosurgery options. In each case the radiation is delivered into the body in a slightly different way. I’m just going to do a comparison between two forms - Linac - delivers a lower dose to the AVM with more radiation impact on surrounding healthy tissue. In my research this is a better option for diffuse AVMs. Gamma Knife - hits the target harder with the precision of a scalpel and little to no impact on surrounding healthy tissue. Better for small, compact AVMs. Proton on the other hand is recommended for larger AVMs. Neutron will be coming in approx. 10 years.

When looking at radiation learn about the structure of your AVM, do your research, talk to doctors from various facilities most especially those at the forefront of research ie. University Hospitals, do not expect your specialist to know the benefits of other systems or even be prepared to acknowledge that different systems will have different outcomes, talk to the scientists that create these radiation systems and be prepared to travel to the system that will give you the best outcome.

I have found hospitals, specialists and scientists more than willing to discuss my son’s case and explore the options including Robert Spetzler (now retired) who created the operability grading scale. All it cost me was the time of uploading his scans to their imaging software or posting the scans to the facilities.

My advice comes from experience, the wrong treatment being used initially for my son, more than a year of not taking what I was being told as fact and learning that I was right, finding the best treatment with the best outcome though that won’t be as good as it should have been, complaints to the health commission about those who have been involved, berating my son’s team that they need to become better informed about the options that are out there and a son who is facing more radiation in the next month that has a reasonable chance of completely paralysing his left side when this risk should NEVER have been on the table!

Linac was the first treatment which has changed his 1:5-10million per year AVM into a landmark world-first case. This second treatment will be (and it should have been the only option ever on the table) Gamma Knife.

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Proton beam therapy now available at Manchester Christie Hospital, UK:

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