Protons 101: Is Proton Therapy the best radiation option for me?

DISCLAIMER / About the Blog – Please read once if you haven’t.

Today we will touch on really one the first questions that a cancer patient might have – are protons right for me?

Notice that in that statement I use the phrase “we will touch” with regards to this topic. This is a big and complicated topic so to try and cover it comprehensively in a blog post just isn’t feasible. Hopefully, what we can do here is highlight a few points and give you some context or questions to ask your care team to see if maybe, it is worthwhile for you to explore proton therapy on a deeper level.

Proton Therapy’s Role in Cancer Treatment

First. The most basic concept is that Proton Therapy replaces traditional radiation. It doesn’t remove the need for surgery or chemotherapy. It is simply a more targeted form of radiation that reduces side effects by significantly reducing the amount of normal tissues that unnecessarily get treated with x-rays.

Of course, there are exceptions to this very general rule OR there are cases where your team is considering the best path and that might be chemotherapy or radiation or surgery. The largest single category exception to this would be re-treatment or re-irradiation where traditional x-rays are not feasible but proton therapy might very well be possible. So the general statement is correct, but as with most things in medicine, there are a number of special case exceptions.

Comparison Between Protons and X-Rays

The best answer to the patient specific “Are protons better for me?” question comes from a direct comparison plan between protons and x-rays.

Experienced proton physicians have a real good idea who can benefit but for many cases, I still think the best answer for the patient and for insurance and for other physician members of your care team to see is the head to head comparison demonstrating clear benefit in better dose distributions.

As we touched on above, this is why it is hard to get a good valid opinion without traveling to a proton center. I’m pretty confident that there are many regions across the country that do not have a single person with hands on proton–experience that can really help you answer this question themselves. They might have someone they could call, but there are many locations that operate in a vacuum of knowledge regarding which cases benefit and the details of what make a strong proton case vs a more technically difficult case. To make things more complicated, proton machines are often somewhat unique and even within the proton world, your best answer might come from someone with specific knowledge of that particular machine.

Proton Therapy – Who Benefits:

Some simple answers – finally 🙂

1) Current Widely Accepted tumors to treat with Proton Therapy basically irregardless of details: (straight from ASTRO Model Policies)

Ocular tumors, including intraocular melanomas • Tumors that approach or are located at the base of skull, including but not limited to: • Chordoma • Chondrosarcomas • Primary or metastatic tumors of the spine where the spinal cord tolerance may be exceeded with conventional treatment or where the spinal cord has previously been irradiated • Hepatocellular cancer • Primary or benign solid tumors in children treated with curative intent and occasional palliative treatment of childhood tumors when at least one of the four criteria noted above apply • Patients with genetic syndromes making total volume of radiation minimization crucial such as but not limited to NF-1 patients and retinoblastoma patients • Malignant and benign primary CNS tumors • Advanced (eg, T4) and/or unresectable head and neck cancers • Cancers of the paranasal sinuses and other accessory sinuses • Non-metastatic retroperitoneal sarcomas • Re-irradiation cases (where cumulative critical structure dose would exceed tolerance dose)

That list actually covers a lot of disease sites. I’ll highlight a few large categories: brain / CNS tumors, advanced or unresectable head and neck cancers, retreatment cases, and primary or metastatic cases where spinal cord is near tumor.

2) Breast cases where mean heart doses are higher than 1 Gy.

These are primarily more locally advanced or lymph node positive breast cancer cases.
In these scenarios, proton therapy consistently produces lower heart doses than IMRT or external radiation. Radiation dose leads directly to heart disease so using proton therapy in these instances is easy to justify. As a breast cancer patient it is important to know your mean heart dose in gray – and if it is above 1 protons likely significantly reduce your risk of long term damage from the radiation.

3) Patients who receive concurrent chemotherapy and radiation (concurrent means given together).

This really represents the newest and largest new category. New data presented at ASCO this year looking at 1500 patients showed that the severe side effect rate dropped from 28% to 12% simply by going from traditional with IMRT to Proton Therapy. The main disease sites are head and neck, CNS, lung, esophagus, pancreatic cancer, and rectal cancer. Really remarkable data, but it makes sense – less radiation to the normal tissues in the patient result in better outcomes. Especially in cases that are difficult enough to warrant both radiation and chemotherapy together, results are better when you use proton therapy than traditional x-rays.
This isn’t something radiation oncologist who is “well-trained” or “very good” or has “lots of experience” can accomplish without proton therapy. The baseline of 28% was a top 10 cancer facility. They have great physicians and great x-ray technology and, very likely, have more and better resources on the x-ray side than a non-NCCN member institution. The difference happened because protons are better. They deliver less radiation to the patient and each year more and more data helps to better define the patients that benefit more from the use of this technology. This appears to be the next large subset of cancer patients to broadly benefit.

So who probably does not benefit from Proton Therapy:

Again, general rules – the best answer is a comparative plan between protons and x-ray radiation.

If you have metastatic cancer and have greater than 5 spots, then proton therapy probably holds a very very low chance of benefit.

There might be exceptions here, but really, you need a doctor on your care team to say that proton therapy makes sense and they need to write the referral before you spend too much time, energy and effort on researching proton therapy. Generally, your time will be better spent looking at chemotherapy options and or targeted therapies sometimes combined with traditional x-ray radiation.

Very small or multiple small brain lesions are likely better treated on a linear accelerator.

If you have lesions in your brain that are <1cm or multiple small brain lesions, then traditional radiation machines have gotten very good at streamlining the treatment procedure for the patient and they generally will deliver excellent treatment that is difficult to outperform with protons.

Early, node negative breast cancer is very well treated with simple 3D treatment.

Again, generally, 3D conformal treatment is very good. If your heart dose is near zero (less than 1), then there is likely little chance to improve the plan with proton therapy to the whole breast. That said, we have a trial looking at 10 treatments for partial breast treatment so there might some options, but the benefit is less in this group than for many cancers.

If your team is considering IMRT for your breast cancer treatment – that is different. Protons will generally outperform an IMRT breast plan.

Disease in the abdomen that has spread to the surface of the liver or lining of the bowel (peritoneum).

If you have more widespread disease in the abdomen, proton therapy is not the primary option. Sometimes, hormonal treatment or chemotherapy or newer options like HIPEC might be benefit for you but in general, proton therapy is not the correct choice.

Hopefully this might give some people at least a bit more information. I encourage you to call a proton facility directly and be an advocate for your own care if you think you might benefit.

Dr Mark Storey MD