Radiation Therapy sends energy to cancer cells to kill them. Traditional Radiation Therapy is delivered by photons, the same photons that are light. Proton Therapy delivers the killing energy with protons, the large, heavy elements in the nucleus of an atom. The names are very similar — photon versus proton — but there is a world of difference. Photons, or X-Rays, are pure energy and have no mass (although Einstein would disagree as energy does have mass). Protons have energy and are relatively massive and heavy. Think of photons as "fluffy" and protons as "portly."
While radiation therapy delivers energy to kill cancer, it also affects healthy cells. In the breast, the most common side effect, a sunburn, occurs as the proton or photon travels through the skin and into the breast. This burn can be a short term problem but also a long term complication leading to skin discoloration and occasionally a thickened breast.
But the real concern is when the radiation energy affects the heart and lungs. The problem arises when radiation energy exits the breast and passes through the heart or lungs, and deposits enough energy to permanently damage the heart or lungs.
Significant delivery of radiation energy to the heart can lead to heart attacks and heart failure (Risk of Ischemic Heart Disease in Women after Radiotherapy for Breast Cancer). Likewise, significant radiation doses to the lungs can lead to chronic inflammation of the lungs, loss of lung function and difficulty breathing.
The promise of proton therapy is that protons, as opposed to photons, deposit most of the energy in the breast. Very little radiation energy from protons goes beyond the breast and therefore, theoretically, there should be less energy delivered and less complications to the heart and lungs (Promise and Pitfalls of Heavy-Particle Therapy).
So why doesn’t everyone use proton therapy then? Cost. The cost of proton therapy can be significantly more than for photon radiation therapy. If the side effects of proton radiation therapy are significantly less, then the increased cost of proton radiation therapy could be worth the benefit of decreased complications and organ injury (Reducing the Cost of Proton Radiation Therapy: The Feasibility of a Streamlined Treatment Technique for Prostate Cancer).
Modern photon radiation therapy is delivered with much safer techniques today. Better planning with CT scanners and more accurate delivery with Intensity-Modulated Radiation Therapy (IMRT) can lead to significantly less radiation energy to the heart and lungs. Therefore, proton radiation energy may not prove to be safer than standard photon radiation therapy and not warrant the additional costs.
Unfortunately, very few, if any, clinical trials have been done with proton radiation therapy for breast cancer. Computer models predict less radiation to unwanted organs with proton radiation therapy, but few clinical studies have examined these models.
On the other hand, no clinical trials have disproved the benefits of proton radiation therapy, which, in the end, may prove to be substantial. Studies using proton radiation therapy for breast cancer need to be done and currently, some Proton Therapy Centers are participating in a Medicare registry to monitor effectiveness and side effects of proton therapy.
For cancers including prostate, lung and brain tumors, studies do exist proving the benefit of proton radiation therapy. Likewise, in children, proton radiation therapy is the modality of choice, especially for brain and spinal cord tumors.
The one clinical scenario where proton radiation therapy may be worth the extra costs in breast cancer is in women with pre-existing heart or lung disease and perhaps those with breast implants. Studies using proton therapy for accelerated partial breast irradiation do demonstrate some promise for proton therapy being safer (Chang JH, Lee NK, Kim JY, et al. (2013) Phase II trial of proton beam accelerated partial breast irradiation in breast cancer. Radiother Oncol 108:209–214).
In summary, proton therapy is just as effective as photon (traditional) radiation therapy, but the question remains as to whether the increased cost of proton therapy is worth the anticipated lessening of side effects.