Myth #8: Proton is radiation, and that means I am at risk for secondary cancer from proton treatment later in life, just as I am with IMRT.
It's Ubiquitous!
The fact is, radiation is everywhere in the cosmos. It’s in the air, the soil, some foods, water, and sunlight. Our own bodies—muscles, bones, and tissue—contain naturally occurring radioactive elements.
Small amounts of radiation can come from medical X-rays, airport scanners, police speed monitors, radio waves, cell phones, solar flares, and there is even some lingering trace radiation from A-bomb tests of the 1940s and 50s.
Eighty-two percent of the radiation we are exposed to comes from natural sources, and the most significant natural source is radon gas from the earth. Most of the remaining 18% is from medical diagnosis and treatments, with less than 1% coming from nuclear power and fallout.
Latitude and Altitude Matter
Naturally occurring radiation is stronger at the poles and weaker at the equator according to studies. But even more of a factor is altitude. Cosmic radiation increases with altitude. People who live in Denver at 1,600 m (1 mile) receive nearly twice the dose than those who live at sea level. People who live in Leadville, Colorado at 3,200 m (2 miles) receive more than four times the annual sea level dose of cosmic rays.
We have lots of airline pilots in our group. Some have speculated that the thousands of hours they have spent flying may have contributed to their prostate cancer diagnosis. No studies have ever confirmed this to our knowledge.
The Good and Bad of Radiation
We know that radiation exposure can be harmful. And we also know that radiation can be beneficial. It can destroy cancer (curative) and relieve pain (palliative).
There is one thing about which radiation oncologists, scientists, and physicists agree: The only safe dose of radiation to healthy tissue is a zero dose. So, using radiation in medicine is a double edged sword. Send it to the target and it does its job. Send it to healthy tissue and you run the risk of collateral damage, including the possibility of causing secondary cancers.
The Beauty of Proton
Because of the special characteristics of the proton particle (the Bragg Peak), it can be sent deep inside the body, releasing minimal energy on the way in, and then depositing all remaining energy at the target volume. There is essentially a zero dose beyond the target, or tumor.
X-rays, or photons, which are used in essentially all other forms of radiation treatment (IMRT, CyberKnife, TomoTherapy, RapidArc, etc.), deposit radiation on everything in their path—both on the way to the target, and on the way out of the body. With proton, most of the radiation is deposited on the target volume. With X-rays (photons), most of the radiation is deposited on healthy tissue. It’s that simple.
Protons Clearly Superior
Studies have shown that two to three times less radiation is deposited on healthy tissue with protons than with X-rays. That being the case, the likelihood of secondary cancers is considerably lower with protons than with X-rays. This was confirmed in a study titled, Comparative Analysis of Second Malignancy Risk in Patients Treated with Proton Therapy versus Conventional Photon Therapy, and was presented September 2008 at the American Society for Therapeutic Radiology and Oncology (ASTRO) in Boston.
No Secondary Malignancies with Protons
In a letter to the Wall Street Journal, Dr. Andrew Lee, from MD Anderson, said,
We’ve also published an analysis comparing IMRT vs. Protons in prostate cancer and the potential impact on 2nd radiation-associated malignancies. We found that proton therapy may decrease the rates of 2nd radiogenic cancers by up to 30-40% compared to IMRT. (Fontenot et. al. IJROBP 2009)
This is corroborated by the clinical experience at Mass General: When they reviewed their 2nd cancer rates with protons, it was significantly lower than the national average with X-rays, and interestingly, the patients who received proton therapy alone (not mixed x-rays and protons) had no 2nd malignancies. (Emphasis added)
Bottom Line
Is a secondary cancer possible several years after proton therapy? Theoretically, yes. But, the likelihood is significantly lower than with any other form of radiotherapy. In fact, we have not seen a single case of secondary cancer from protons within our group of 6,400 members, which represents more than half the men in the world who have been treated with protons for prostate cancer.
Myth #8: BUSTED!
Any questions? Just ask.