3D- Radiation versus IMRT versus Proton Therapy
Dear OncoLink "Ask The Experts,"
What are the differences between IMRT, 3-D radiation, and proton therapy? What is the best choice for a patient?
Eric Shinohara, MD, MSCI Radiation Oncology Section Editor for OncoLink responds:
The issue of IMRT vs. 3-D conformal radiation is a hot one, and unfortunately there are no randomized trials comparing the two methods. Some experts think that IMRT is better because it allows the radiation dose to be shaped more precisely around the prostate. Also, some people with prostate cancer may require radiation treatment to their whole pelvis in order to treat surrounding lymph nodes. IMRT may allow the dose to be shaped such that normal organs, such as intestines, can be avoided. By shaping the dose to avoid normal organs, toxicity may be decreased. Some studies have suggested that IMRT decreases toxicity. However, since IMRT is a new technology and most people treated with this technique were treated more recently, other advances, besides IMRT, could potentially account for some of the reduced toxicity.
Another issue to consider is that it can take longer to deliver IMRT - about 15 to 20 minutes/fraction, compared to about 5 minutes with 3-D conformal. During this extra time needed for IMRT delivery, some people worry that there may be repair of the damage caused by the radiation in the tumor cells (i.e.: the "good damage" from radiation that you actually want to occur in order to fight the tumor). Therefore, this would mean that higher doses of radiation are needed to achieve tumor control and counteract any unwanted tumor repair. This is purely theoretical, and there is no objective data to support this theory.
There is also the potential risk of increased rates of secondary cancers with IMRT, because it spreads a low dose of radiation around a larger area of normal tissue. This lower dose to a large amount of tissue may put a greater amount of normal tissue at risk for developing cancer compared with 3-D conformal radiation.
Nonetheless, based on the data that are currently available about the theoretical risks and benefits of IMRT, the majority of centers in the United States do treat prostate cancer with IMRT.
At present, there are only a limited number of radiation oncology departments with proton therapy. There are currently operational proton facilities in Loma Linda, CA ( Loma Linda University), Boston, MA ( Massachusetts General Hospital), Houston, TX (MD Anderson Cancer Center) , Jacksonville, FL ( University of Florida), and Bloomington, Indiana ( Indiana University). However, there are numerous proton facilities in development in various parts of the U.S., including: Philadelphia, PA (University of Pennsylvania), Hampton, Virginia (Hampton University), Seattle, WA (Seattle Cancer Center Alliance), St. Louis, MO (Siteman Cancer Center), Oklahoma City, OK (ProCure Treatment Center), and Dekalb, IL (The Northern Illinois Proton Treatment and Research Center), among others. Hence, the availability of proton therapy is gradually expanding and a greater number of people will have access to it in the near future. Protons may allow for even more precise shaping of the radiation beam around the tumor compared with IMRT. This has the potential to further decrease toxicity to the bowel and bladder. Another potential benefit to protons is that they deliver less radiation to normal tissues. This may reduce the risk of secondary cancers by decreasing the volume of normal tissue that sees radiation, particularly if a special type of proton beam, a scanning beam, is used. However, as with IMRT vs. 3-D conformal therapy, there have not been large trials which compare 3-D conformal radiation or IMRT with protons. However, studies that have compared the amount of side effects that people experience with protons vs. with IMRT and 3-D conformal therapy suggest that protons cause fewer side effects. It may be possible to go to higher radiation doses more safely with protons and these studies, among others, are ongoing.
The bottom line is that all experts believe in what they do, and there will most likely never be a randomized trial directly comparing 3D conformal radiation, IMRT and protons. What we do know is that both 3-D and IMRT are superior to non-CT-based conventional radiation treatment (which was used in the past), and that the cure rate for prostate cancer is substantially higher when the radiation dose is higher than 75Gy (7500 cGy), no matter whether you use IMRT or standard 3-D conformal. In theory, we may be able to achieve higher doses with proton therapy due to its ability to better shape the beam and reduce toxicity. Numerous studies are currently ongoing.