The Truth About Protons
The illustration above was adopted from a prominent U.S. proton facility. Note the inability to target specific tumor sites within the prostate gland to higher dose levels while lacking the ability to minimize doses to the urethra. Protons can also not be utilized to target lymph nodes.
Proton-beam vs Intensity-Modulated Radiation Therapy: Which is best for treating Prostate Cancer? Here are two recently published articles:
(Proton) Prostate Cancer Therapy Too Good to Be True Explodes Health Cost by Robert Langreth on March 26th 2012 explores increasing numbers of “breathtakingly expensive” proton beam facilities across the country and their questionable treatment efficacy related to “favorable reimbursement”.
To read the complete article, click here.
The June 2008 issue of Oncology Journal (volume 33, number 7, pages 748-753) presents the most current assessment of the heated battle between protons and photons. The article is authored by radiation oncologists at Harvard Medical School, the first institution in the world to utilize proton therapy (primarily for small brain tumors). The abstract follows:
“There is a growing interest in the use of proton therapy for the treatment of many cancers. While much evidence supports this notion in context of many oncologic sites, only limited clinical data have compared protons to photon in prostate cancer. Therefore, the increasing enthusiasm for the use of protons in prostate cancer has aroused considerable concern. Some have questioned its ability to limit morbidity. Theoretical concerns over potential additional risks for developing secondary malignancies (i.e., cancer in other areas of the body), as well as promoting hip fractures. In this article, we review the current status of the evidence supporting the use of protons in prostate cancer and discuss the active controversies that surround this modality.”
To read the complete article, click here
The Size of the Proton Beam Requires “Scattering” to treat Prostate Cancer
We believe that the best use for protons is in the treatment of tiny brain tumors, and not for treating a gland the size of the prostate. Because protons travel in tiny, straight beams they must be “scattered” to form a large enough beam to treat the entire area of the prostate. In passive scattering, or “passive modulation” (the most common proton method utilized), “scattering foils” are added to produce a beam of large enough size to cover the entire target. Unfortunately, once the proton beam encounters the filter or scanner, it becomes cone-shaped and results in spreading the radiation dose outside of and beyond the target area. In other words, the beam cannot be manipulated into a spherical shape as can be done with the photon beam (or IMRT) using the multi-leaf collimators.
Eric J. Hall, D.Phil, D.Sc, of Columbia University, widely regarded as the world’s premiere radiobiologist, writing in The International Journal of Radiation Oncology, Biology and Physics (2006;65:1-7), states:
“Protons emerging from a cyclotron form a narrow pencil beam. To cover a treatment field of practical size, the beam must be either scattered by a foil or scanned. Passive scanning is by far the simplest technique but suffers the disadvantage of increased total-body effective dose to the patient… Passive modulation results in (NEUTRON) doses distant from the field edge that are 10 times higher than those characteristic of IMRT.”
Dr. Hall continues, …
“the scattering foil becomes a source of neutrons, which results in a total-body dose to the patient.”
Because of this widening cone-shaped bean, the side effects with protons will be greater even at lower dose levels than with our high energy photons, the complete opposite of what most patients currently are told and/or what patients think!
Protons versus photons: Which is state of the art?
Many people believe that proton therapy causes significantly fewer complications than does traditional external radiation therapy (EBRT), which utilizes photons. However, the newest technology in external radiation therapy with photons surpasses both proton therapy and traditional external radiation therapy. It is called 4 Dimensional Image Guided Intensity Modulated Radiation Therapy (4D-IG IMRT) with Dynamic Adaptive Radiotherapy (DART). This advanced technology allows for “inverse treatment planning” utilized for the initial IMRT planning phase. Inverse treatment planning provides the oncologist with the ability to plan for and control the amount of radiation received by the tissues surrounding the prostate while maximizing the dose to the prostate. Thereafter, a number of technological advances, including but not limited to SonArray, Cone Beam Tomography, Exact Couch ™, Portal Vision, On-Board Imaging, and Respiratory Gating are combined to allow for the analysis of organ motion in real-time (the 4th dimension) to achieve unsurpassed accuracy. Once the motion is detected, numerous software programs activate to adapt the radiation to target the organ site which may have moved. This is true Dynamic Adaptive Radiotherapy (DART). Using gating technologies, 4D IG-IMRT can even hit a continually moving target! This ability to optimize and adapt to changes is the basis for DART. None of this is even remotely possible with protons.
Cancer Killing Effects Compared
It should be emphasized that the radio-biological effects (RBE) or cancer killing ability of photons in the high dose range used at our institute with 4D IG-IMRT are identical to that of protons. This being the case, we strongly favor 4D IG-IMRT because of the highly sophisticated beam arrangements which are available here, state-of-the-art prostate targeting, as outlined above, and far more mature research data which has been accrued with high energy photons in general compared to the short history of protons.
What the Research Says
Published research series have already demonstrated advantages of IMRT over 3-D conformal radiation at higher dose levels. Meanwhile, several recent studies have even demonstrated the lack of superiority of protons over 3D conformal radiation. We are unaware of any proton study series utilizing higher doses than 3D conformal radiation. The preponderance of data suggests that higher doses equal higher cure rates. It is not possible to safely escalate protons to doses as high as those used with 4D IG-IMRT coupled with a Palladium-103 brachytherapy. It has been well documented that it requires far higher doses of radiation to truly eradicate prostate cancer. This is accomplished only with 4D IG-IMRT utilizing DART and Pd-103 brachytherapy, which also has the advantage of maximally sparing adjacent normal tissues — neither is achieved with protons.
How effective is the High Energy Photon beam used in 4D IG-IMRT?
This is an image that shows the effective radiation dose around the prostate using 4D IG-IMRT. With this method, we are able to control or modulate the beam in such a way that ‘structures within structures’ can receive a lower or higher dose while maintaining an adequate dose to the target area (the prostate plus a margin – or possibly even lymph nodes). For example, in this illustration the urethra (shown in yellow) in the middle is receiving a far lower dose than the surrounding prostate tissue, while the tumor areas (outlined in blue and green) receive a much higher dose than the surrounding prostate tissue. In proton therapy there is always a need for a “compensating filter” in order to expand the beam width to treat the prostate to appropriate dose levels. In doing so, control over the modulation of the beam is completely lost and it becomes impossible to target small areas within the prostate to receive lower or higher doses as is possible with 4D IG-IMRT (that is, dose escalate or dose demodulate). Therefore, with protons, the entire prostate receives the same doses and this dose is minimal compared to what can be achieved with 4D IG-IMRT. While the overall dose to the prostate may look the same as with proton therapy, the ability to control the dose to the critical structures within the target (prostate) is lost. These functional results from proton filtering or scanning make 4D IG-IMRT a more versatile and therefore superior treatment to proton therapy. Additionally, because of the need for the “compensating filter”, the normal adjacent tissues including the bladder and rectum through which the proton beam enters the pelvis receive far higher dose than with 4D IG-IMRT. (quite opposite to what most people think!) Exit dose from protons is primarily minimized or even negated secondary to electron capture.
