The following is a summary of a presentation by James McDonough, Ph.D. from a panel session at the ASTRO 2007 Annual Meeting.
Current Status of Protons in America
There are currently five high energy proton facilities in the United States
The first was in Loma Linda and started in 1990 and has the largest experience treating patients at present.
Mass General Hospital had previously run a cyclotron facility for approximately 40 years, treating ~10,000 patients. In 2001 MGH opened its high energy proton facility.
Indiana opened a facility in 2004
Florida and MD Anderson opened facilities in 2006
There are several additional facilities being planned, such as at the University of Pennsylvania.
There appears to be a clustering effect with proton facilities being opened close to one another.
There appears to be no correlation with population of the area and the development of proton facilities.
The number of planned proton facilities continues to expand.
Use of Protons in United States
Protons allow superior dose delivery to the target, allowing less entrance and exit dose.
Presently, in the United States, scatter systems are used which have several requirements:
Shaping of the beam laterally:
A scatter foil to broaden the proton beam laterally.
An aperture is needed to allow greater conformality of the beam laterally. These are generally very heavy and hard to manipulate as they are usually made of brass or similar materials.
There is currently a MLC system in development for protons which would prevent the need for custom apertures.
Shaping of the beam along the deep axis:
The depth at which a proton can treat is based on its energy. Protons must be guided in a mono-energetic form because the bending magnets can only bend mono-energetic beams. To allow the beam to be spread to multiple depths a modulator wheel is used which changes the energy of the beam such that it can treat a larger area along the depth axis.
A compensator, usually made of wax or acrylic is used to control where the dose goes along the distal edge of the tumor. However, this results in some areas proximal to the tumor, which we do not wish to be treated, receiving 100% of the dose.
Problems with the scatter system:
Due to the aperture and compensator, the proton must travel through several layers, creating more neutrons.
Due to the compensator, there are areas proximal to the tumor, which receive unwanted high doses.
There are many custom components which need to be made as each compensator and aperture must be custom made for each field used to treat each patient. The aperture becomes radioactive after treatment and need to be stored until they are safe to dispose of. All of these components also need to have quality and assurance checks performed which is time consuming.
Advantages to scanning beam:
Scanning beams can “paint” each depth, then the energy can be changed and another layer can be painted, allowing more conformality and sparing more normal tissue proximally.
Scanning beams allow the use of IMPT
Fewer neutrons are produced with scanning beams as a compensator and an aperture are not needed.
Scanning beam allows treatment of larger fields without matching
Scanning beams are more widely used in Europe.
There is some uncertainty in the range of protons, approximately 1% (about 3 mm).
There needs to be further development of existing setup technologies, such as orthogonal imaging, to improve setup with proton therapy. This is crucial as with greater conformality there needs to be precise daily setup. Furthermore, if we want to decrease margins setup become even more critical.
Adaptation of setup imaging to proton technology is limited by the small number of centers, leading to large development costs.
Accounting for respiratory motion will also be important to decrease margins in proton therapy and decrease toxicity.
During proton therapy nuclear reactions can cause positron emitting isotope formation which can allow PET imaging. This can be used to confirm depth and confirm that protons are treating the appropriate area. Further technological development potentially will allow dosimetric evaluation during while patients are on treatment.
Currently there are several single room proton gantries in development.
Still River has been developing a cyclotron which is small enough to mount onto a single room gantry.
Dielectric wall accelerators are in development as a single room unit.
Questions include: Will the energy be high enough to treat deep tumors?
As a scanning beam is used, will the pulse rate be high enough to treat patients in a reasonable time?
Laser based protons are being developed at Fox Chase Cancer Center.
Partially funded by an unrestricted educational grant from Bristol-Myers Squibb.
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