About Proton Therapy
The therapeutic radiation dose is limited by the unavoidable dose to healthy tissue surrounding the target. The mandate of modern radiation therapy has been to increase the dose to the target while reducing the dose to the surrounding tissue. Proton therapy addresses this mandate because the physics of proton therapy is fundamentally different from the physics of traditional radiation.
As illustrated left, traditional radiation (green) deposits most of the dose near the skin (0 depth), and about 20% of the maximum dose to tissues beyond the target. Electrons (yellow) do not deliver dose to tissues beyond the target, but the maximum dose is still delivered short of the falloff, and electron beam radiation is not suitable for targets deeper than about 5 cm. Proton radiation (white) delivers maximum dose at the target depth and falls off rapidly protecting tissue beyond the target. The maximum dose range (Bragg peak) can be extended - the spread out Bragg peak (SOBP) - to cover the tumor.
The result is illustrated by this comparison of treatment plans for a hypothetical prostate tumor. The image is a cross section of a patient, head toward the reader, front toward the top of the page. The circular outline surrounds the prostate and the triangular outline surrounds the rectal portion of the bowel (which must be protected from radiation). Traditional radiation (top) deposits most of the dose (yellow) in front of the prostate and about 50% of the maximal dose to the rectum.
Proton beam radiation delivers most of the dose to the prostate and less than 10% of the maximum to the rectum.
By using proton therapy it is possible to deliver more dose to the tumor without delivering more dose to the healthy tissue. For tumors that are close to radiation sensitive tissue and for tumors that are radiation resistant an increase in dose is crucial to successful therapy.
Beam Production System Development
The Trunkline at IUCF forms a doubly achromatic waist at the southern most end of a beam line running the length of MPRI and ending in a beam dump at the northern most extreme. The line consists of vacuum pipe, through which the proton beam travels, steering magnets (red) and shaping magnets (blue). The gating magnets (green) "kick" the beam into an Energy Selection Line (ES).
The doubly achromatic waist is repeated at each "kicker" magnet; the beam optics at the entrance to each ES line are identical. The Trunkline is controlled by cyclotron operations. The optics are evaluated at each waist and the energy (205MeV) is evaluated at the beam dump.
The Energy Selection Line (ES) begins at the kicker magnet (green in the images and on the diagram). When energized,
this magnet switches the beam from the trunkline into the Energy Selection Line. The beam passes through a berillium double wedge
degrader that can be set to reduce the beam energy continuously from 205 to 0 MeV. The position of the degrader is coordinated with the
magnetic fields of two bending magnets (Energy Spectrometer Dipole Magnets) so that protons of any energy other than that selected will
not pass into the treatment room.
The beam energy and current are checked by a multi-layer Faraday cup (MLFC) located after the first bending magnet. After the Faraday cup is removed, the verified treatment beam enters the Treatment Room for final shaping and administration.
Proton Therapy Project Progress
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