Proton Beam Radiosurgery Comparison of Charged Particles
History of Stereotactic Radiosurgery Gamma Knife Website
Gamma Knife® Radiosurgery
by Stephen B. Tatter, M.D., Ph.D.
Comparison of Charged ParticlesComparison of
Charged Particles (e. g. Protons) and Photons (Gamma particles)
Photons produced in Gamma Knife® and linear accelerator radiosurgery units do not produce a peak of interaction of a single beam with tissue as do charged particles such as protons. There is also no rapid fall off after the target is reached. Thus, with Gamma Knife and LINAC radiosurgical doses must be achieved by superimposing a large number of beams on the target while only a small number of beams reach other areas. The large number of different target paths required makes milling of individual collimators impractical, a limitation that will not be overcome until variably-gated (or dynamically-collimated) linear accelerators are widely available.
The circular collimators currently in use in photon radiosurgery units result in roughly spherical or elliptical target volumes. Most lesions can still be satisfactorily covered using multiple superimposed ellipsoids. However, this may significantly increase the dose inhomgeneity leading to the potential for side effects in regions recieving particularly high doses. This also suggetsts that proton radiosurgery will be particularly appealing in applications where dose fractionation is of benefit, since no portion of target in a proton field is expected to receive a complication producing dose because of superimposition of high dose areas. Fractionated particle beam radiosurgery is, therefore, an area of active current research.
The Proton Beam Unit was founded in 1962 and has the largest experience with stereotactic radiosurgery of any center in the United States. Proton beam offers certain theoretical advantages over other modalities of stereotactic radiosurgery (i.e. Gamma Knife® and linear accelerators) because it makes use of the quantum wave properites of protons to reduces doses to surrounding tissue beyond the target to a theoretical minimum of zero. In practice, the proton facility offers advantages for the treatment of unusually shaped brain tumors and arteriovenous malformations. The homogeneous doses delivered also makes fractionated therapy possible. Proton beam radiosurgery also has the ability to treat tumors outside of the cranial cavity. These properties make it the ideal post-resection therapy for many chordomas and certain chondrosarchomas of the spine and skull base as well as an excellent mode of therapy for many other types of tumors.
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