A study of light ion accelerators for cancer treatment Page: 4 of 34
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Radiation therapy has become one of the most important modalities in the treatment of cancer.
It is estimated that a person has one out of three chance to be confronted with the disease during his
or her lifetime and that less than half of them will be cured. While the surgery is still the most
successful treatment, radiation therapy either alone or in combination with other modalities
contributes to about 40% of the overall cure rates. It is interesting to note that chemotherapy alone
results in a rather small part of cancer cures; it is used mostly as an adjuvant therapy. All other
modalities contribute only a few percent to the cure rates.
Ideally, the objective of any cancer treatment method is to remove or destroy the tumor while
preserving at the same time the healthy tissue as much as possible. It was with this idea in mind that
almost a hundred years ago low energy X-rays began to be used for this purpose, although their
penetration was poor and therapeutic effect debatable. In early 1920's radium units came into use,
producing deeper penetrating gamma rays; this was followed by electron accelerators providing
higher energy X-rays. Nuclear reactors made radioactive cobalt sources available and they became
a standard gamma ray source for radiotherapy, used until the present (e.g., gamma knife). Most
modern and very widely used machines for X-ray therapy are compact linear accelerators and it is
estimated that there are up to 4000 of them around the world. Over the years this technique has been
constantly improved, the machines have been adapted to the hospital environment and the delivery
of the radiation to the tumor has become more and more accurate, trying at the same time to spare
the healthy tissues. However, there are still many cases where it is not possible to avoid irradiation
of critical organs in the vicinity of the tumor; the maximum dose allowed for critical organs would
in such cases limit the dose given to the tumor, leading to a possible failure of the local control.
About fifty years ago R. Wilson remarked that the Bragg peak of monoenergetic protons
(and other, heavier ions) would allow the radiation dose to be preferentially delivered at the end of
their path, in the tumor itself where most damage has to be done. By modulating the proton (or ion)
energy it would in principle be possible to irradiate the whole volume of the tumor with a uniform
and sufficient dose, while keeping the dose delivered to other organs at a lower value. This
characteristic together with a high lateral beam accuracy is the basis of conformal treatment of
tumors, which is an important step toward the ideal method. Since this, first, proposal there were
a number of proton machines either adapted or specifically built for tumor treatment and proton
beams have established their place in the radiotherapy.
The most recent and quite promising introduction into the range of types of radiation for
cancer treatment have been energetic ions in the mass range from carbon to neon. They will be
referred to as light ions which is the accepted use in physics, although in the medical literature they
are usually called heavy ions. In addition to the advantage of showing a Bragg peak which is a
similar characteristic of protons, and an even better lateral beam accuracy than protons, ions have
other characteristics which could make them more suitable for treatment of some types of tumors
than any other radiation. The Linear Energy Transfer (LET) or the rate of energy deposition along
the path of a particle is higher for light ions (fast neutrons have a similar property) than it is for
conventional radiation, including protons; the Relative Biological Effectiveness (RBE) tends to be
higher if LET values are higher. Treatment of cancer fails very often because of the failure of the
local tumor control; in some cases this may be due to the fact that in some tumors cells are anoxic
and as such are more resistant to conventional radiation.The oxygen effect is characterized by the
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Prelec, K. A study of light ion accelerators for cancer treatment, report, July 1, 1997; Upton, New York. (digital.library.unt.edu/ark:/67531/metadc691514/m1/4/: accessed February 18, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.