Precise Ion Irradiation Dosing Method Developed for Cancer Therapy

Scientists are employing nuclear physics principles to provide more effective approaches to radiotherapy treatment for cancer patients.

Radiation therapy using heavy ions is best suitable for cancer patients with tumors that are difficult to access, such as in the brain. These particles scarcely damage the penetrated tissue around the tumor, and can be used in such a way that they deliver their maximum energy only directly at their intended target: the tumor.

New research in this comparatively new technology is concentrated on the exact dosing: the investigators must figure out how precisely to set the radiation parameters to destroy the cancerous cells directly with as little damage as possible to the surrounding tissue. The solution depends on the extent to which the ions can be decelerated by body tissue on their way to the tumor.

Scientists from the Physikalisch-Technische Bundesanstalt (PTB; Braunschweig, Germany) have established a strategy for the more precise determination of the deceleration of carbon ions in the tissue in the therapeutically relevant area, which is totally a new approach. Although the measurement data available up to now must still become more exact, the technique has been show to perform well and can, in the future, contribute to improving the dosing for cancer therapy with carbon ions. The first findings of this research have been published June 13, 2014, in the journal Physics in Medicine and Biology.

Human tissue mostly consists of water. It can, therefore, be simulated in liquid water in which form accelerated ions can be blocked on their way and at which target they deliver their maximum energy quantity—at least hypothetically, because up to now research findings have existed only for water vapor. Scientists, however, assume that if the aggregate state is neglected, the data for the determination of the radiation dose become too imprecise.

PTB scientists have now succeeded for the first time in determining the decelerating power of liquid water for carbon ions with kinetic energies in the range of the maximum energy dissipation by experiment. These findings indicate that carbon ions are less strongly blocked in liquid water, per molecule, than in water vapor. As soon as more definitive data become available, the findings will include the calibration of ionization chambers, which are used to determine the dose in therapy planning. Currently, the Heidelberg Ion-Beam Therapy Center (HIT; Germany) is the only institution in Europe that irradiates patients with heavy ions.

The procedure applied by the researchers is based on a method that originates from nuclear physics: the inverted Doppler shift attenuation technique. While the carbon ions excited by a nuclear reaction move through the water volume, they are stopped and fall back into their ground state. The energy distribution of the gamma quanta emitted thereby is recorded with the aid of an ultra-pure germanium detector. The Doppler Effect, which leads to the displacement of the gamma energy, and the exponential-decay principle allow for the development of the velocity of the carbon ions with time to be studied, and therefore, conclusions about the stopping process can be drawn, according to the researchers.

Related Links:
Physikalisch-Technische Bundesanstalt
Heidelberg Ion-Beam Therapy Center