Heavy ion therapy

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Treatment center for heavy ion therapy at the GSI with a sample mask for the radiation of brain tumors
Therapy place at the Heidelberg Ion Beam Therapy Center (HIT)

The heavy ion therapy is a form of external radiation therapy and reaches for especially radiation-resistant forms of tumors greater chances of recovery than other therapies. The tumor is with a beam of carbon - ions irradiated to kill the modified cells.

As an extended form of proton therapy , the method belongs to particle therapy . It was developed and tested at the GSI Helmholtz Center for Heavy Ion Research in Darmstadt and has been used in its own therapy center ( Heidelberg Ion Beam Therapy Center , HIT) at Heidelberg University Hospital since 2009 , mainly against tumors in the head area. A similar system went into operation in the Marburg ion beam therapy center in October 2015 .

history

After ten years of basic research in radiation biology and physics , the prototype of an intensity-modulated grid irradiation was set up and tested at the GSI heavy ion accelerator SIS from 1986 to 1991 . A therapy unit was set up from 1993 to 1997. Research showed that carbon ions are the ideal candidate for treating deep-seated tumors because they offer a high dose at the end of the radiation, i.e. H. a very pronounced Bragg peak with minimal scattering losses in the healthy tissue in front of it . In addition, there is a higher biological effectiveness on tumors than protons and a special side effect: the carbon beam allows precise beam localization through the carbon isotopes that are produced , the emissions of which are used in positron emission tomography (PET) as a safety check and detection monitor. This enabled the first patient to be treated in a very short time after a test phase on December 13, 1997 . This was the first tumor therapy with carbon ions in Europe and the first applied intensity-modulated heavy ion therapy worldwide.

functionality

When planning the radiation therapy , the radiation therapist uses an X-ray computed tomography to determine the target volume, i.e. the tumor plus a safety margin. The desired dose distribution takes into account particularly radiation-sensitive organs at risk, such as the spinal cord. For the purpose of irradiation, the target volume is computationally divided into layers of equal thickness that are perpendicular to the planned beam direction. These layers are then scanned by the ion beam point by point with a precisely defined energy and dwell time, so that each point receives a previously calculated radiation dose . For this purpose, the particles circulating in the accelerator ring are disturbed in their path in such a way that some of them emerge from the ring and can be directed towards the target; the remaining particles initially remain in the ring. The number of particles irradiated per grid point is controlled by means of multi-wire chambers .

The beam entrance in the patient room runs horizontally and was initially immobile. However, the gantry has been in operation at HIT since October 2012, a beam guidance that can be rotated through 360 ° . The deflection of the beam for scanning the target volume is achieved by two dipole magnets . The beam is precisely delimited with collimators . By rotating the patient table, the patient can be irradiated from different directions.

advantages

More targeted energy delivery

Electrons with 4 MeV penetrate tissue only 1 cm deep, but also generate far-reaching bremsstrahlung . Photons with 20 MeV damage from 3 cm and deeper. Protons with 150 MeV damage mainly spatially limited at a depth of 12 cm. "Intensity" here means dose per unit of path length

Therapy with ions and especially with heavy ions has the advantage over conventional radiation therapy with photons (X-rays) that the spatial energy output of the particles can be controlled much better. Photon radiation emits a large part of its energy as it enters the tissue, the tumor is shielded by the tissue in front of it and receives a significantly lower radiation dose , while the healthy tissue is damaged. In contrast, protons and, even more pronounced, heavy ions initially release very little energy (at high speed) to the surrounding tissue; their energy output per path only increases very sharply when the residual energy is low. This area of ​​the particle trajectory, the Bragg peak , can be placed after the tumor has been measured by choosing the incident energy of the particles so that most of the energy is transferred to the tumor, while the healthy tissue is hardly damaged.

Direct beam monitoring

On their way through tissue, some carbon ions are converted into lighter carbon isotopes that emit positrons . When these positrons meet electrons , they emit two characteristic photons at an angle of 180 °. On their way into the body, some particles in a gas-flowed detector cause statistical ionization on a metal grid. The angle of the beam and thus its position in the tissue can be calculated from the data from the detector and compared with the target values ​​specified in the therapy plan and the deflection corrected if necessary.

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