Proton Therapy
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Right column (Protons): Compared with the X-ray plan, the proton therapy plan shows the superiority of the three-dimensional targetability of proton therapy.
The underlying principle of proton therapy
The slower the particles, the greater the linear energy transfer and the braking effect become. This leads to an "energy explosion" known as the Bragg peak at the end of the particle's path, i.e. a characteristic tissue depth of 1-4 mm for monoenergetic particles.
Figure 1 - click to enlarge
Right column (Protons): Compared with the X-ray plan, the illustrated proton therapy plan shows the superiority of the three-dimensional targetability of proton therapy.
Figure 3 provides a direct comparison of the local dose distribution for photon beams versus X-rays as shown in Figure 4.
Left column (X-rays): Two different perspectives of an X-ray treatment plan for a relapsing nasopharyngeal tumour with radiation from several directions.
At the end of their penetration depth at the Bragg peak, protons penetrating the tissue release similar amounts of energy to molecules as photons (X-rays) do, at least with respect to the hydrogen present in cellular water. In both cases, this causes molecules to lose electrons.
Comparison between X-ray and proton therapy
Figures 6 and 7 present comparisons of dose distributions in the same patients. The left-hand image in each figure shows the conventional photon radiation actually received by the patients. The right-hand image shows the exposure that would have been possible with proton therapy. The fine inner line within each image indicates the boundary of the target area (the tumour), while the other colors correspond to the local dose delivered.
X-rays Protons
Using proton beams instead of X-rays allows medical personnel to increase the therapeutic dose, which is limited due to side effects, while simultaneously reducing the dose deposited in healthy tissue.
Protons accelerated to 60% of the speed of light (180,000 km/s, 250 MeV of kinetic energy) by cyclotrons and synchrotrons penetrate approximately 38 cm into the body).
Initially, they transfer relatively small amounts of energy to the molecular electron clouds they pass through (low degree of ionization). However, this process slows them down (see Figure 1).
Unlike X-rays, proton radiation deposits a lower dose in front of the tumour. The tissue behind the tumour is not exposed to any radiation at all. This physical phenomenon makes it possible to determine the depth of the Bragg peak through modulation of the particle velocity and focus the radiation "three-dimensionally" onto the tumour with precision, greatly improving the ratio of “good radiation” to “bad radiation.”
The Bragg peak is so sharply defined that it must be “spread out” across the tumour by varying the particle speed. Figure 2 shows the resulting dose distribution for a large tumour. The reduced upstream dose is maintained while no radiation is deposited downstream of the tumour.
Figure 2 - click to enlarge
Figure 3 (proton) and Figure 4 (X-ray) - click to enlarge
Figure 5 - click to enlarge
The tissue subsequently "forgets" the cause of the electron loss (whether protons or photons) and the resulting ionization. Ionization, which is identical for both types of radiation but is more effectively targeted in the case of protons, acts as a cellular toxin as illustrated in Figure 5.
Thanks to the identical biological effects of these two radiation types, therapists can draw on the entire body of more than 100 years of empirical clinical knowledge on X-rays and thus apply clinical experience in X-ray dosing to the use of protons.
In clinical practice, this proton beam control, which is three-dimensional rather than two-dimensional thanks to lateral bundling, has reduced the radiation deposited in healthy tissue by roughly 43% to 78%, depending on the tumour geometry.
Figure 6 - click individual images to enlarge
Conventional radiotherapy with X-rays results in an unacceptable exposure of surrounding healthy tissue. In this case, the saliva glands are severely damaged.
Exposure of the tissue surrounding the tumour is minimised so that the tumour can be treated with higher doses, increasing the chance of recovery of the patient.
Figure 7 - click individual images to enlarge
Left column (X-rays): Three different perspectives of an X-ray treatment plan for a patient with a lung tumour are shown.
Irradiation takes place from several directions. Both lungs are exposed to high levels of radiation.
Adjusting the penetration depth of the proton beams allows the heart and the healthy lung to be spared to a large extent.
NOTE: The following text is provided courtesy of the Rinecker Proton Therapy Centre, Munich, Germany
A higher dose of radiation to the target but reduced exposure around it
Rinecker Proton Therapy Centre in Munich reports a reduction in the dose of radiation in healthy tissue to only 36% to 29% of the X-ray dose, for ts first 500 cases treated.
They state that the long list of damage to surrounding tissue, e.g. functional damage to healthy brain tissue, dessication of the salivary glands, radiation-induced pulmonary inflammation, kidney damage, vascular damage, impaired fertility and much more is significantly minimised.
Traditional radiotherapy uses beams of X-rays. These damage the DNA of the target tumour and of healthy tissue alike, both on the path to the tumour and beyond it, as the beam passes through the body. Various techniques and proprietary technologies have been adopted and improved in order to minimise damage to healthy tissue, including more accurate targeting and treating from many different angles, but the same basic problem is inherent in all external X-ray based treatments.
The problem with conventional radiotherapy
The advantages of proton therapy
Proton therapy uses protons (hydrogen ions) instead of X-rays. These have considerably less impact on the path to the tumour, deposit a high dose in the tumour and then almost nothing beyond. This means that the dosage is much more concentrated in the desired area, so that:
Potentially serious and distressing side effects which are common in some conventional radiotherapy treatments are greatly reduced.
Proton therapy can treat tumours where X-rays cannot be used because they would damage or destroy sensitive tissue beyond.
The danger of the radiation therapy itself causing cancers later in life is much less. For this reason proton therapy centres initially prioritised treatment of children, but proton therapy is now available to adults too.
The number of days required for treatment can be much less (this varies with different cases and between centres).
Proton therapy can safely deliver higher doses of radiation to stubborn, deep-seated tumours than is possible with conventional radiotherapy
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