A1.3 Particle radiotherapy

Topic

Charged particles—protons and carbon ions—have a physical advantage over photons regarding how they deposit energy along their path through the body: instead of losing energy evenly, charged particles concentrate most of that energy at the end of their trajectory—at a point known as the Bragg peak—and the dose delivered drops to nearly zero beyond that peak.

Photons, by contrast, do not concentrate their dose at a single point; they deliver a high entrance dose immediately upon entering the body, and this dose decreases exponentially as they travel, resulting in an exit dose beyond the tumor—something the charged particle profile virtually eliminates. This difference in dose distribution along the path makes it possible, in principle, to deliver the same dose to the tumor using charged particles as with photons, while significantly reducing the dose to surrounding healthy tissues.

Among charged particles, carbon ions offer a second advantage—this time biological—that protons lack to the same extent: carbon ions transfer energy to tissue in a much more concentrated manner along their trajectory—a property known as linear energy transfer. This concentrated transfer results in much higher relative biological effectiveness: the same physical dose delivered via carbon ions causes two to three times more biological damage than the same dose delivered via photons.

The greater biological damage caused by carbon ions has an additional characteristic: unlike photons—whose biological damage depends on the presence of oxygen in the tumor to fully manifest—carbon ions are virtually independent of oxygen, meaning the biological damage they cause remains nearly the same regardless of whether oxygen is present in the tumor tissue. This independence from oxygen is precisely what gives carbon ions particular efficacy in hypoxic tumors—that is, tumors characterized by low oxygen levels, where photons lose a significant portion of their capacity to damage tumor tissue. Taken together, the concentration of the dose at the Bragg peak (a characteristic of all charged particles) and the enhanced, oxygen-independent biological damage specifically delivered by carbon ions constitute the two advantages—one physical, the other biological—that enable charged-particle radiotherapy to treat the tumor with a higher effective dose while causing far less damage to surrounding healthy tissues than photon therapy does.

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