Secondary Particle Energy Deposition for Proton and Carbon Therapy Beams

Article Outline

• Purpose/Objective(s)
• Materials/Methods
• Results
• Conclusions
• Copyright

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Purpose/Objective(s) 

One of the radiobiological difficulties associated with proton and carbon ion therapy is the production of secondary particles. When heavy charged particles collide with stationary target nuclei, part of the nuclear volume of the target can be sheared away by the collision. This fragmentation process (known as spallation) results in the emission of a large number of nucleons. The rate of fragmentation depends on the composition of the medium and the energy and mass of the accelerated particles. The purpose of this work was to evaluate the energy deposition contributions from primary particles versus the contribution of secondary particles.

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Materials/Methods 

The energy deposition contribution for primary beams of heavy charged particles was calculated using HETC-HEDS (High Energy Transport Code for Human Exploration in Deep Space). HETC-HEDS simulates particle cascades using Monte Carlo to compute the trajectories of the primary particle and all secondary particles produced in nuclear collisions. The particles considered by HETC-HEDS (protons, neutrons, π⁺, π⁻, μ⁺, or μ⁻, light ions and heavy ions) can be arbitrarily distributed in angle, energy, and space. Each particle in the cascade is followed until it disappears by escaping from the boundaries of the system, undergoes a nuclear collision or absorption, or comes to rest due to energy losses from ionization and excitation of atomic electrons in the target medium. Calculations were performed for incident proton beams between 90 and 200 MeV/Nucleon in 5 MeV/Nucleon increments incident on 10 g/cm² of A-150 Tissue Equivalent Plastic. Calculations were also performed for 150–600 MeV/Nucleon carbon ion beams in the same treatment geometry.

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Results 

For proton beams, secondary particles contributed approximately 50% of the total energy deposited into the Tissue Equivalent Plastic. For the carbon beams, secondary particles contributed less than 10% of the total energy deposited. While it is well known that spallation can occur during proton and heavy ion therapy, the distributions of the spallation products after the Bragg peak are often ignored in treatment planning calculations. For a 90 MeV proton beam, only 29% of the charged particles just beyond the Bragg Peak are protons. The remaining 71% of the charged particles after the Bragg peak are heavy recoil nuclei ranging in mass from helium (21%) to Carbon (8%). For an incident carbon ion beam in the same treatment geometry, the distribution of particles just beyond the Bragg Peak are 34% protons, 29% helium, 11% lithium, 10% beryllium, 16% boron, and 1% carbon ions. Given that critical structures are often located near the Bragg peak, the effects of these particles should be taken into consideration.

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Conclusions 

Biological effects in tissue depend on absorbed dose and the distribution of the Linear Energy Transfer (LET) along the particle tracks. For proton therapy beams, Monte Carlo simulations indicate that about 50% of total energy deposition originates from secondary particles. Furthermore, a significant fraction of the fluence beyond the Bragg Peak for proton and heavy ion therapy comes from secondary particles. These secondary particles will have different LET spectra than the incident beam. A model is currently under development to incorporate the biological effect of secondary particles in the treatment planning process.