Basics of particle therapy I: physics

Abstract

With the advance of modern radiation therapy technique, radiation dose conformation and dose distribution have improved dramatically. However, the progress does not completely fulfill the goal of cancer treatment such as improved local control or survival. The discordances with the clinical results are from the biophysical nature of photon, which is the main source of radiation therapy in current field, with the lower linear energy transfer to the target. As part of a natural progression, there recently has been a resurgence of interest in particle therapy, specifically using heavy charged particles, because these kinds of radiations serve theoretical advantages in both biological and physical aspects. The Korean government is to set up a heavy charged particle facility in Korea Institute of Radiological & Medical Sciences. This review introduces some of the elementary physics of the various particles for the sake of Korean radiation oncologists' interest.

Keywords: Carbon ion; Neutron; Particle therapy; Proton.

Fig. 1

Standard Model of particles. Hadron is particles composed of quarks; meson has two quarks (quark and antiquark) and baryon has three quarks (up and down quarks).

Fig. 2

Interaction of neutrons. Elastic scattering: a neutron hits nucleus and bounce off in a different direction. Target nucleus gains energy from neutron and then increases speed. Inelastic scattering: a neutron hits a nucleus and is temporarily absorbed, forming a compound nucleus. An excited nucleus de-excites by emitting another neutron of lower energy and γ-ray. Nuclear capture: This is the most common nuclear reaction. The product nucleus becomes an isotope with increased mass. The interaction emits only γ-ray (no particles are emitted).

Fig. 3

Interaction of protons. The Coulomb interaction slows the velocity of protons before Bragg peak. As the stopping power increases, the energy of proton lowers at the Bragg peak where the proton interacts with nucleus to emit secondary neutron and γ-rays.

Fig. 4

Interaction of carbon. Carbon hits oxygen and both atoms are fragmented into boron and nitrogen generating delta radiation. The delta radiations decay to emit gamma radiation which can be used as the source of PET-CT in treatment field. Due to locally absorbed radiation around and after the Bragg peak, relative biological effectiveness (RBE) increases abruptly.

Fig. 5

Interactions of low linear energy transfer (LET), high LET and heavy particles with DNA. The low LET radiation generates radicals to cause single strand break while the high LET radiation causes multiple lesions to cause double strand break. During the heavy particle interactions, the fragmentation of elements atoms occur resulting isotopes (e.g., ¹²C + ¹²C → ⁴Li + ²⁰F and ¹²C + ¹⁶O → ²⁴Na + (α , d, p)). (The base image of DNA structure is from Wikimedia Commons. Permission is granted to copy under the terms of the GNU Free Documentation License).