Radiation Therapy Lab Report

Physics Background

Radiation therapy is used to destroy cancer cells because when an ionizing radiation beam interacts with matter, such as tissue cells, ions produced by the interaction damages DNA (International Atomic Energy Agency, n.d.). Ionizing radiation is produced through the products of radioactive decay. Directly ionization radiation includes alpha decay and beta decay products.

Alpha decay occurs when an unstable nucleus emits an alpha particle (also known as a helium nucleus) in order to produce a more stable atom according to:

(_Z^A)X → (_Z-2^(A-4))Y+ (_2^4)He

There are two types of beta decay; beta minus and beta plus decay (or positron decay). In beta minus decay, a neutron is converted to a proton while an electron and

an antineutrino are emitted:
(_Z^A)X → (_Z+1^A)Y+ e^-+ v ̅_e
Beta plus decay involves the conversion of a proton to a neutron, with the emission of a positron and a neutrino:
(_Z^A)X → (_Z-1^A)Y+ e^++ v_e

Indirectly ionizing radiation does not interact as strongly with matter and arises from the production of gamma rays, X-Rays, and electron capture due to secondary radiation interactions.

Gamma rays and X-Rays are more generally called photon radiation, with gamma rays arising from interactions inside the nucleus, and X-Rays arising from interactions outside the nucleus (Grupen, 2005).

Gamma rays are emitted through either an excited atom returning to the ground state, or annihilation of a positron with an electron. When an excited atom returns to the ground state, the excess energy is released as a gamma ray:

(_Z^A)X^* → (_Z^A)X+

(_0^0)γ
In the annihilation process, an electron and a positron collide, and their energy is converted into two antiparallel gamma rays.

Generally, X-Rays have a lower energy than gamma rays, and can be produced through electron capture, X-Ray fluorescence, or Bremsstrahlung radiation.

Electron capture occurs when the nucleus of an atom absorbs an electron from the inner K or L shell, converting a proton to a neutron and emitting a neutrino:

(_Z^A)X+ e^- → (_Z-1^A)Y+ v_e+ (_0^0)γ

An outer shell electron will then replace the captured inner electron, emitting characteristic X-Rays in the process. If the daughter nucleus is in an excited state, gamma rays will be also emitted according to equation (4). X-Ray fluorescence arises when a sufficiently energetic electron knocks an orbital electron out of an inner shell, followed by subsequent electron capture and X-Ray emission. Bremsstrahlung radiation is produced by the scattering of electrons due to a strong electric field surrounding high Z nuclei.

Based on the type of IORT being used, the desired products of these reaction will be focused into a beam directed at the treatment site during surgery. The ionizing radiation beam will then interact with the soft tissue based on the energy of the beam. While the photoelectric effect and pair production do occur during radiation therapy, Compton effect is the most significant interaction. Compton effect (or Compton scattering) is the inelastic scattering of X-Ray photons by loosely bound outer electrons. The photon transfers some of its energy to the electron, which ionizes the

atom.

Ionized atoms can directly damage DNA through breaking the structure at the backbone or base pairs or affecting the binding of base pairs. Indirect damage can also occur through the production of free radicals which can cause harmful chemical reaction in the cell and lead to damage or death (International Atomic Energy Agency, n.d.). Once the DNA of the cancer cells is damaged, the cells will die and prevent further growth or metastases of the cancer.