Radiation Biology Lecture 2 PDF

Radiation Biology Lecture 2 : Physics and Chimistry of Radiation Absorption , Radiation interaction with mater Dr. Abdulaziz Alshihri Assistant Professor Diagnostic Radiology Department Faculty of Applied Medical Sciences Absorption Ioniazing Radiation • Radiation can be directly or indirectly ion...

Radiation Biology Lecture 2 : Physics and Chimistry of Radiation Absorption , Radiation interaction with mater Dr. Abdulaziz Alshihri Assistant Professor Diagnostic Radiology Department Faculty of Applied Medical Sciences Absorption Ioniazing Radiation • Radiation can be directly or indirectly ionizing. • Directly ionizing radiation: Charged particles with sufficient kinetic energy disrupt the atomic structure of the absorber, causing chemical and biological changes. • Indirectly ionizing radiation: Electromagnetic radiations like X-rays and γ-rays do not directly cause chemical and biological damage. Instead, when they are absorbed in a material, they produce fast-moving charged particles that can induce damage. 2 Linear Energy Transfer Linear energy transfer (LET) is the average amount of energy of a particular radiation imparts to the local medium per unit length; ie: Energy per Length Low LET • • • • Doc let High LET Electrons • Alpha Particles • Ions Of Heavy Nuclei Positrons Gamma Rays• Low Energy Neutrons X-rays x mass 8000 = 1 electron mass lany distinct . Ha distanstinotic short more with . 3 Absorption of X-Rays • X-ray photon absorption depends on photon energy and the chemical composition of the absorbing material. ⑪• At high energies (e.g., from cobalt-60 or linear accelerators in radiotherapy), the Compton the camptar out process is dominant. react with • Compton process: Photon interacts with a She "free" electron, transferring some of its energy as kinetic energy to the electron, and then continues its path, potentially interacting further. • Result: Production of fast electrons that can ionize other atoms, break chemical bonds, and initiate biologic damage. 4 Absorption of X-Rays • In diagnostic radiology, both Compton and photoelectric absorption processes occur. • Compton dominates at higher energies, while photoelectric absorption is important at lower energies. • Photoelectric process: X-ray photon interacts with a bound electron in the atom's shell, transferring all its energy to the electron, which may be released inner it from its orbit -O 5 Absorption of X-Rays • The mass absorption coefficient: • Compton process: Independent of the atomic number (Z) of the absorbing material. • Photoelectric absorption: Varies rapidly with atomic number (Z), approximately proportional to Z^3. depends 7 an SissSt - : 3 meterich o In diagnostic radiology, where photoelectric absorption matters, materials with higher Z absorb X-rays more effectively (e.g., bone contains calcium with a high Z). • For radiotherapy, high-energy photons in the megavoltage range are preferred because the Compton process dominates, ensuring a more uniform absorbed dose in different tissues. ⑱y 0 ↑pair production . "oner patient - - - absariste oversized 6 Absorption of Neutrons • Neutrons interact differently from x-rays. • Neutrons interact with atomic nuclei rather than electrons. • The way they interact is by collision with other atomic particles and this produces secondary particle ③ “spallation products” (protons, alpha particles or nuclear fragments )which are able to do the ionization process. • For instance, a high-energy neutron hitting a carbon atom can cause it to break up into three α-particles (alpha particles). ① 7 Absorption of Protons and Heavier Ions (e.g., Carbon): • Protons passing through matter undergo three significant interactions. • Coulomb interactions with atomic electrons, leading to the ionization of atoms. • Coulomb interactions with atomic nuclei, resulting in the deflection of protons. • Nuclear interactions with atomic nuclei, often leading to tow the ejection of a proton or an α-particle. q needs ⑧ zelectro stable • For heavy particles, like x-rays, the biologic effect can result from direct or indirect action. + 2 to become AT ch 8 DeoxyriboNucleic Acid (DNA) • DNA is the most important material making up the chromosome and serve as the master blue print of the cell. • It consists of two strands held together by hydrogen bonds between bases. • The "backbone" of each strand consists of alternating sugar and phosphate groups. • Four bases, adenine, thymine, cytosine, and guanine, determine the genetic code. • Bases on opposite strands must be complementary, with adenine pairing with thymine and guanine pairing with cytosine. • In Radiation Biology, DNA is the critical target for radiation. only together 9 sugarproa , T 8, · Direct and Indirect Action of Radiation • Biological effects of radiation primarily result from DNA damage. • Direct action: Radiation interacts directly with critical targets in cells, possibly ionizing or exciting target atoms, initiating biological changes. • Dominant for radiations with high linear energy transfer (LET), like neutrons or α-particles. 10 Direct and Indirect Action of Radiation (cont.) • Indirect action: Radiation interacts with other atoms or molecules in cells, especially water, producing free radicals that can damage critical targets. • Free radicals are atoms or molecules with unpaired orbital electrons, making them highly reactive. • Indirect action is dominant for sparsely ionizing radiation (low LET), such as xrays and Gamma rays. 11 Direct and Indirect Action of Radiation • (cont.) Ionization and Production of Free Radicals • In the absence of direct interactions, radiation may interact with water molecules, which compose 80% of a cell. • Free radicals have unpaired outer-shell electrons, making them highly reactive and chemically unstable. • Primary ion radicals have an extremely short lifetime (about 10^-10 second) and decay into free radicals. • H* and OH* can diffuse to reach critical cellular targets. • Estimated that about two-thirds of x-ray damage to DNA in mammalian cells is caused by OH*. H2O H2 O + + e - H2O+ H+ + OH* OH + OH H2O + e H2O - H + H H2 H2 O 2 H2O- H* + OH - H + O2 HO2 12