Radiation Biology for Medical Physicists

C. S. Sureka, Christina Armpilia

• Publisher: CRC Press
• Year: 2017
• ISBN:
  9781351649827
• Edition: 1
• Topic: Médecine
• Subtopic: Oncologie

This book is designed to convey as much information as possible in a concise and simple way to make it suitable for students, researchers and clinical medical physicists. Better meanings, codes and examples are included. Most of the basics are also covered for easy reference along with a glossary of objective-type questions. Upon completion of this textbook, the readers will gather knowledge about the physics, chemistry and biology of the human body towards cancer treatment using radiation.

1
Cell Biology

Objective

This chapter discusses the basic biological background of the cell that is required to understand the remaining chapters.

1.1Cell Biology

Cell biology is the study of biochemistry of cells, structure of the cell, cellular components, cell communication with their environment, cell metabolism, their life cycle, cellular abnormalities, etc. In order to understand the biological effects of radiation, it is necessary to review cell biology. Cell theory says that cells are the basic units that perform the functions of living things, so they are called the “building blocks of life.” Their major functions include metabolism, growth, irritability, adaptability, repair, reproduction, etc.

An adult human body consists of approximately 100 trillion (100 × 10¹²) cells. These cells are different in size, shape, and function. Regarding their size, most of the cells are visible only under the microscope, with dimensions between 1 and 100 μm (10⁻⁶ m) except human ovum or egg cells. Cells have different shapes like spherical, rectangular, or irregular. Each cell type has its own function in helping our bodies to work properly, and a specific shape and size that helps it to carry out these functions effectively. Our body consists of many specialized cells including bone, muscle, fat, blood, endothelial, exocrine, endocrine, and nerve cells. However, all cells have some similarities in their structure and metabolic activities.

The human body consists of around 200 different kinds of specialized cells. When many identical cells are organized together, it is called a tissue. Tissues are grouped into four basic types, including connective, muscle, nervous, and epithelial tissues. Various tissues organized together for a common purpose form an organ, for example, stomach, skin, brain, uterus, heart, lung, etc. There are almost 78 organs in the human body that are different based on their sizes, shapes, and functions. Two or more organs working together to perform a particular function comprise an organ system. The human body consists of the following 11 organ systems and an individual organism is formed when all these organ systems are grouped together:

1. The circulatory system, which includes heart, veins, arteries, etc.
2. The lymphatic system, which includes nodes, lymph vessels, spleen, etc.
3. The respiratory system, which includes lungs, trachea, etc.
4. The digestive system, which includes stomach, liver, intestines, etc.
5. The endocrine system, which includes pituitary gland, thyroid gland, etc.
6. The nervous system, which includes brain, spinal cord, nerves, etc.
7. The reproductive system, which includes the male testes; the female ovaries, uterus; etc.
8. The skeletal system, which includes bones, cartilage, etc.
9. The muscular system, which includes muscles, tendons, etc.
10. The urinary system, which includes kidneys, bladder, etc.
11. The integumentary system, which includes skin, hair, etc.

1.2Biochemistry

Biochemistry is the study of life at the molecular level. It is generally classified into three major areas. They are (1) “structural and functional biochemistry,” which deals with the discovery of chemical structures, 3D arrangements of biomolecules, and chemicals that are present in living things; (2) “informational biochemistry,” which defines the language(s) for storing and transmitting biological data in cells and organisms; and (3) “bioenergetics,” which describes the flow of energy in living organisms and how it is converted from one form of energy into another. Since biochemistry is a wide area, a brief review on molecules necessary for life is presented here (Forshier, 2009).

Out of more than 100 chemical elements, only about 31 (28%) occur naturally in plants and animals. Out of 31, 6 of the elements, namely, C, H, O, N, P, and S, are present in major quantities and are very important for life. Twelve of the elements, namely, Na, Mg, Cl, K, Ca, Mn, Fe, Co, Ni, Cu, Zn, and I, are present in small quantities for a specific function. Also, 13 more elements, namely, B, F, Al, Si, V, Cr, Ga, As, Se, Br, Mo, Cd, and W, are present in some organisms. The selection and interaction of these elements during the early stages of evolutionary development of life is not yet unknown. However, it is known that some of these elements are bonded together to produce both inorganic and organic molecules of any organism.

An inorganic molecule is generally termed as a substance that does not contain carbon and hydrogen atoms. However, many inorganic compounds like water (H₂O) and hydrochloric acid (HCl) do contain hydrogen atoms. But, only a few inorganic compounds like carbon dioxide (CO₂) contain carbon atoms. Hence, inorganic compounds include water, salts, acids, and bases, which are essential in all living organisms. Water (more than 80%) plays a tremendous and sensitive role, namely, (1) it dissolves and transports various substances that are necessary for life, (2) it removes waste products from the cells and is hence effectively involved in chemical and metabolic activity, (3) it regulates body temperature, (4) it is involved in the biochemical breakdown of food that we eat, and (5) it serves as an effective lubricant around joints. Salts like NaCl are dissolved in water and dissociate into ions other than H⁺ or OH^–. These ions are electrolytes, so they are capable of conducting an electric current in solution. This property is important in transmitting nerve impulses and prompting muscle contraction. Unlike salts, acids release H⁺ in solution (quantified in terms of pH), making it more acidic, which is needed for digestion and to kill microbes. But, bases accept H⁺, thereby making the solution more alkaline.

An organic compound is a substance that contains both carbon and hydrogen. Organic compounds are synthesized by grouping elements via covalent bonds. For example, C can form multiple (4) covalent bonds with other C atoms and functional groups such as H, N, O, or S and hence lead to the formation of long and complex chains. This nature of the carbon atom induces the generation of four most important types of organic macromolecules, namely, nucleic acids (1% of the cell), proteins (15%), carbohydrates (1%), and lipids (2%).

Nucleic acids are complex, high-molecular-weight macromolecules made up of thousands of nucleotides (polynucleotide chain). The main function of nucleic acids is to store and transfer genetic information. Each nucleotide is made up of (five carbon) pentose sugars (deoxyribose in DNA and ribose in RNA), phosphate groups, and nitrogenous bases, either purine or pyrimidine. Adenine (A) and guanine (G) are purine bases. Cytosine (C), thymine (T)/uracil (U) are pyrimidine bases. Examples of nucleic acids are DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). RNA and DNA are discussed later in this chapter.

Proteins are large organic molecules built from a set of amino acid...