The smallest structural and functional component of protoplasm that can exist freely is the cell (Suntharalingam et al, 2005, p. 485). It is just necessary to study the effects of radiation at the cellular level to truly understand the factors that affect biological tissues. Cells are of two types - somatic cells and germ cells. Of these somatic cells have three subtypes - stem cells (cells that generate other cells through differentiation), transit cells (cells that are in the state of being transformed from one type of cell to another) and mature cells (cells that are fully differentiated and are relatively stable in that state) (Suntharalingam et al, 2005, p. 487). Somatic cells proliferate through two well-defined time periods - mitosis (M), when cell division takes place while maintaining the species chromosome number; and the period of DNA synthesis (S). (Suntharalingam et al, 2005, p. 487). Before S, there is a gap (rest period) when DNA is not yet synthesized. After S there is another gap (when DNA is synthesized but other metabolic processes are taking place). After M takes place. Thus, the cell proliferation cycle is - S M. In time this whole process is - (1-8h) S (6-8h) (2-4h) M (>1h) (h = hours). Thus, the entire cell proliferation cycle can take between 10 to 20 hrs (Suntharalingam et al, 2005, p. 487).
When there is death among non-prol...
It is notable that cells are the most vulnerable to radiation (radiosensitive) in the M and phases while they are the most resistant in the late S phase (Suntharalingam et al, 2005, p. 487).
When there is death among non-proliferating cells (static) there is said to be loss of a specific bodily function while death of proliferating cells such as stem cells and others are taken to be loss of reproductive integrity (Suntharalingam et al, 2005, p. 487).
In cases where a certain radiation-damaged cell survives and begins to proliferate indefinitely it is termed as a 'clonogenic' cell (Suntharalingam et al, 2005, p. 487) with changed cellular characteristics (the change is to the DNA components of the cell).
When cells are exposed to radiation the usual physical effects of the radiation on the atoms and molecules of the cells is immediate. Effects on biological function may follow later. Radiation effects on biological function are most pronounced when there is structural damage to DNA - the most critical target within cells (Suntharalingam et al, 2005, p. 488). It is obvious that some physical factor defines the quality of the ionizing radiation beam that may damage biological tissue. In radiobiology and radiation protection this physical factor is the 'linear energy transfer (LET).
According to the ICRU it may be defined as - "LET of charged particles in a medium is the quotient dE/dl, where dE is the average energy locally imparted to the medium by a charged particle of specified energy in traversing a distance of dl." (Suntharalingam et al, 2005, p. 486)
Thus, the impact that radiated packets may have with the atoms and molecules of the biological tissues is not as important as the