X- and g -rays are called sparsely ionizing because along the tracks of the electrons set in motion, primary ionizing events are well separated in space.
Alpha-particles and neutrons are densely ionizing because the tracks consist of dense columns of ionization.
X-rays, gamma rays, electrons, and protons are all low LET forms of radiation in that their density of ionization is sparse. In general, they penetrate tissues deeply and result in less intracellular radiation injury. High LET forms of radiation, such as heavy nuclear particles (e.g. fast neutrons), penetrate tissues less deeply and cause more radiation injury to biologic material.
LAW OF BERGONIE & TRIBONIDEAU"
Cells are most sensitive to Radiation when:
they are actively proliferating.
they are undifferentiated.
Exceptions to this Law:
X-rays and gamma rays show latent injury that is residual tissue damage even after the initial radiation reaction is subsided. (Shafer, 1993)
Proteins tend to be more radiosensitive than carbohydrates and lipids.
Most radiosensitive tissue-small lymphocyte
Most radioresistant tissue- brain
But Alan Jackson, 2001 from Seibert, 1996.gives muscles to be the most radioresistant.
RELATIVE RADIO SENSITIVITY OF THE TISSUES. (FROM SHAFER, 1993.)
Radiosensitive (2500 r or less kills or seriously injures many cells)
Lymphocytes and lymphoblasts
Bone marrow (myeloblastic and erythroblastic cells)
Germ cells (testes and ovary)
Radioresponsive (2500-5000 r kills or seriously injures many cells)
Epithelium of skin and many appendages.
Endothelium of blood vessels
Growing bone and cartilage.
Conjunctiva, cornea and lens of eye
Collagen and elastic tissue(fibroblasts themselves are resistant)
Radioresistant (over 5000 r are required to kill or injure many cells)
Adrenal and parathyroids
Mature bone and cartilage
Brain and other nervous tissue.
(From internet: The numbers represent the minimum damaging doses; a gray and a sievert represent roughly the same amount of radiation:
Fetus--2 grays (Gy).
Bone marrow--2 Gy.
Lens of the eye--5 Gy.
Child cartilage--10 Gy.
Adult cartilage--60 Gy.
Child bone--20 Gy.
Adult bone--60 Gy.
Child muscle--20-30 Gy.
Adult muscle--100+ Gy.
Embryonic, immature or poorly differentiated tissues are more easily injured by radiation, but they also show greater recovery properties. (Shafer, 1993)
All cells show increased susceptibility to radiation at the time of mitotic division and if the cells are irradiated during the resting phase, mitosis is delayed or inhibited. (Shafer, 1993)
In general, cells are most radiosensitive in late M and G2 phases and most resistant in late S.
for cells with a longer cell cycle time and a significantly long G1 phase, there is a second peak of resistance late in G1
the pattern of resistance and sensitivity correlates with the level of sulfhydryl compounds in the cell. Sulfhydryls are natural radioprotectors and tend to be at their highest levels in S and at their lowest near mitosis.
To produce its effect. Oxygen must be present during the radiation exposure or at least during the lifetime of the free radicals (10-5 sec).
Mandible is more susceptible to radiation injury than maxilla due to the denser structure and poorer blood supply. (Burkett, 1977)
Salivary glands though an organ with a low turnover rate, was unusually sensitive to radiation
Liposarcoma tumors are the most radiosensitive soft tissue tumors
Exophytic tumors are usually more easily controlled with radiation while infiltrative and ulcerative lesions are more radioresistant. The infiltrative and ulcerative lesions are more likely to be larger than clinically apparent and contain a larger proportion of hypoxic cells.
shafer, 1993-A TB of oral pathology.4th ed.
burkett, 1977-Burket's Oral Medicine. 7th ed