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To understand more clearly at a higher level, there are some defined parameters,

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and the parameters characterizing ionizing effects are the following three.

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One is called the linear energy transfer, or also called as LET.

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The second one is the relative biological effectiveness, RBE.

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The third one is the weighting factors.

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There are two type of weighting factor, radiation quality factor and the

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tissue weighting factor. In order to quantify or better understand the radiation

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effect, there are some well-defined parameters in the radiation biology.

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Among the important topic to be understood are, first is the linear energy

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transfer, also called as LET. The second one is called relative biological

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effectiveness, also called as RBE. The third one is a weighting factor.

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It is two types of weighting factor.

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One is called the radiation quality factor or the radiation weighting factor.

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The second one is the tissue weighting factor, WR.

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Let's examine each one of them and how it relates to the radiation dose magnitudes

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and so forth.

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In general, the understanding of the radiation interaction with the cell or

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tissue is defined by the law of Burgoyne and Tribondeau, two French

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radiation biologists. They defined the radio sensitivity of any

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cells is directly proportional to their reproductive activity

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and inversely proportional to their degree of differentiation.

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And that is defined by what is called the cell cycles or cell survival cycles.

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This is basically showing here the fraction of the cell surviving versus the dose

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radiation it was exposed to.

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So you can see here the fraction of the cell surviving.

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The cells have survived up to here, there's no loss, but then the

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loss goes faster with the cells which are more radio sensitive,

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and it goes lesser with the cells which are more radio resistant, and that is

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defined in the cell survival cycles. So how does it matter?

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So let's examine the cells in our body which are more resistant to cells

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compared to the sensitive cells. So the resistant cells are the following,

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cartilage, bone, liver, nervous tissue, kidney, and muscle, and so forth.

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They can take more radiation to have the same effect shown compared

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to the sensitive cells such as lymphoid cell tissue, epithelium in the GI tract,

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hematopoietic tissues, basal cells, germinal cells, and so forth.

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You're going to see later how this translate into the tissue weighting factor when

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we examine, and that depends on the type of resistance or sensitivity.

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So the linear energy transfer is one of the important topic of the

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parameter defined to characterize radiation effect.

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The linear energy transfer, also called as LET, is defined as follows.

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The average energy deposited per unit path length of incident radiation

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described in units of kilo electron volt per micrometer,

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which is important. This is defined by the experts, and it is accepted the

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definition by the ICRU and other places, and it is the average energy deposited

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by the radiation per unit path length of incident radiation described in

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units of keV. On this diagram shown here also is the fraction of cell surviving

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versus the dose. So there are two types of LET.

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One is called a low LET, the second one is called high LET.

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When we say low LET, we're talking about X-rays and gamma rays.

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They're considered as a low LET, means it takes a lot more radiation to destroy the

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cells, means it has a broader shoulder in the cell survival curve and drops off

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deeply as the radiation level goes higher after some level.

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The high LET is neutrons and alpha particle, an example, and they have

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very short shoulder in the cell survival curve, but the fraction of the cells death

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goes very fraction. Usually, when we talk about the cell survival, we

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define the amount of radiation to kill 90% of the cells, and that's what marked

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in this one. For example, in this particular diagram, from 100% cell to

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90%, that is 1 to 0.1, if you draw across this line, the high LET

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requires just 4 gray of dose to kill 90% of the

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cells, whereas the low LET type of radiation, such as X-rays and gamma

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ray, require up to 12 gray of radiation to make the same damage of 90%

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cell damage. So that is very important to understand because that's how you

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distinguish between the type of radiation, low LET or high LET.