0:01

The second important parameter to understand the radiation biology is the relative

0:06

biological effectiveness of radiation.

0:10

It is defined as the relative capability of radiation with different

0:15

LET to produce particular biological reaction.

0:19

And that is defined as RBE as a ratio of dose of 250 or DR

0:25

by dose of any radiation, where dose of 250 and dose of

0:31

DR are respectively the doses of 250 kV

0:37

X-rays and the test radiation required for equal biological

0:42

effectiveness. And that's called the relative biological effectiveness.

0:46

Why is this important? That is very important in understanding the effect of the

0:51

radiation on the system, on the body, because that'll give us an idea

0:57

of what is the damaging capability of the different type of radiation.

1:02

In this diagram, shown the relative biological effectiveness of various

1:08

tissues, where various type of radiation and at the bottom is called the LET.

1:14

If the LET, the kilo electron volt for micrometer of tissue, the linear energy

1:20

transfer through the tissue, that is keV by micrometer, is given in the logarithmic

1:25

scale of one, 10, 100, and 1,000. If you examine this curve, the

1:32

relative biological effectiveness is quoted at one for Cobalt-60 gammas

1:38

and diagnostic X-rays. That's why in radiology or diagnostic radiology, we don't

1:43

focus so much on RBE because the RBE value is one.

1:47

But in radiation therapy, where we are interacting at higher doses and type of

1:51

radiation, this is very important because RBE for one kilo electron volt is almost

1:57

close to two, fast neutrons is three, and alpha particle is given high, and

2:03

so forth. This is important in choosing the type of radiation for radiation

2:08

treatment planning. In radiology, the relative biological effectiveness is one.

2:15

The third important factors which is useful for understanding radiation biology is

2:20

the radiation quality factor and the tissue weighting factor.

2:25

Let's examine the first radiation quality factor, is called QF.

2:30

The radiation quality factor is defined as follows.

2:33

The different type of radiation and the quality factor is given in this table.

2:38

X-rays, gamma rays, beta particles, or electron all are given a quality factor of

2:43

one, while protons are given a quality factor of two.

2:48

Neutrons, depending on the energy, have the quality factor of five to 20, and alpha

2:53

particle, heavy nuclei, and fission products have a quality factor of 20.

3:00

What does that mean? Which means for the same radiation dose exposure to the

3:06

cells or to the body from an X-ray versus the proton, the damage caused

3:12

is twice with the proton compared to the X-rays.

3:15

For example, with alpha particle compared to X-rays, it is the same amount of

3:19

radiation, let's say one gray of radiation of alpha particle or one gray of X-rays.

3:25

The biological damage by alpha particle is 20 times than the biological effect from

3:31

the X-rays. That's how we should be understand what this radiation quality factor

3:36

really means. In fact, as a digression, one of the current challenges

3:42

for human space travel is how to shield the spacecraft to protect from high energy

3:48

radiation such as neutrons, alpha particle, heavy nuclei and so forth, because that

3:54

has a large magnitude of biological damage, even at the lower level of radiation

3:59

exposure, and that's the challenge which scientists are still working.

4:04

Now, the second type of weighting factors are called the organ or tissue weighting

4:10

factor. Earlier I said there are some cells which are sensitive and some cells

4:15

which are resistant to radiation.

4:18

Means for the same radiation exposure, some cells have a lot more sensitivity,

4:23

therefore have a lot more damaging capability compared to the other cell.

4:28

That is reflected in this particular table by providing a weighting factor.

4:33

But the confusion factor of this weighting factor is this is defined by the

4:38

International Commission of Radiation Protection, and depending on the prevailing

4:42

radiation research information, that committee decides on the weighting factor.

4:48

That's why I've given here both weighting factor from two different reports.

4:53

For disclosure, I'm now one of the elected member of the ICRP committee.

4:58

Shown here is ICRP 60 and ICRP 103, which is published in 2007.

5:04

So what does it mean? Does it mean the tissues change their sensitivity? No.

5:09

The understanding of the tissue sensitivity have changed over a period of time for

5:15

the scientist. For example, the breast organ tissue, which had a lower weighting

5:20

factor, is now elevated to a higher level because the later understanding of the

5:25

breast tissue being more sensitive compared to the other tissue highlights in this

5:31

assignment of this weighting factor. The same thing with the gonads.

5:35

Gonads were always considered as the most sensitive to radiation.

5:40

But as you can see here, the gonads are no longer considered the most sensitive

5:45

organ in our body. It is more like that's one of the reason why the weighting

5:50

factor for the gonads were decreased from 0.2 to 0.08.

5:57

And this very much applies when we are calculating the effective dose of a

6:02

particular procedure or a risk for a particular organ and so forth.

6:07

This tissue weighting factor is applied and so forth.

6:11

In radiology, especially, for example, if you want to compare the effective doses

6:16

from chest X-ray versus chest CT, one of the easier way to compare is to estimate

6:21

the effective dose for the chest CT versus chest X-ray, and we are calculating the

6:26

effective dose, we need to take into account the tissue weighting factor, and the

6:30

tissue weighting factor need to be based on the current ICRP report, and the

6:36

current ICRP report is ICRP 103 weighting factor, which are shown here.