0:01

We are also in radiology, we are interested in what is called radiation-induced

0:04

carcinogenesis.

0:07

Looking at the bigger picture, the stochastic effect are the cancer in exposed

0:12

individual or this radiation-induced carcinogenesis.

0:16

Now, cancer indication is one of the most important delayed somatic effect of

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radiation exposure.

0:24

The probability of developing radiation-induced cancer depends on three things,

0:29

dose rate, radiation quantity, and the interval between exposure, which is a very

0:35

important one.

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As of now, there are no conclusive proof exists that low level radiation below 0.1

0:43

sievert or 10 rem causes significant increase in the risk of malignancy.

0:49

That has been well established now.

0:53

Where did we get the data for radiation exposure to humans?

0:57

Survivor of atomic bomb detonation in Hiroshima and Nagasaki and their offspring

1:02

has been one of the biggest data source for understanding radiation effects and

1:06

radiation biology. Second one is medical exposure.

1:10

Third one is occupational exposure.

1:12

And the later is epidemiological comparison of areas with high background

1:16

radiation.

1:19

According to this report, biological effect of ionizing radiation put forward by

1:24

the United Nation, BE7 risk estimation.

1:27

Accordingly, it is listed as 8% per sievert for high dose rate

1:33

and 4% per sievert for the low dose rate.

1:37

So the linear dose threshold model is applied for all the cancer, exposed leukemia

1:42

and bone cancer. So the non-fatal risk is 2 the power -4 cancers per

1:48

rem, and for the fatal cancer is four times 10 to the power -4 fatal cancers

1:54

per rem. The risk of radiation-induced mortality is given here per sievert as

1:59

follows. That translate to four sieverts per for low dose rate.

2:05

And there are a variety of model to understand the radiation carcinogenesis.

2:10

As you can see here, this dark blue is called the linear no-threshold model,

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where no matter how small the radiation level is, there is some risk involved, and

2:22

that's the drive. But the data which we have available can fit any of the

2:28

other three across this to match the 100 millisievert background radiation level.

2:35

Risk model for radiation-induced cancers can be absolute risk model and relative

2:39

risk model. I'm not going to go into detail of this one, but it is important to

2:43

understand.

2:45

But in general, the average risk of cancer is 5% per sievert.

2:51

Shown here, 5% per sievert. For females, it's slightly higher than the males.

2:57

And then this also tells us the pediatric population is more sensitive than the

3:02

older population, and so forth.

3:06

The last one is called the genetically significant dose or GSD.

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It is defined as the equivalent dose, if received by every member of the

3:16

population, would be expected to produce same genetic injury as

3:22

neutral doses received by the radiated person.

3:25

And the general significant dose is 0.3 millisievert.

3:30

Finally, there is also called as a negligible individual doses.

3:35

It is an effective dose corresponding to level of average annual exposure, excess

3:40

risk of fatal health effect attributable to radiation exposure, below which effort

3:46

to further reduce is not warranted.

3:49

The negligible individual dose is set at an annual minimum of 10 millisievert.

3:54

This concludes the topic which I will talk about radiation biology.

3:58

So we looked, examine at the cellular level, at the tissue level, at the organ

4:03

level, and the bigger picture of the population level.

4:06

All understanding the basics of these things is very critical in order to

4:10

understand the radiation effects through the medical imaging. Thank you.