Radiation Oncology/Radiobiology/Basic Radiobiology

4 R's of Radiobiology

• Repair

• Reassortment

• Repopulation

• Reoxygention

Basic Radiobiology

Here I would like to explain the basic concepts in radiobiology in a way, I never forget and in a way that you certainly understand. I am positive that if you understand, then you will never forget as well:

OK!
Deep Breath! :D
1. Remember the concept of Logarithm!
When base is 10:

$Log1000=3$

So

$10x10x10=1000$

What does that do with radiobiology and radiation oncology?!

2. Think about fractional cell kill!

What the hell is that then?!
Let's say each Gy of dose kill 90%(0.9) of cells —> 10% will be left —> 0.10 will be left

then after the second next Gy —> 0.9x0.10 will be left

or 0.9x(1-0.9)

and then after the third dose of Gy —> 0.9x0.9x(1-0.9) … etc would be the amount of cells that will be killed.

That's were the log comes in! After certain number of dose==>

0.9^{certain number} x (1-0.9) should be calculated.

or

0.9^{certain number} - 0.9 ^{certain number+1} should be calculated

ɑ/β ratio

Alpha

• Intrinsic radiosensitivity of the cells
• Defined as how many logs (to the exponential base “e”) are killed (sterilized) per gray, in a “non- repairable” way.

Beta

• It reflects cell repair mechanisms.Repairable portion of the radiation damage, requiring 6 h or more for complete repair.
• It can be regarded as the result of two charged-particle tracks passing through a sensitive target in the cell nucleus in less than 6 h, so this term has to be multiplied by d squared.

Alpha/Beta

• Think about small alpha/beta ==> So beta should be big ==> that means repair mechanisms is large ==> that means time to carry out repair should be long ==> that means cell cycle is long ==> that means tissue is slowly proliferating and evetually it's a late reacting tissue.
• The shoulder will be broad too.

Famous rule of Bergonie and Tribondeau:

• They did experiments on testis Rat.
• Radiosensitivity of cells or tissues is correlated with the frequency of mitoses which they undergo ==> poorly-differentiated and fast-proliferating tissues are more radiosensitive than well-differentiated and slow-proliferating tissues

Also look at Radiobiology and Lung