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:

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

\begin{equation} Log1000=3 \end{equation}


\begin{equation} 10x10x10 = 1000 \end{equation}

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.


0.9 certain number - 0.9 certain number-1 should be calculated

alpha/beta ratio


  • 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.


  • 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.


  • Think about small alpha/beta ==> So beta should be big ==> that means repair capacity 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