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: So 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==> 0.9certain number x (1-0.9) should be calculated. or 0.9certain 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