LET & RBE
Track Average
- divide the tract into equal lengths, calculate the energy deposited in each length and find the mean
Relationship between LET and RBE
-direct relationship **lower the LET=Lower the RBE
Relationship between energy and LET
inverse relationship between energy and LET ** higher the energy= lower the LET
W(r)
weighting factor and is used in radiation protection to reduce radiations of different biological effectivness to a common scale
LET (Linear Energy Transfer)
-the rate at which energy is deposited by charged particles -the average value calculated by dividing the energy deposited in keV by the distance traveled in micrometers -function of the mass and charge of the radiation
RBE depends on...
1. Radiation Quality (LET) 2. Dose=RBE increases as dose decreases 3. Number of Dose fractions= RBE increases of high LET, dose fx decreases 4. Dose Rate=high rate, more killing 5. Biological System (endpoint)=RBE is higher for tissues that accumulate and repair sublethal damage
RBE is the ratio....
RBE= ratio D250/Dr
Energy Average
-divide the tract into equal energy increments and average the lengths of tract over which the energy increments are deposited
Relationship between OER and LET
-inverse relationship **OER for Low LET= High, 2.5 and 3 **OER for High LET= Low
Low LET
-less damage -produce fewer ionizations -sparsly ionizing -small particles deposit their energy over great distance
High LET
-more damage -produce many ionizations -densly ionizing -large particles travel short distances in tissue
Oxygen Enhancement Ratio (OER)
-ratio of dose required to produce a given biological response in the absence of oxygen to the dose required to produce the same response in the presence of oxygen
Dose Rate
-slope of dose curve for gamma and xray= varies widely -slope of high LET depend little on dose rate
Relative Biological Effectiveness (RBE)
-the comparison of a dose of test radiation to the dose of 250 keV xray that produces the same biological response -the ratio of doses requires by two radiations to cause the same biological level of effect -DEPENDS ON: Dose and the Biological endpoint
