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