Relative biological effectiveness

RBE

1. RELATIVE BIOLOGICAL EFFECTIVENESS DR.ASMITA RAYAMAJHI 1 ST YR RESIDENT

2. Ratio of biological effectiveness of one type of radiation relative to another,given the same amount of absorbed energy. Eg,.1 Gy of neutrons produces a greater biologic effect than 1 Gy of Xrays . Empirical values that varies depending on the particles,energies involved.

3. Comparison of different radiation it is customary to use x-rays as standard. The National Bureau of Standards in 1954 defined RBE as Ratio of D 250 and D r where D 250 :dose of x rays D r :dose of test radiation required for equal biologic effect.

4. Example: Comparison:RBE of fast neutron vs 250kV x-rays Test system:plant seedlings Observation:dose of x rays that result in death of half of the plants and then of neutrons that result in death of half of the plants. If LD 50 for xray:6Gy and LD 50 for neutrons:4Gy RBE:6:4 or 1.5

5. RBE DEPENDS ON Radiation Quality Radiation Dose Number of dose fraction Dose rate Biologic system or end point

6. LET:Energy transferred per unit length of tract. LE:dE / dL , where dE is average energy locally imparted to the medium by the charge particle of specified energy in traversing distance of dL .

7. As LET increases from about 2keV/ μ m for xrays upto 150keV/ μ m for alpha particles,the survival curve becomes steeper and the shoulder of curve becomes progressively smaller.

8. RBE AS A FUNCTION OF LET For cell to be killed enough energy must be deposited to produce sufficient double strand breaks Low LET radiation is inefficient coz more than one particle may have to pass through the cell to produce enough DNA breaks

9. Densely ionizing radiation also is inefficient coz more densely ionizing radiation readily produces DNA break but energy is wasted as are too close together. Radiation of optimal LET deposits right amount of energy per cell Optimal LET is around 100keV/ μ m. Radiation having optimal LET are few hundred kilo-electron volts,low energy protons and α particle.

10. As LET increases,the RBE increases slowly at first,and then more rapidly as LET increases beyond 10keV/ μ m. RBE reaches maximum at about 100keV/ μ m Beyond this value RBE falls.

11. THE OPTIMAL LET At 100 keV / μ m LET is optimal? At this density of ionization average separation between the ionizing events coincides with diameter of DNA helix(2nm) causing DNA break.

12. RBE DEPENDS UPON DOSE Xrays and neutron survival curve have different shapes,the xray survival curve having an initial shoulder and neutron curve being an exponential function of dose,resultant RBE depends on the dose level chosen

13. RBE AND FRACTIONATED DOSES X ray curve has larger initial shoulder and neutron curve has smaller shoulder and steeper slope. RBE increases with decrease in dose. RBE is larger for small dose.

14. RBE FOR DIFFERENT CELLS AND TISSUES RBE varies greatly according to tissues Depends on intrinsic radiosensitivity that differs from each other. X ray survival curve have large and variable initial shoulder whereas for neutrons it is small and less variable.

15. Cells characterized by x ray survival curve with large shoulder,indicates they can accumulate and repair,shows larger RBE For neutrons it is small and less variable RBE is different for different cells.

16. OXYGEN ENHANCEMENT RATIO Ratio of the doses under hypoxic condition to those under oxygenated conditions to produce same biologic effect. OER is measure of radiosensitizing effect of oxygen

17. As LET increases OER falls slowly at first. LET exceeds 60keV/ μ m,OER falls rapidly Rapid increase in RBE and rapid fall of OER occurs at same LET 100keV/ μ m Two curves are virtually mirror image of each other.

18. RADIATION WEIGHTING FACTOR Term radiation weighting factor has been introduced by International Commision of Radiological protection. Independent of tissue type Equivalent dose is the product of absorbed dose and the radiation weighting factor If absorbed dose is expressed in Gy , Unit of equivalent dose sievert .

19. Radiation weighting factor is set at unity for low LET radiations(x rays, γ rays and electrons)with a value of 20 for maximally effective neutrons and α particles. Equivalent dose is designated by ICRP as the limiting quantity to specify exposure limits Used for risk and exposure assessment in radiology and nuclear assessment.

20. Quality factor for different type of radiation TYPE OF RADIATION Q FACTOR X RAY 1 BETA PARTICLES 1 GAMMA RAY 1 THERMAL NEUTRONS 5 FAST NEUTRONS 10 ALPHA PARTICLES 20

21. THANK YOU