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Relative Biological EffectivenessPARAG ROYLOK NAYAK HOSPITAL

2Introduction

Absorbed dose - measure of the energy absorbed per unit mass of tissue.

But equal doses of different types of radiation do not, however, produce equal biologic effects.

For example, 1 Gy of neutrons produces a greater biologic effect than 1 Gy of x-rays.

The key to the difference lies in the pattern of energy deposition at the microscopic level.

3Relative Biologic Effectiveness(RBE)

The National Bureau of Standards in 1954 defined RBE as: The RBE of some test radiation(r) compared with x-

rays is defined by the ratio D250/Dr, where D250 and Dr are, respectively, the doses of x-rays and the test radiation required for the equal biologic effects.

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• RBEt=D250/Dt (same biological end-point, therefore it is end-point dependent)

• Reference: 250 kV x-ray

Equation

5Example

To achieve 50% survival fraction, 250 kV x-ray needs 2 Gy, but the tested particle needs 0.66 Gy only

RBE = D250/Dt 2 = 2 / 0.66 = 3 RBE at survival fraction of 0.5 for the tested particle is

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6RBE Is End-point Dependent• The survival curve for x-rays

has a large initial shoulder;• For fast neutrons, the initial

shoulder is smaller and the final slope is steeper.

• Because survival curves have different shapes, RBE does not have a unique value but varies with dose, getting larger as the size of the dose is reduced.

7RBE & Fractionated Doses• RBE for a fractionated regimen With neutrons

is greater than for a single exposure,• Because a fractionated schedule consists of

several small doses and the RBE is large for small doses.

• Neutrons Become progressively more efficient than X-rays as the dose per fraction is reduced and the number of fractions is increased

• The shoulder of the survival curves is re-expressed after each dose fraction; the fact that the shoulder is larger for x-rays than for neutrons results in an enlarged RBE for fractionated treatments.

8RBE for different cells and tissues The intrinsic radiosensitivity

among the various types of cells differ from each other.

The curves demonstrate the variation of radiosensitivites for x-rays and markedly less variation for neutrons.

X-ray survival curves have large and variable initial shoulder whereas for neutrons ,it is small and less variable

Hence RBE is also different for different cell lines.

9RBE as a function of LET • As the LET increases from about

2keV/µm for x-rays upto 150 keV/µm for α-particles, the survival curve becomes steeper and the shoulder of the curve becomes progressively smaller.

• Larger shoulder indicates the accumulation and repair of the large amount of sub-lethal radiation damage

10RBE as a function of LET

Linear Energy Transfer (LET keV/mm))

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RBE

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Fast Neutrons

Alpha Particles

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As the LET increases, the RBE increases slowly at first, and then more rapidly as the LET increases beyond 10 keV/µm.

Between 10. and 100 keV/µm, RBE increases rapidly with increasing LET and in fact reaches a maximum at about 100 keV/µm. Beyond this value for the LET, the RBE again falls to lower values.

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RBE

LET

100 keV/mm

The spatial distribution of ionizing events varies with the type of radiation and can be defined by LET.

12RBE and OER

Oxygen is a powerful oxidizing agent and therefore acts as a radiosensitizer if it is present at the time of irradiation (within msecs).

Its effects are measured as the oxygen enhancement ratio (O.E.R.) O.E.R. = the ratio of doses needed

to obtain a given level of biological effect under anoxic and oxic conditions.

O.E.R. = D(anox)/D(ox) For low LET radiation the O.E.R. is

2.5-3.0 It is in the higher range at higher

doses For neutrons, O.E.R is about 1.6

.

13Factors That Determine RBE

•Radiation quality (LET)•Radiation dose•Number of dose fractions•Dose rate•Biological system or end-point

14OER as a function of LET• At low LET (x- or y-rays) with

OER between 2.5 and 3, as the LET increases, the OER falls slowly until the LET exceeds about 60 keV/µm, after which the OER falls rapidly and reaches unity by the time the LET has reached about 200keV/µm.

15OER and RBE as A Function of LET

• The rapid increase in RBE and the rapid fall of OER occur at about the same LET 100keV/µm .

• Two curves are virtually mirror images of each other.

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