Hall Book Ch 7 (Linear Energy Transfer and Relative Biologic Effectiveness)

Question 1

X- and γ-rays are said to be ( ) ionizing because along the tracks of the electrons set in motion, primary ionizing events are well ( ) in space.

Answer 1

sparsely, separated

Question 2

α-Particles and neutrons are ( ) ionizing because the tracks consist of ( ) of ionization.

Answer 2

densely, dense columns

Question 3

LET (Linear Energy Transfer) is the ( ).

Typical values are ( ) for cobalt-60 γ-rays, ( ) for 250-kV x-rays, ( ) for 2.5-MeV α-particles, and 1,000 keV/μm for heavy charged particles encountered in space.

Answer 3

energy transferred per unit length of track

0.2 keV/μm

2 keV/μm

166 keV/μm

Question 4

RBE (Relative Biologic Effectiveness) of some test radiation (r) is the ratio D250/Dr, in which D250 and Dr are the doses of ( ) and the ( ) radiation, respectively, required to produce equal biologic effect.

Answer 4

250-kV x-rays, test

Question 5

RBE increases with LET to a maximum at about ( ), thereafter decreasing with ( ) LET.

Answer 5

100 keV/μm, higher

Question 6

For radiation with the optimal LET of ( ), the average separation between ionizing events is similar to the ( ) so that DSBs can be most efficiently produced by a ( ) track.

Answer 6

100 keV/μm, diameter of the DNA double helix (2 nm), single

Question 7

The RBE of high-LET radiations compared with that of low-LET radiations ( ) as the dose per fraction ( ).

This is a direct consequence of the fact that the dose–response curve for low-LET radiations has a ( ) than for high-LET radiations.

Answer 7

increases, decreases

broader shoulder

Question 8

RBE (Relative Biological Effect) varies according to the tissue or end point studied. In general, RBE values are ( ) for cells or tissues that accumulate and repair a great deal of ( ) damage so that their dose–response curves for x-rays have a ( ) shoulder.

Answer 8

high, sublethal, broad initial

Question 9

RBE (Relative Biologic Effectiveness) depends on the following 5 parameters:

Answer 9

Radiation quality (LET)
Radiation dose
Number of dose fractions
Dose rate
Biologic system or end point

Question 10

The OER has a value of about ( ) for low-LET radiations, ( ) when the LET rises more than about 30 keV/μm, and reaches ( ) by an LET of about 200 keV/μm.

Answer 10

3, falls, unity, unity

Question 11

The radiation weighting factor (WR) depends on ( ) and is specified by the ICRP (International Commissions on Radiological Protection) as a representative RBE at ( ) for biologic effects relevant to radiation protection, such as cancer induction and heritable effects.

It is used in radiologic protection to reduce radiations of different biologic effectiveness to a common scale.

Answer 11

LET, low dose and low dose rate

Question 12

Equivalent dose is the product of ( ) and the ( ).

The unit of equivalent dose is the ( ). (In the old units, absorbed dose was expressed in rads and equivalent dose was expressed in rem.

Answer 12

absorbed dose, radiation weighting factor

sievert (Sv)

Question 13

Note that for a given type of charged particle, the higher the energy, the lower the LET and, therefore, the lower its biological effectiveness.

Compare the LET of 10 MeV protons vs 150 MeV protons.

Answer 13

10 MeV protons LET is 4.7

150 MeV protons LET is 0.5

Question 14

Because the RBE of more densely ionizing radiations, such as ( ), varies with the (
), the RBE for a fractionated regimen with neutrons is ( ) than for a single exposure because a fractionated schedule consists of several small doses and the RBE is ( ).

Answer 14

neutrons, dose per fraction, greater, large for small doses

Question 15

Figure 7.3B illustrates a hypothetical treatment with neutrons consisting of ( ) fractions. For a surviving fraction of 0.01, the RBE for neutrons relative to x-rays is about ( ).

The RBE for the same radiations in Figure 7.3A at the same level of survival was about ( ) because only ( ) exposures were involved.

Answer 15

four, 2.6, 1.5, single

Question 16

This is a direct consequence of the larger shoulder that is characteristic of the xray curve, which must be repeated for each fraction. The ( ) of the shoulder represents a part of the dose that is ( ); the larger the number of ( ), the ( ) the extent of the wastage.

Answer 16

width, wasted, fractions, greater, greater

Question 17

An important relationship exists between LET and the oxygen enhancement ratio (OER). Figure 7.7 shows mammalian cell survival curves for various types of radiation that have very different LETs and that exhibit very different OERs.

Figure 7.7A refers to x-rays, which are ( ) ionizing, have a ( ) LET, and consequently exhibit a large OER of about 2.5. Figure 7.7B refers to neutrons, which are intermediate in ionizing density and characteristically show an OER of 1.6. Figure 7.7D refers to 2.5-MeV α-particles, which are densely ionizing and have a high LET; in this case, survival estimates, whether in the presence or absence of oxygen, fall along a common line, and so the OER is unity.

Figure 7.7C contains data for 4-MeV α-particles, which are slightly less densely
ionizing; in this case, the OER is about 1.3.

Answer 17

sparsely, low