Medical Archive

Hall 7 Linear Energy Transfer and Relative Biologic Effectiveness

1 THE DEPOSITION OF RADIATION ENERGY

  • – radiation is absorbed in biologic material → ionizations and excitations occur
  • – not distributed at random but tend to be localized along the tracks of individual charged particles
  • – depends on the type of radiation involved
  • – spatial distribution of ionization events produced by different particles varies (Fig 7.1.)
  • 1) sparsely ionizing
    • – χ-and γ-rays
    • – ionizing events are well separated in space
  • 2) densely ionizing
    • – α-particles and neutrons
    • – dense columns of ionization

2 LINEAR ENERGY TRANSFER

– definition : energy transferred per unit length of the track (keV/μm)

L = dE/dl
(L; linear energy transfer
dE ; the average energy locally imparted to the medium by a charged particle of specified energy
dl ; traversing distance)

– LET is the average energy deposited per unit length of track (Fig 7.2)

  • 1) track average
    • – obtained by dividing the track into equal lengths
    • – calculating the energy deposited in each length
  • 2) energy average
    •  – obtained by dividing the track into equal energy increments and averaging the lengths of track

– LET is useless and very misleading in some circumstances, but is useful as a simple and naive way to indicate the quality of different types of radiation

– for a given type of charged particle, the higher the energy, the lower the LET and therefore the lower its biologic effectiveness (Table 7.1.)

3 RELATIVE BIOLOGIC EFFECTIVENESS

  • – absorbed dose : term of amount or quantity of radiation (unit : gray or rad)
    • – a measure of the energy absorbed per unit mass of tissue
  • – equal doses of different type of radiation do not produce equal biologic effects
    • – difference : pattern of energy deposition at the microscopic level
  • – RBE (relative biologic effectiveness) : some test radiation compared with x-rays (x-ray as standard)
    •       i.e.) D250/Dr = X-ray dose / tested
    •       D250; doses of 250-kV x-rays
    •       Dr; dose of the test radiation(r) required for equal biologic effect
  • study of RBE differs according to the endpoint (Fig. 7.3)
    • fast neutron, 250-kV x-rays, mammalian cells
      • surviving fraction of 0.01 : RBE=1.5
      • surviving fraction of 0.6 : RBE=3.0
    • RBE는 dose가 작아질수록 커지는 경향이 있다. fractionation하는 경우에는 그렇지 않은 경우보다 크다.
    • endpoint에 따라 RBE가 다르기 때문에, endpoint를 가지고 얘기해야 한다. 매우 중요!
  • because the x-ray and neutron survival curves have different shapes (shoulder), the RBE depends on the level of biologic damage chosen
  • generally, dose ↓ →  RBE ↑

4 RELATIVE BIOLOGIC EFFECTIVENESS AND FRACTIONATED DOSES

  • RBE for a fractionated regimen with neutrons is greater than for a single exposure (because fractionated schedule consists of a number of small doses → the RBE is large for small doses)
  • Fig 7.3B
    • surviving fraction of 0.01 ; RBE=2.6 (> Fig 7.3A)
    • → this is a direct consequence of the larger shoulder of the x-ray, repeated for each fraction
  • neutrons become progressively more efficient than x-rays as the dose per fraction is reduced and the number of fraction is increased

5 RELATIVE BIOLOGIC EFFECTIVENESS FOR DIFFERENT CELLS AND TISSUES

  • RBE varies greatly according to the tissue or endpoint studied
  • cells characterized by x-ray survival curve with large shoulder  larger RBEs for neutrons
  • cells characterized by x-ray survival curve with little shoulder  smaller RBEs for neutrons

6 RELATIVE BIOLOGIC EFFECTIVENESS AS A FUNCTION OF LET

(Fig 7.4) LET↑ → survival curves become steeper, shoulder of the curve becomes smaller

Fig 7.5) LET↑ → RBE increases slowly at first
→ more rapidly increased beyond 10keV/μm
→ rapidly increased between 10 and 100keV/μm (maximum at 100keV/μm)
→ beyond this value → RBE falls to lower values

7 THE OPTIMAL LINEAR ENERGY TRANSFER

왜 100keV/mL LET에서 가장 RBE가 클까? 아래 설명은 절대적인 사실은 아니고 유력한 가설임.

  • – why radiation with an LET of about 100keV/μm is optimal (Fig 7.6.)
    • – at this density of ionization, the average separation between ionizing events just about coincides with the diameter of the DNA double helix (2nm) → highest probability of causing a double-strand break
    • – the most biologically effective LET is that at which there is a coincidence between the diameter of the DNA helix and the average separation of ionizing events

8 FACTORS THAT DETERMINE RELATIVE BIOLOGIC EFFECTIVENESS

RBE depends on the following
– Radiation quality (LET)
– Type of radiation and its energy
– Radiation dose
– Number of dose fractions
– Dose rate
– Biologic system or endpoint

9 THE OXYGEN EFFECT AND LINEAR ENERGY TRANSFER

Important relationship between LET and the OER (Fig 7.7.)
– mammalian cell survival curves for various types of radiation
– very different LETs and OERs

OER as a function of LET (Fig 7.8)
– at low LET (x-ray, γ-ray) : OER is between 2.5 and 3
– as LET increases : OER falls slowly at first
– after LET 60 keV/μm; OER falls rapidly and reaches unity about 200keV/μm

OER and RBE as a function of LET (Fig 7.9)
– virtually mirror images of each other
– rapid increase of RBE and the rapid fall of OER occur at about the same LET, 100keV/μm

10 RADIATION WEIGHTING FACTOR

 – simpler way to consider differences in biologic effectiveness of different radiations
 – Equivalent dose = the absorbed dose * radiation weighting factor
 – Gy : unit of absorbed dose
 – Sv (Sievert) : unit of equivalent dose
 – weighting factor : chosen by ICRP
         – 1 (low-LET; x-ray, γ-ray, electron) ∼ 20 (neutron, α-particle)