Energy Transfer and Relative Biological EffectivenessYvonne Pham and Chirag Shah

LINEAR ENERGY TRANSFER AND RELATIVE BIOLOGICAL EFFECTIVENESS

YVONNE PHAM AND CHIRAG SHAH

Question 1

What is the definition of linear energy transfer (LET)?

Question 2

What are two different methods to calculate linear energy transfer (LET)?

Question 3

What is considered high (densely ionizing) linear energy transfer (LET) radiation versus low (sparsely ionizing) LET radiation?

Question 4

What happens to the density of ionization and linear energy transfer (LET) as the energy increases within a certain particle type?

Question 1 What is the definition of linear energy transfer (LET)?

Answer 1

LET is the average energy transferred per unit path length of travel within the local medium, expressed in kiloelectron volt (keV) per micrometer (μm). Energy deposition per path length varies widely due to the primary radiation and electrons set in motion; thus, LET is an average quantity.

Hall EJ, Giaccia AJ. Linear energy transfer and relative biologic effectiveness. In: Hall EJ, Giaccia AJ, eds. Radiobiology for the Radiologist. 7th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2012:104–113.

Question 2 What are two different methods to calculate linear energy transfer (LET)?

Answer 2

a. Track average: Obtained by dividing the track into equal lengths, calculating the energy transfer in each length, and deriving the average energy deposition.

Example: /x x xx /xx /xx x x / xx x /x /xxxxx/

b. Energy average: Obtained by dividing the track into equal energy intervals, calculating the path length for each energy increment, and deriving the average of track lengths.

Example: /xxxxx/xx x x x/xx xx x/x x x x x /x x x xx/ xx xxx /

Question 3 What is considered high (densely ionizing) linear energy transfer (LET) radiation versus low (sparsely ionizing) LET radiation?

Answer 3

Radiation with LET values less than 10 keV/μm are considered low LET while radiation with LET values greater than 10 keV/μm are considered high LET.

Podgorsak, EB. Introduction to modern physics. In: Greenbaum E, ed. Radiation Physics for Medical Physicists. 2nd ed. New York, NY: Springer Science & Business Media; 2010:1–76.

Question 4 What happens to the density of ionization and linear energy transfer (LET) as the energy increases within a certain particle type?

Answer 4

The density of ionization and LET decreases as the energy increases for a given particle type. For example, 10 MeV protons have an LET of 4.7 keV/μm, whereas 150 MeV protons have an LET of 0.5 keV/μm.

Question 5

What are some linear energy transfer (LET) values for different radiation types?

Question 6

What does a unit of gray (Gy) represent?

Question 5 What are some linear energy transfer (LET) values for different radiation types?

Answer 5

LET Values
Radiation Type	LET (keV/μm)
Cobalt-60 γ-rays	0.2
250-kV x-rays	2.0
1 MeV electrons	0.25
10 keV electrons	2.3
150 MeV proton	0.5
2 MeV proton	17
14 MeV neutrons	12
100 MeV carbon ion	160
2.5 MeV α-particle	166
75 MeV argon ion	250
2 GeV Fe ions	1,000

Podgorsak EB. Introduction to modern physics. In: Greenbaum E, ed. Radiation Physics for Medical Physicists. 2nd ed. New York, NY: Springer Science & Business Media; 2010:1–76.

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