Electromagnetic Radiation

Ioniz i ng radiation : shown here for photon energies g r eater than about 1 keV.

Different types of ionizing radiation:

 Photons

 neutrons

 charged particles

 visible spectrum

[Image removed due to copyright concerns]

[Alpen, 1998]

Absorbed Dose

 Dose is a measure of the am ount of energy from an ioniz i ng radiation

deposited i n a mass of some material.

Form ally, absorbed dose at a point is defi ned by the ICRU as

D   

 m

where  

is the mean energy transfe rred by the radiation to a m a ss

 m .

 The biological effect is relate d to the dose and depends on the nature of the radiation.

Units

RAD (Ra d iation Absorbed Dose)

 Old, but still co mm o n ly used.

 1 rad = 100 ergs/g

Gray (Gy)

 SI unit used to measu r e absorbed dose is the gray (Gy).

 1 Gy =

1 J  10 7 erg 

4 erg 

kg 10 3 g

10 100 rad g

 Gy can be used for a ny type of radiation.

 Gy does not describe the biological effects of the different radiations.

Dose Cal c ulati o ns: Examp l e

Alpha and Low energy Beta em itters distributed in tissue.

A radionuclide, ingested or in haled, and distributed in various parts of the body is called an internal emitter .

Many radionuclides follow specific metabo lic pathways, acting as a chem ical element, and localize in specific tissues.

E.g. , iodine concentrates i n the thyroid

radium and strontium are bone seekers

tritium will distribute throughout the whole body in body water

cesium tends to dist ribute throughout the whole body.

If an internally deposited ra dionuclide em its particles that have a short range, then their energies will be absorbed in the tissue that contains them .

Let:

A = the activity concentration in Bq g -1 , of the radionuclide in the tissue

E = the average alpha or beta par tic le energy, in MeV per disinte g ration The rate of energy absorpt i on per gram tissue is A E (MeV g -1 s -1 ).

The absorbed dose rate is:

D ˙  A E MeV

g s

x 1 . 60 x 10  13

J

MeV

x 10 3 g

kg

= 1.60 x 10 -10 A E Gy s -1

Health effects depend on how radiation interacts w i th bio l ogical material at the microscopic level.

Radiation delivers energy in “small pa ckets”, not uniformly distributed throughout the entire mass.

[Image removed due to copyright concerns]

[Hall, 2000]

Energy Deposition by Ioniz ing Radiation

 The biological effects of radiation are the end product of this long series of phenom ena, which are set in motion by the passage of radi ation through the mediu m .

 The interaction of radiation with ma tter produces excited and ionized atom s and m o lecules as well as large num bers of secondary electrons.

 The secondary electrons produce additio nal ionizations and excitations until the energies of all el ectrons fall belo w the threshold necessary for further interactions.

Dose must be applied to different endpoints… at vastly different scales.

organism tissue cell DNA

(atom?)

Deficiency of the dose conc ept at the microscopic level

As early as the 1920s, experiments with i rradiation of virus pa rticles with x-rays illustrated the paradox that continues today.

 The viruses appeared to be both sensitive and highly resistant at the same time.

 Even at lo w doses some were k illed.

 On aver age they tolera ted very large doses.

 A paradox? Responding di fferently?

 No, actual ly receiving different doses.

 The radiation is actually deliv ered in s m al l packets.

 Dose was seen to be a statistic al expectation of being hit.

 The virus particles were demonstrating thi s random statistical fluctuation.

 “Protection by virt ue of being small.”

[Image removed due to copyright concerns]

Distribution of absorbed energy and ioniz a tion on a microscopic scale.

[Tubiana, 1990]

Given: 1 Gy = 1 J/kg; 1 eV = 1.6 x 10 -19 J

Assume: 1 ionization = 33 eV; 1 nucleus = 10 -10 gram or about 5 µm 3 Therefore:

1 Gy ≈ 2 x 10 17 ionizations/kg ≈ 2x 10 14 ionizations/g ≈ 20,000 ionizations /

10 -10 g

In crossing the 5 µm nucleus:

1 MeV electrons lose 200 eV in 6 ionizations/µm ,

700 tracks ≈ 20,000 ionizations ≈ 1 Gy

30 keV electrons lose 1 keV in 30 ionizati ons/µm ,

140 tracks ≈ 20,000 ionizations ≈ 1 Gy

4 MeV protons l o se 10keV in 300 ionizations/µm 14 tracks ≈ 20,000 i onizations ≈ 1 Gy

The dose to the nucleus is the same. Th e biological effect is ver y different .

 In reality,  

depends on the number of particles traversing t h e mass  m

and the en ergy that these particles deposit.

The Probl e m:

 Dose represents the density of energy absorbed per unit mass at a point in the m e diu m .

 On a very small scale (on the order of µm) the energy distri buti on in not uniform .

 Energy is localized near the tracks of particles.

 Energy transfer takes place in a discont i n uous manner, in discrete am ounts.

 Dose is a statistical quantity and doe s not express these characteristics.

 Dose gives a global value determ ined in a mass su fficiently large that the statistical fluctuation in not significant.

However , the biological effect i s related to the dose depositi on at the microscopic level.

At the lev e l of the cell or the DNA, where the size of th e “target” is measured in µm or nm , the non-uniform and discontinuou s nature of the energy deposition can be very important.

At the m i croscopic level, the biological effect depends on the nature and the energy of the ionizing particles.