L 7: Measurement of absorbed dose

Question 1

Absorbed dose

Answer 1

Energy absorbed per unit mass
dE/dm

dE= mean energy, dm=material of the mass.
unit rads

Question 2

SI unit of Absorbed dose

Answer 2

Gray (old unit is rad)
* 1Rads = 1cGy
* 1Gy = 100cGy
* 1Gy = 100 rads
* 1 Gy = 1J/kg

Question 3

Kerma

Answer 3

• Kerma is the sum of kinetic energies of electrons and positrons released by photons in a medium per unit mass.
• Kinetic (k) energy(e) released(r) in the medium(m)
• SI Unit is Gray
• Unit is J/Kg
• Directly proportional to fluence.
• Implies only to photon beams

Question 4

Formula for Kerma

Answer 4

dE/dm

dm=uncharged

dE= Sum of initial kinetic energy of all charged particles

dm = uncharged particles in a material of mass

Question 5

Absorbed dose to air

Question 6

Roentengen to Rad conversion factor for air

Board question

Answer 6

• 0.876
• Roentgen-to-rad conversion factor for air (fair) = 0.876 rad/R for all x and g radiation.

Question 7

Absorbed dose to any medium

Answer 7

For any medium, the roentgen-to rad conversion factor, fmed = 0.876 × (ratio of average mass energy absorption coefficient for medium to that for air).

Question 8

Absorbed dose for low energy/non-MeV beams

Answer 8

• For low-energy x-ray beams such as orthovoltage and superficial, the absorbed dose in bone is two to four times the absorbed dose in soft tissue for the same exposure.
• This is because of photoelectric effect, which is dependent
• on Z3 (Zeff for bone is 12.3 and for soft tissue it is 7.6).
• That is why is diagnostic scans bones are brighter

Question 9

Absorbed dose for Megavoltage MeV beams >3MeV

Answer 9

• For megavoltage photon beams, the absorbed dose in bone is slightly less than that in soft tissue because of the predominance of the Compton effect.
• Compton effect is independent of Z, is dependent on the number of electrons per gram (bone has slightly fewer numbers of electrons per gram than soft tissue).

Question 10

Bragg gray cavity theory

Only name is imp no other questions form this topic

Answer 10

the ionization charge produced per unit mass of cavity gas
and the ratio of average mass stopping power of electrons traversing the medium to that of electrons crossing the cavity gas.

Question 11

B-G cavity theory

Answer 11

• Roentgens cannot be measured accurately for photon beams of
  energy above 3 MeV.
• The B-G cavity theory has no limitation of energy or the type of ionizing radiation in the measurement of absorbed dose.
• All recent calibration protocols (TG-21, TG-51, and IAEA TRS-398) use B-G cavity theory.

Question 12

Other methods of measuring absorbed dose

Answer 12

1. Ionization chamber
2. Calerimetry
3. Clinical Dosimetry
4. TLDs/OSLs
5. Silicon diodes

Question 13

X-ray film OD formula

Answer 13

OD=log Io/It

Io = Initial, OD=optical density

It= transmission