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Depth Dose Profiles - Photons

Surface dose D

s

(

skin sparing effect)

photons scattered from collimators,

flattening filters, air

backscatter from patient

High E e

-

produced in air and in any

shielding structure close to the pt

MV Xrays

Radiation

D

s

Z

max

(cm)

100 kVp Xrays 100% 0

Co-60

30% 0.5

6 MV

15% 1.5

18 MV

10% 3.5

Build up

z < z

max

Absorbed dose is much smaller than

the transferred energy (KERMA)

Dose buildup results from the

relatively

long range

of secondary

charged particles that first are released

in the patient by photon interactions

and then deposit their kinetic energy

in the patient

D

max

-> z=z

max

range of

secondary particles

z > z

max

Dose decrease for photons attenuation

in the patient

E>

<

t 

>

s

C

less attenuation

higher penetrating power

more secondary e

-

higher electron energy