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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