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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
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
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
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