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Soft tissue – lung interface
→
Less attenuation
→
Increase in primary beam intensity
→
Reduction of scattering material
→
Different electron transport and longer range
→
lateral loss of e
-
equilibrium
e
-
disequilibrium at tissue interfaces
NB: Higher
E
→
higher e
-
range
→
penumbra broadening
→
wider margins required to compensate for e
-
disequilibrium
(increased dose calculation uncertainty – worst for smaller target
volumes)