Physics Athens 2016

Depth Dose Profiles - Photons

Build up

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 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

MV Xrays

in the patient

Radiation

D s

Z max

(cm)

E> → < t  > s C • less attenuation • higher penetrating power • more secondary e - • higher electron energy

D max

-> z=z max

≈ range of

100 kVp Xrays 100% 0 Co-60 30% 0.5 6 MV 15% 1.5 18 MV 10% 3.5 secondary particles z > z max Dose decrease for photons attenuation in the patient