Dose Course 2018_Flipping book

Origin of peripheral dose contributions

Patient scatter

– Unavoidable – From AA : Decrease from target essentially as:

r

 − 

e

n =1 column shaped scatter source

n =2 “blob” shaped scatter source

n r

Head scatter −

Main source the flattening filter, but other parts of linac head too − Machine design dependent (FFF machines have less)

Leakage, including scatter leakage −

Depends on collimator and shield thicknesses and material (density) − Collimator design dependent − Treatment technique dependent

Neutrons −

Mainly a high Z phenomena at photon energies above photonuclear threshold values

− Not considered at all by TPS