Dosimetric Characteristics of Clinical Photon Beams

Measurement of Attenuation
FFactors t for f Beam B Modifiers M difi
� The attenuation factor for a beam modifier is defined as
the ratio of the dose rate at the point of calculation for a
given field with and without the modifier in place.
� Attenuation factors for devices such as block trays, y , accessories
etc. are often assumed to be independent of field size, depth and
SSD.
� These factors should be measured at a depth well beyond the
maximum range of electron contamination
� The attenuation devices that are in contact with the
patient skin (immobilization apparatus, table top, etc.)
req require ire additional considerations
considerations.
� These devices not only attenuate the incident beam but they
introduce scatter radiation that increase the scatter to primary
ratio within the patient patient.
� It is best to include such attenuation devices as a part of the patient
in 3DRTPS