Premium
Electrons as the cause of the observed d max shift with field size in high energy photon beams
Author(s) -
Biggs Peter J.,
Ling C. Clifton
Publication year - 1979
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.594580
Subject(s) - electron , photon , physics , beam (structure) , linear particle accelerator , atomic physics , field (mathematics) , field size , dosimetry , optics , materials science , nuclear physics , nuclear medicine , medicine , mathematics , pure mathematics
For megavoltage x‐ray beams, it is well known that the percent depth‐dose increases considerably with field size in the buildup region, resulting in a significant shift in the apparent position of maximum dose, d max . The nature of this increase has been investigated using a sweeping magnet placed just below the treatment head of a 25‐MV linac. Measurements show that for increasing magnetic fields the dose in the buildup region is continually reduced, until a point is reached beyond which no additional reduction is observed. Here the buildup curve is essentially field size independent. These results clearly show that electrons are the primary cause of dose increase with field size in the buildup region, in contrast to a recent publication claiming that scattered photons are the cause. Further measurements were made by blocking out the primary beam at the level of the jaws and measuring the depth dose of the scattered electrons originating from the jaws. The results show that a thickness of approximately 1 g cm −2 , of either polystyrene or lead, reduces the dose by a factor of two, providing further evidence that the scattered component of the beam consists of low energy electrons.