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Buildup region and depth of dose maximum of megavoltage x‐ray beams
Author(s) -
Sixel Katharina E.,
Podgorsak Ervin B.
Publication year - 1994
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 - 10.1118/1.597305
Subject(s) - imaging phantom , collimator , monte carlo method , linear particle accelerator , beam (structure) , optics , field size , physics , dosimetry , flattening , percentage depth dose curve , field (mathematics) , electron , materials science , ionization chamber , nuclear medicine , nuclear physics , ionization , mathematics , medicine , ion , statistics , quantum mechanics , astronomy , pure mathematics
The depth of dose maximum, d max , of megavoltage x‐ray beams was studied as a function of beam energy and field size for 6‐, 10‐, and 18‐MV x‐ray beams and field sizes ranging from 1×1 to 30×30 cm 2 . For a given beam energy, d max increases rapidly with increasing field size at small fields, reaches a maximum around 5×5 cm 2 and then gradually decreases with increasing field size for large fields. Monte Carlo simulations combined with measurements verified that the effect observed at small field sizes is caused by in‐phantom scatter, while at large fields the effect is due to scatter contamination of the primary beam from the linac head. A comparison between the d max behavior of flattened beams to that of unflattened beams indicates that the d max decrease at large fields for flattened beams is caused mainly by contamination electrons which are produced in the flattening filter and further scattered by collimator jaws and air.