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SU‐E‐T‐343: Monte Carlo Study on a Novel Direction‐Selective Flattening Filter System
Author(s) -
Chofor N,
Harder D,
Willborn K,
Poppe B
Publication year - 2011
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 - 10.1118/1.3612297
Subject(s) - flatness (cosmology) , monte carlo method , physics , optics , imaging phantom , photon , flattening , attenuation , linear particle accelerator , dosimetry , materials science , nuclear medicine , beam (structure) , mathematics , medicine , statistics , cosmology , quantum mechanics , astronomy
Purpose: Monte Carlo evaluation of a newly‐designed narrow steel cone flattening filter, which functions via the direction‐selective attenuation of the photons produced in the target. Methods: Using BEAMnrc and DOSXYZnrc (EGSnrc), novel 6 and 15 MV bremsstrahlung beams were modeled for a Siemens Primus linac. Under flattening filter free (FFF) conditions, the classical flattening filter was omitted. For the direction‐selective filter (DSF) configuration, an axis‐centered bullet‐shaped narrow steel cone filter is now mounted directly below the graphite electron absorber of the tungsten target. The neutron source strength was modeled by assessing photoneutron production at all tungsten surfaces hit by photons, assuming total photon absorption and using the macroscopic photoneutron cross section of tungsten. Results: Photon spectra for FFF and DSF conditions are less filtered compared to classical flattening filter conditions, while depth dose curves are very similar. Under DSF conditions, field flatness has been achieved for a circle of diameter 15 cm at 100 cm SSD, and the normalized peripheral dose was reduced by factor 1.5. For FFF and DSF, the dose rate at 10 cm water phantom depth was enhanced by factors 2 and 1.7 (6 MV) and 4 and 2.5 (15 MV) respectively, while the corresponding beam head photon leakage dose remained unchanged. The photoneutron source strength produced/Gy was reduced by factors of 3.5 (FFF) and 2.8 (DSF). Conclusions: With the DSF system, the simplicity of treatment techniques and workflow is maintained, because field flatness up to 15 cm field diameter is achieved at all depths, while the advantages characteristic for FFF conditions, the dose‐rate enhancement, the reduction of photon and neutron leakage from the beam head/Gy in the treatment volume, and the reduction of the normalized dose levels in the far periphery are maintained.