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Energy spectrum control for modulated proton beams
Author(s) -
Hsi Wen C.,
Moyers Michael F.,
Nichiporov Dmitri,
Anferov Vladimir,
Wolanski Mark,
Allgower Chris E.,
Farr Jonathan B.,
Mascia Anthony E.,
Schreuder Andries N.
Publication year - 2009
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 - 10.1118/1.3132422
Subject(s) - proton therapy , range (aeronautics) , percentage depth dose curve , optics , proton , beam (structure) , monte carlo method , ionization chamber , materials science , spectral line , energy (signal processing) , cyclotron , imaging phantom , detector , dose profile , computational physics , atomic physics , physics , ionization , nuclear physics , ion , electron , statistics , mathematics , quantum mechanics , astronomy , composite material
In proton therapy delivered with range modulated beams, the energy spectrum of protons entering the delivery nozzle can affect the dose uniformity within the target region and the dose gradient around its periphery. For a cyclotron with a fixed extraction energy, a rangeshifter is used to change the energy but this produces increasing energy spreads for decreasing energies. This study investigated the magnitude of the effects of different energy spreads on dose uniformity and distal edge dose gradient and determined the limits for controlling the incident spectrum. A multilayer Faraday cup (MLFC) was calibrated against depth dose curves measured in water for nonmodulated beams with various incident spectra. Depth dose curves were measured in a water phantom and in a multilayer ionization chamber detector for modulated beams using different incident energy spreads. Some nozzle entrance energy spectra can produce unacceptable dose nonuniformities of up to ± 21 % over the modulated region. For modulated beams and small beam ranges, the width of the distal penumbra can vary by a factor of 2.5. When the energy spread was controlled within the defined limits, the dose nonuniformity was less than ± 3 % . To facilitate understanding of the results, the data were compared to the measured and Monte Carlo calculated data from a variable extraction energy synchrotron which has a narrow spectrum for all energies. Dose uniformity is only maintained within prescription limits when the energy spread is controlled. At low energies, a large spread can be beneficial for extending the energy range at which a single range modulator device can be used. An MLFC can be used as part of a feedback to provide specified energy spreads for different energies.