Premium
Dose–response and ghosting effects of an amorphous silicon electronic portal imaging device
Author(s) -
McDermott L. N.,
Louwe R. J. W.,
Sonke J.J.,
van Herk M. B.,
Mijnheer B. J.
Publication year - 2004
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 - 10.1118/1.1637969
Subject(s) - ghosting , ionization chamber , dosimetry , linear particle accelerator , image guided radiation therapy , nuclear medicine , pulse repetition frequency , imaging phantom , optics , materials science , detector , dose profile , beam (structure) , biomedical engineering , medicine , ionization , medical imaging , physics , radiology , computer science , ion , telecommunications , radar , quantum mechanics
The purpose of this study was to investigate the dose–response characteristics, including ghosting effects, of an amorphous silicon‐based electronic portal imaging device ( a ‐ Si EPID ) under clinical conditions. EPID measurements were performed using one prototype and two commercial a ‐ Si detectors on two linear accelerators: one with 4 and 6 MV and the other with 8 and 18 MV x‐ray beams. First, the EPID signal and ionization chamber measurements in a mini‐phantom were compared to determine the amount of buildup required for EPID dosimetry. Subsequently, EPID signal characteristics were studied as a function of dose per pulse, pulse repetition frequency (PRF) and total dose, as well as the effects of ghosting. There was an over‐response of the EPID signal compared to the ionization chamber of up to 18%, with no additional buildup layer over an air gap range of 10 to 60 cm. The addition of a 2.5 mm thick copper plate sufficiently reduced this over‐response to within 1% at clinically relevant patient–detector air gaps ( > 40 cm ) . The response of the EPIDs varied by up to 8% over a large range of dose per pulse values, PRF values and number of monitor units. The EPID response showed an under‐response at shorter beam times due to ghosting effects, which depended on the number of exposure frames for a fixed frame acquisition rate. With an appropriate build‐up layer and corrections for dose per pulse, PRF and ghosting, the variation in the a ‐ Si EPID response can be reduced to well within ± 1 % .