Dosimetric verification of 6 and 18 MV intensity modulated photon beams using a dedicated fluoroscopic electronic portal imaging device (EPID).
OBJECTIVE
Intensity modulated radiation therapy offers a dose distribution improvement by modulating the two-dimensional X-ray fluence. This increases the treatment complexity from the planning phase to the verification one. Pre-treatment dosimetric verifications of each treatment field are strictly necessary, films and EPIDs are generally used for this purpose. In this paper we investigated the dosimetric characteristics of a commercially available fluoroscopic EPID "I'mRT QA" (Scanditronix-Wellhöfer) (I-QA) based on charge-coupled device camera (CCD) and we present a new method for optimising the EPID's response.


METHODS
I-QA is an optically sealed EPID and it is designed for on-line two-dimensional measurements of relative dose distribution. Dose profiles measured in water with diodes and dose distributions measured in water-equivalent phantoms with films were compared with those obtained with the I-QA for homogeneous and intensity modulated 6 and 18 MV photon beams.


CONCLUSIONS
I-QA measurements depend on the field size and on the two-dimensional energy spectrum of the beam. The incoming beam was modified positioning a series of lead and plastic (RW3) slabs above the fluorescent screen to obtain a homogeneous response of the I-QA over the whole sensitive area. The thickness of lead and RW3 sheets was optimised to get the best matching between diodes, films and the I-QA measurements for each energy. Gamma index evaluations showed a correspondence between I-QA measurements and diode ones within 3% or 2 mm for homogeneous and simple modulated fields, and within 5% or 3mm for complex modulated fields.
Monti
Angelo F
AF
Department of Medical Physics, Ospedale S. Anna, Como, Italy. amonf@iol.it
Frigerio
Giovanna
G
D005453:Fluorescence; D010315:Particle Accelerators; D017785:Photons; D011834:Radiation Monitoring / Q000295:instrumentation; D011874:Radiometry / Q000295:instrumentation*; D050397:Radiotherapy, Intensity-Modulated; D012542:Scattering, Radiation