Dosimetric verification of 6 and 18 MV intensity modulated photon beams using a dedicated fluoroscopic electronic portal imaging device (EPID).

BACKGROUND AND PURPOSE: 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. MATERIALS AND 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. RESULTS AND 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.

Visual verification of linac light and radiation fields coincidence.

X-ray and light field alignment evaluation is carried out during linac quality assurance programs. In this paper, we compare the size of the light field measured by a photodiode and by a more traditional visual observation with the size of the x-ray field. The comparison between actual light field size, measured with the photodiode, and light field size measured by human eye allow us to verify the reliability of human eye in the evaluation of this parameter. The visual field is always larger than real light field; however, it agrees better with the x-ray field. It matches the light field if we take into account the 25% (+/- 1%) of the decrement line of the maximum central lightening; however, this method simulates better the actual field employed in radiation treatments.