Analysis of the sources of uncertainty for EDR2 film-based IMRT quality assurance.

In our institution, patient specific quality assurance (QA) for intensity modulated radiation therapy (IMRT) is usually performed by measuring the dose to a point using an ion chamber and by measuring the dose to a plane using film. In order to perform absolute dose comparison measurements using film, an accurate calibration curve should be used. In this paper, we investigated the film response curve uncertainty factors including film batch differences, film processor temperature effect, film digitization, and treatment unit. In addition, we reviewed fifty patient-specific IMRT QA procedures performed in our institution in order to quantify the sources of error in film based dosimetry. Our study showed that the EDR2 film dosimetry can be done with less than 3% uncertainty. The EDR2 film response was not affected by the choice of treatment unit provided the nominal energy was the same. This investigation of the different sources of uncertainties in the film calibration procedure can provide a better understanding of the film based dosimetry and improve quality control for IMRT QA.