AbstractID: 6817 Title: Evaluation and development of film dosimetry accuracy in IMRT
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G X-ray film overresponds to low-energy photons in relative photon beam dosimetry. This overresponse
becomes a significant problem in clinical photon beam dosimetry especially in regions outside
penumbra. In intensity modulated radiation therapy (IMRT), the radiation field consists of multiple
small fields and their outside-penumbra regions. Thus, the film dosimetry for it involves the source of
overresponse in its radiation field. This study is aimed at verifying and possibly improving film
dosimetry for IMRT. Modulated beams were constructed by the combination of five different static
radiation field using 6 MeV X-rays. For verifying film dosimetry, we used x-ray film and ion chamber
to measure dose profiles at various depths in a phantom and percentage depth dose at central axis. In
addition, to reduce overresponse we placed 0.1 mm thick lead filters on both sides of the film. Measured
dose profiles showed the film overresponse at outside-penumbra and low-dose regions. The error
increased with depths and approached 10% as a maximum for the field size of 15 x 15 cm at 10 cm
depth. The use of filters reduced the error down to 3%, but caused underresponse of dose. We found
that the film overresponse in IMRT diminished in comparison with conventional radiation beams. It is
due to the combinatory effect of multiple radiation fields and their associated outside-penumbra regions,
that mixes out the predominant overresponse occurring outside penumbra. The use of filers can enhance
accuracy in film dosimetry for IMRT.
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