Absolute dose verification in intensity modulated radiation fields.

摘要

This work is to device a method for measuring the absolute dose for Intensity modulated radiation fields. An accurate verification and physical measurement of the computed absolute dose in Intensity modulated radiation therapy (IMRT) is a challenging task. There are many sources of errors associated with IMRT measurements using ionization chambers in steep dose regions. Some of these errors are due to the lack of electronic lateral equilibrium and detector position-orientation effect. These two limitations were experimentally verified and critiqued in the light of other findings by different studies. On the other hand, there has been a growing and renewed interest in using radiographic films as a potential dosimeter. This is due to their high spatial resolution and relatively short measurement time. Film dosimetry is suited to deal with the low dose rate problem because it integrates the dose and acquires an entire plane of data in a single exposure. However, radiographic films have their own limitations. Most film types become increasingly sensitive and tend to over respond at low photon energies. This limitation is even more problematic in IMRT where, the spectrum varies significantly within the field, producing considerable amounts of low photon energies at the point of measurements. All measurements were obtained using a Varian Clinac 21000 Varian Medical Systems Inc., Palo Alto, CA. The dosimetric property of Kodak XV film was verified for 6 and 18 MV photon beams. A technique to quantify film over response to low photon energy was presented. Monte Carlo calculations were used to validate measurements obtained by this technique and thereafter, it was applied to several clinical IMRT fields. Results came within 3% of the absolute date computed by the treatment planning system.