Rapid radiographic film calibration for IMRT verification using automated MLC fields

Abstract

A method for measuring a film sensitometric curve using a single sheet of film exposed with a two field step-and-shoot MLC treatment was developed and tested with Kodak XV2 and EDR2 films. With this technique a film sensitometric curve can be completed in only 10 minutes, making it practical to generate new film calibrations daily. This method is applicable to film calibrations for all purposes, but is particularly useful in IMRT treatment verification due to the method's use of small fields. This method agrees with the traditional large-field multifilm calibration within 0.5% and will produce sensitometric curves with errors less than 1% throughout the dose range, including uncertainties in dose delivery, film response, and optical density measurements. OD values for XV2 and EDR2 films were consistent in the middle of exposure areas at high depths, but the XV2 film penumbra regions showed large amounts of over-response as the calibration depth increased. If XV2 film is used for IMRT treatment verification, it is necessary to reduce the fluence of low energy photons in areas around the film by using thin lead shields. EDR2 film was shown to have minimal energy dependence, as it accurately represented penumbra areas and yielded identical sensitometric curves generated with 6 and 18 MV photons. However, its darker tint may make it more sensitive to scanning laser film digitizers' horizontal nonuniformities. This single film method proved to be superior to the traditional calibration method and allows fast daily calibrations of films for highly accurate IMRT delivery verifications.

Accuracy of rapid radiographic film calibration for intensity‐modulated radiation therapy verification

“…Digitizer error can be due to random readout error and systematic response error. The systematic error may be film‐dependent, (1) but can be controlled by periodic recalibration or correction factor measurement. Spectral matching error can be minimized by measurement matching or spectral filtering (4) …”

“…The right side was irradiated similarly, except the initial irradiation of all four squares received a greater dose. The irradiation sequence was very similar to that followed by Childress et al (1) except that here the jaws as well as MLC were used to block out the squares that had already received the planned dose. The four‐field, two‐ film exposures were performed similarly (Fig.…”

“…Here, we report on verification measurements performed for the implementation of a variant of the single‐film method suggested by Childress et al (1) and without the introduction of scatter filtering. We also investigate and discuss film response differences for the single‐film eight‐field method as compared to the standard film calibration method using $10\times 10{\text{cm}}^2$ fields on multiple films.…”

“…With the increased use of radiographic film dosimetry for intensity‐modulated radiation therapy (IMRT) treatment verification, ( 1 – 4 ) a quick and reliable method of generating film response calibration is desirable. Due to the various contributing errors in film dosimetry (e.g., film manufacture variability, processing conditions variability, densitometer readout errors, and film energy dependence), optimal accuracy of film response requires accompanying calibration films.…”

“…Recently, Childress et al (1) reported on a method of using a single calibration film for IMRT quality assurance (QA) measurements. This method of generating a calibration film saves time and radiographic resources.…”