Abstract
The objectives of this study are to identify and quantify factors that influence radiochromic film dose response and to determine whether such films are suitable for reference dosimetry. The influence of several parameters that may introduce systematic dose errors when performing reference dose measurements were investigated. The effect of the film storage temperature was determined by comparing the performance of three lots of GAFCHROMIC EBT2 films stored at either 4°C or room temperature. The effect of high (>80%) or low (<20%) relative humidity was also determined. Doses measured in optimal conditions with EBT and EBT2 films were then compared with an A12 ionization chamber measurement. Intensity‐modulated radiation therapy quality controls using EBT2 films were also performed in reference dose. The results obtained using reference dose measurements were compared with those obtained using relative dose measurements. Storing the film at 4°C improves the stability of the film over time, but does not eliminate the noncatalytic film development, seen as a rise in optical density over time in the absence of radiation. Relative humidity variations ranging from 80% to 20% have a strong impact on the optical density and could introduce dose errors of up to 15% if the humidity were not controlled during the film storage period. During the scanning procedure, the film temperature influences the optical density that is measured. When controlling for these three parameters, the dose differences between EBT or EBT2 and the A12 chamber are found to be within ±4% (2σ level) over a dose range of 20–350 cGy. Our results also demonstrate the limitation of the Anisotropic Analytical Algorithm for dose calculation of highly modulated treatment plans.PACS numbers: 87.55.Qr; 87.56.Fc
Highlights
Radiochromic films offer two-dimensional measurements, a high spatial resolution, quasi-energy independence, and water equivalence
These characteristics render them ideal for intensitymodulated radiation therapy (IMRT) quality control (QC), among other tasks. Their use is at present limited to relative dose measurements[1] using dose normalization in order to reduce dose errors introduced by the film themselves. This can potentially drown out systematic errors that are not associated to film dosimetry.[2]. QC tests would greatly benefit from reference dose measurements if the errors from the film can be adequately controlled
Recent work has demonstrated that under optimal conditions, absorbed dose can be determined with a film measurement uncertainty of less than 1% using EBT.[3]. EBT films are no longer commercially available, and no studies have been made to evaluate if EBT2 can be used for reference dosimetry
Summary
Radiochromic films offer two-dimensional measurements, a high spatial resolution, quasi-energy independence, and water equivalence These characteristics render them ideal for intensitymodulated radiation therapy (IMRT) quality control (QC), among other tasks. Their use is at present limited to relative dose measurements[1] using dose normalization in order to reduce dose errors introduced by the film themselves. It is very important to use a consistent orientation (i.e., portrait/landscape and face up/face down) when digitizing the film
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