EPIDose™ for Mapcheck™

©JRT Associates 2011

EPIDose is for any EPID user wishing to use their EPID for IMRT QA. EPIDose is necessary because it directly resolves two significant problems with EPID based IMRT QA.

EPIDose™ is a completely independent method of converting MV EPID images to dose for analysis in the MapCHECK software. EPIDose compares your EPID output to planned dose, not predicted images as other systems do, and therefore EPIDose performs QA of the TPS system as well as the delivery. Keep your IMRT QA independent and absolute dose based with EPIDose, the missing link for EPID based IMRT QA.

How EPIDose Works
EPIDose introduces an EPID-to-Dose conversion that allows your IMRT QA to remain dose-based, resulting in accurate conversions of EPID images to "Dose Planes". EPIDose operating in the MapCHECK analysis environment, offers complete independence from the delivery and planning systems along with a suite of familiar and proven analysis options.

EPID Based QA with EPIDose

• Using a raw EPID file, the EPID image is projected to the desired dose plane location. Corrections for output factors and the source distribution of scattered photons are applied to each MLC sub-field.
• The results are convolved with a dose redistribution kernel which converts the dose from EPID response (sharper than water) to a water equivalent scattering of dose.
• The results are then automatically calibrated to dose using data in the EPIDose model (based on MapCHECK absolute dose planes).

Conversion of EPID Images to Dose
Comparisons of images to predicted images do not audit the TPS dose calculation algorithm. TPS dose modeling is the most common source of error in IMRT dose QA. EPIDose incorporates output factor variation (per IMRT segment), dose distribution kernels (phantom scatter) and baseline absolute dose measurements from the MapCHECK to compute absolute dose.

Physics Modeling
Physics Modeling is quickly performed for each linac and energy combination.

After EPIDose physics modeling is initially done, it is not required again unless there is a change in the EPID.

Independent Analysis
Exposing errors through independent and rigorous QA is the most important goal of medical physics. Relying on one system for planning, delivery, and QA reduces the likelihood of error discovery due to shared components and conflict of interest. EPIDose operates in the MapCHECK software, achieving complete independence from planning and delivery.

1214000-0 EpiDOSE (to be used with MapCHECK)
1214000-1 EPIDose: Additional License (One EPIDose license required per EPID)

Click on images to enlarge

EPID image vs. TPS image

EPID image (dotted) agrees with TPS predicted image (line). The TPS dose algorithm is not audited.

EPIDose vs. TPS dose

The same field with EPIDose algorithm and the TPS algorithm applied, a difference is clearly evident.

Click on images to enlarge

EPID image vs. TPS image

EPIDose (upper left) vs. TPS (upper right). The TPS penumbra is very smooth - caused by using volume-averaged ion chamber profiles for beam model commissioning. The TPS is underestimating the profile gradients.

EPIDose vs. TPS dose

The same field, after an improved beam model was made in the TPS using a smaller detector. EPIDose revealed the volume averaging error in the commissioning data and confirmed the correction.