 The three advantages of the EPID are: 1) the online convenience, 2) the data resolution (small pixels), and 3) the data density. However, MV EPIDs are not dosimeters; the interactions of photons in an EPID are notably different than interactions in water/tissue. Medical physics and QA relies on “apples to apples” analysis, therefore comparison of a measured image to a predicted image is an undesirable shift from comparing a measured dose plane to calculated dose plane. Additionally, percent difference, DTA, and gamma criteria are based on dose-in-tissue/water rationale.
EPIDose introduces an EPID-to-Dose conversion algorithm that allows your IMRT QA to remain dose-based. EPIDose features a Physics Modelling module that accounts for differences such as output factor variation and dose distribution kernels, between EPID response and tissue dose. EPIDose physics modelling incorporates baseline absolute dose measurements from the high resolution MapCHECK diode array to create a unique calibration to predict absolute dose. This EPIDose Physics Model and algorithm, when applied to a raw EPID image in the EPIDose software, allows the image to be converted to a dose plane within seconds.
Any QA solution that is built into the delivery system is a “self check” – which is a fundamental conflict of interest because it lacks 3rd party independence. Exposing potential errors via independent and rigorous QA is the single largest goal of medical physics. Relying completely on one system for planning, delivery, and QA reduces the likelihood of catching errors due to shared components, internal biases, and conflicting objectives. By opening EPIDose files in the MapCHECK software for analysis, complete autonomy from the delivery planning system is achieved and a complete suite of analysis options is available.
EPID image “QA” without EPIDose
- Units are not cGy; any comparison is quantitative but not dosimetric.
- Usage of standard IMRT QA analysis tools (DTA and gamma especially) is questionable unless acceptance criteria are re-calibrated to the non-dosimetric images.
- The EPID image is measured and predicted at the depth and SDD of the EPID, not at patient isocenter.
EPID dose QA with EPIDose
The EPIDose solution modifies any EPID image from any manufacturer to dose in three steps:
Using raw EPID input, the EPID is projected to the desired dose plane location and corrected for Output
1. Factor differences between EPID and dose. There is a correction for each and every MLC sub-field to correct for the “source distribution” of scattered photons.
2. The results of step 1 are convolved with a “Dose Redistribution Kernel” which converts the dose from EPID response (sharper than water) to a water equivalent scattering of dose.
3. Calibrate of the EPID occurs with the calibration data that was pre-stored in the model using MapCHECK.
Physics Modelling
EPIDose will require Physics Modelling to be done once per Linac/energy combination for a specific EPIDose setup. After EPIDose physics modelling is initially done, it is not required again unless the EPID drifts, is recalibrated or is repaired. All EPID measurements are to be done at the normal EPID SSD, typically 140 or 145 cm.
1. EPID does not need to move to 100 or 105cm SSD.
2. Build-up on top of the EPID is not required
3. Users do not need to account for inherent build-up or scattering of the EPID.
Posted on 28 Aug 2008
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