Varian’s Portal Dosimetry serves as a method to QA IMRT/VMAT plans which have constant dose modulation. As opposed to using a phantom and ionization chamber for pre-treatment verification, a portal imager made of amorphous silicon is used to produce the portal dose image in calibration units (CU). CU is an absolute unit that is calibrated, though will vary between institutions, so that 1 CU equals approximately 1 Gy. The QA is performed by comparing the predicted dose of the plan obtained by the Portal Dosimetry Image Prediction (PDIP) algorithm versus the portal dose recorded by the EPID. For every reference point on the predicted dose, Portal Dosimetry will search the portal dose for a point matching the user set criteria:
- Dose Difference: the percent difference in dose allowed for an evaluated point to pass.
- Distance to Agreement (DTA): the radius around the reference point that an evaluated point can be searched for.
Upon evaluating both the dose difference and DTA, a gamma value will be assigned to the reference point. Gamma represents the shortest distance from the reference point to the evaluated point with a gamma < 1 will passing the test and a gamma > 1 failing. Gamma analysis refers to when this process is performed for all of the reference points. A user set threshold amount of points must pass for the gamma analysis to pass. The following equation is used to determine the gamma of a reference point:
Gamma_minimum = √ {[(distance between reference & evaluated points^2) / (DTA^2)] + [(dose difference between reference & evaluated points^2) / (dose difference^2)]}
To perform an IMRT QA using Portal Dosimetry with Mosaiq, the treatment plan must first be imported into Mosaiq. Head over to the treatment machine, and enter the room to shift the couch out (longitudinal) to prevent any collision. Use Mosaiq to open up the patient’s chart, and enter into QA mode in RO treat. Load the reference data from any field and send over the plan information over to the TrueBeam. On the TrueBeam console screen, complete the following steps:
- Add ‘imaging’ for every field.
- Check off ‘MV’, ‘daily’, and ‘dosimetry’, and ‘apply all’.
- Change imager/panel vertical to 0.0 for every field.
- Acquire couch positions once reference data finishes loading.
- Beam on, and portal dose will send automatically for import in Eclipse.
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