A Quality Assurance (QA) procedure has been implemented to help assure that the software is operating correctly. You should run this QA test any time a dose calculation table and/or constant is modified or you receive a software upgrade. Copies of the QA file are located at Macintosh HD:Documents:Plaque Simulator Patients:QA:Quality Assurance 2009.iplan and/or Macintosh HD:Applications:Plaque Simulator Folder:Plaque Simulator Data:(Quality Assurance):QA:Quality Assurance 2009.iplan.
Open the Quality Assurance 2009.iplan file by selecting Open from the File menu and use the MacOS navigation services to find and open the file.
The QA procedure involves creating a plaque with a single I-125 source on its Central AXis at 0.0 mm offset. When this plaque is placed on the eye, the center of this centered source will be at the external surface of the sclera.
In the file Quality Assurance 2009.iplan a single 1 mCi, model 6711 (TG43U1), I-125 seed, calibrated to the time of implant, has been locked into the central slot of a COMS 12 mm plaque whose carrier has been slightly modified to place the seed center at the plaque surface. The tumor apex has been set to 9.0 mm and the prescription (Rx) point set to the tumor apex, 10.0 mm from the external surface of the sclera.
In the Prescription window, the dose calculation algorithm buttons should be set to USC, point source, anisotropy disabled, carrier, gold, air, shell and slots (ie 'normal' collimation) all disabled. The dose integration (T) time should be one hour. This simple dose calculation mode is easily checked against hand calculations.
A dose rate of 1.225 cGy/h should be displayed for both the Rx point and at 10.0 mm in the Central AXis table. This value represents the dose rate constant for TG43U1 model 6711 seeds (0.965) multiplied by the apparent activity to air kerma strength conversion factor (1.27) and the radial dose function g(r) value at 10 mm (1.0). The dose rate constants and radial dose function for each isotope are loaded from the physics files at launch time. These values can be edited in the Constants and Radial Dose panes.
For a more comprehensive QA check, enable anisotropy correction by clicking the Isotropy button in the Prescription window. The dose rate at 10.0 mm should not change since F[φ] should equal 1.0 at 0 degrees. The dose rate at 0.00 mm may change. Plaque Simulator checks for division by zero and substitutes a value of 0.0001 for zero distance in divisions.
Next, click the Point button in the Prescription window. This enables linear sources and the button changes to read "Line". Treating the source as linear will reduce the dose rate by approximately 1% at 10 mm and 35% at 1 mm on the central axis compared to the point geometry calculation. Click the Line button again to return to Point geometry.
Activating shell collimation should have no effect since central axis points are never obscurred by the shell of the plaque. Activating slots collimation should have no effect since a COMS plaque does not have collimating slots. For off axis points, hardcopy of the QA test is available by printing a dose table using the Dose tables document. These data can be compared to hand calculations.
This is what you should get with these dosimetric factors enabled.
In the Isodose window normalize the isodose plot to the prescription (rx) point (ie 10mm on the plaque axis, also the tumor apex in this case) by checking the Normalize at control and load the normalized isodose values by control-clicking on the Isodose button. The window will look like this. The QA file should do this for you automatically. Calculate the 2D isodose matrices for the standard surfaces from the Dosimetry menu.
Below are isodose plots normalized at 10mm on the plaque axis (also the tumor apex in this case) comparing the simple uncollimated isotropic point source (left) calculation with the gold shell collimated anisotropic linear source (right) calculation for the QA file. (Note that the I-125 seed center in an actual COMS plaque would be offset 1.4 mm from the surface so these isodose plots do not represent a single seed as used in a real COMS plaque.)
This completes the QA test procedure. If at any time the calculated dose rate is not within 1% of a hand calculation, contact the distributor and the author for assistance and do not use this program for clinical calculations until the problem is resolved.