How to determine the reference dose for MatriXX Output Calibration?
There usually are three options to determine the reference dose:
Use PDD. This is the most convenient one. There will be a little deviation because PDDs come from water measurement, not solid water, but usually acceptable.
Here we usually use SSD setup, because PDDs are acquired using SSD setup. Let's say we use SSD 100cm.
Then we need to determine where are MatriXX ion chambers.
We put some buildup materials on top of MatiXX, eg. solid phantom slabs.
The MatriXX ion chambers are about 3mm under MatriXX surface. When we put 4.7cm slabs on it, the ion chambers are at 5cm under 'water surface'.
Like above, SSD is 100cm, MatriXX ion chamber is 5cm under 'water'.
We lookup in the PDD table and find what is the dose percentage. We look for 6MV. 10x10 field size, 5cm underwater, the percentage, let's say the dose percentage is 68%
If we deliver 100MU with this setting, the dose MatriXX should receive is 100cGy*0.68=68cGy.
68cGy is our reference dose.
Calculate in TPS. This is accurate, but complicate compared to option 1
First please scan MatriXX together with the phantom in CT. Import CT slices into TPS, contour them, make necessary electron density override, and then create a square field plan.
In TPS, we can calculate the dose on MatrXX ion chamber center, and that is our reference dose.
Measure with a Farmer chamber in a solid water phantom setup.
First please setup Farmer chamber in a solid water phantom. It is very important to take note of the phantom setup here because when performing output calibration with MatriXX, the exact same setup should be applied. The center of MatriXX ion chambers should be at the same position as the Farmer chamber measurement point. Also, the effective measurement point of the chamber and MatriXX should be taken into account.
Then connect the Farmer chamber to an electrometer and get ready for measurement.
At last, we measure a certain beam, eg. 6MV, 100MU, 10x10 field size. The measured result is your reference dose.
No matter which option you prefer, please follow exactly the same setup during output factor measurement. Only in this way, the measured cnt is able to convert to Gy correctly.