Electron Blocks

WMK:

We read an electron block if the size of the opening in centimeters is less than half the energy in MeV. For example, for a 9 MeV block, if the opening is less than 4.5 cm. near the center of the field we must make a physical measurement. If the opening is greater than 4.5 cm. we can make a calculation as if the block was open.

When measuring a block, we make two readings. One reading is where we know the output at and the other is at the treatment distance. For example, we can take an open field measurement at 100 cm. For a 10 x 10 field at 100 cm. the output is 1 cGy/MU at dmax. We record the electrometer reading. This gives us the reading to deliver 1 cGy/MU. Next we take a reading for the blocked field at the treatment distance. This reading divided by the open field known reading will give us the cGy/MU delivered under the treatment conditions.

We use this information to calculate the monitor units necessary to deliver the prescribed dose.

HRE adds these comments:

We're making a few assumptions here. One is that fractional depth dose doesn't change between your open measurement and your treatment set up measurement. That is, the distance of dmax remains the same for the two measurements. Another assumption is that the electrometer reading is linearly proportional to dose.

DRR adds this:
Note that for purposes of the calculation, we use the electron cone factor from the table at 100 SSD if we measured the open field at this distance. A common mistake when a treatment is at 110 SSD and the block is read would be to use the electron cone factor at 110 SSD. This would be wrong.