Introduction to Corrections-Based Calculations

This calculation method is powerful in that it provides a relatively simple yet accurate way to calculate the dose from a medical linear accelerator with only a small amount of measurements needed.

Corrections-based calculations start with a known absolute dose rate (cGy/MU) under a specific set of measurement conditions and then apply measured correction factors to result in the final dose or MU. Put in another way, the correction factors figuratively move the known calibration point until it is at the same conditions as the desired calculation point.

Before deriving the equations, let's first define a geometry to specify where the calculation point is in relation to the radiation source and patient (or water tank in this case).

Putting this all together, we get the following equation:

D = MU \times \dot{D}_0 \times S_c \times S_p \times DDR \times ISF \times OAR \times \prod_i BM_i

Alternatively, the equation can be inverted to calculate the required MU to deliver a given dose:

MU = \frac{D}{\dot{D}_0 \times S_c \times S_p \times DDR \times ISF \times OAR \times \prod_i BM_i}

In the next several tutorials, we will look at each term in these equations and discuss what measurements and calculations we need to obtain them.