# TDF Plan: Comparing Regimens

One way to compare regimens is to directly manipulate the parameters in the IMR calculator. You can work with any two of the three inter-related parameters: total dose, # fractions, and Rx dose/fraction by locking the parameter you wish to hold constant. Unlocked parameters have enabled editable text fields and "up/down" arrow bumpers. You can modify the parameters in predefined increments using the arrow bumpers, or enter exact values from the keyboard. The BED (E/α) is automatically updated when any of the parameters is changed.

For example, lets begin with a course of 30 fractions of 200 cGy. The total dose is 6000 cGy and the biologically effective dose is 72.00 Gy(10.0).

Now, lock the total dose field and change the new number of fractions to 25. The dose/fraction will change to 240 in order to maintain the locked total dose of 6000 cGy and the biologically effective dose readout will calculate 74.4 Gy(10.0).

Now, lock the number of fractions at 25 and reduce the dose/fraction field (to about 234 cGy) until the biologically effective dose returns to approximately 72 Gy(10.0). If the assumption of an α/β ratio ≈ 10.0 is valid, the regimens 60 Gy delivered in 30 fractions and 58.5 Gy delivered in 25 fractions should result in equivalent acute biological effects.

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