Dear Dr Miller or any other physics enthusiasts out there,

May I trouble you again please as I have become stuck.

I am getting confused as to when the f factor is used and when stopping power ratios are used.

My understanding is that the F factor is used to convert from Roentgen (exposure) to Rad (Gy, dose) from air to any medium, by using some factors to convert units from charge per unit mass, to Joules per unit mass; and correcting for the differences in attenuation of the beam between the two mediums using mass attenuation coefficient ratios. This works out to be 0.876 times the MAC ratios.

Sounds fine to me so far, but when calibrating linacs, we use stopping power ratiosâ€¦

Dose = meter reading x cobalt calibration factor x stopping power ratio of water to air x perturbation factor

The ionization chamber is still collecting ions in a known mass of air, so we are still getting exposure and need to convert to dose. So why is the f factor not used, and stopping power ratios used here?

Any help would be much appreciated.

Parev.