If dmax does not equal d0 there must be a correction.

Standard SSD is 100cm right and for the ion chamber measurement, depth at dmax needs no correction.

However, for in-vivo dosimetry calibration, the ISF accounts for any changes produced as a result of the differences between the point source and the surface of calibration (point/depth of calibration).

Hence the logic is to ensure that machine output gives 1cGY/MU irrespective of in-vivo dosimetry, LINAC calibration, and patient treatment.

I hope this helps!

Abayomi Opadele Thank you so much for your comment.

As in the dose calculation formula, the same ISF is multiplied that was employed in the calibration coefficient in the denominator. Apparently these two cancel out, and it seems no correction is needed as in both cases, calibration of in vivo dosimeter and dose measurement for patients, the setup is same except the change of phantom and patient. So the logic that to ensure the machine output gives 1cGy/MU is obvious but insufficient to justify the presence of this factor in both formula. Looking forward to your valuable comment!

When doing diode calibration, I use certain "source to diode distance" 100cm, to get "1 reading = 1 cGy in dmax", and for SSD photon dose calibration and10x10 field size, that is "1 reading = 1 MU".

For patient, same 10x10 field, the predicted diode reading is MU*ISF, there is no cancel out.

Nazia Toor I do understand your point now, thanks for elucidating your thoughts.

While the addition of ISF in the dose calibration formulae may lack significance due to the cancellation during the calculation. However, I think the ISF further acts as a factor to correct for any (if at all) change during setup.

Also, I tried to find relevant articles that have evaluated the ISF effect in in-vivo dosimetry calibrations but to no avail. This may suggest again that both accuracy and precision are needed during calibration procedures.

I hope this helps!