The energy fluence of the photons fall off according to the Inverse square law of distance with the source or the target position as the origin.
The dose measured at the dmax point in a phantom by varying the SSD, roughly follows (to better than 2%) the inverse square law, for moderate changes in SSD (say within about 20 cm). This however must be verified for the treatment machine before putting it into clinical use.
SIDE NOTE:
Note that photons do not have a finite range in the patient.
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