Energy fluence of an accelerator electron beam and dose fall off.
IT DOSE NOT FALL OFF ACCORDING THE ISL. The dose measured at dmax point in a phantom, by varying the SSD, roughly follows (to better than 2%) the ISL for small changes in distance (say within about 5 cm), but not with respect ot the target position, but with respect to a point much closer than the target. This apparent distance, which is also a function of field size, must be experimentally determined for every electron beam quality of the machine (See Khan 2003 for more details)
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