A patient is to be treated with a 6MV photon beam for a field size of 12cmx12cm on the patient surface, at an extended SSD of 125 cm. (PLEASE SKETCH DIAGRAM). The depth of target center is 9 cm. In this case (compared to a standard SSD treatment):
1) The reference dose rate (or dose/MU) at the input port (dmax position) decreases relative to a shorter SSD
2) the PDD (9, 12x12, 125) increases due to the increased SSD
3) the PDD correction is approximately given by Mayneords F Factor
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