With external pituitary irradiation using a linear accelerator, a mean fall in GH levels of seventy-seven per cent after five years has been obtained (Fig 3.22), and, at the end of ten years, eighty-one per cent of patients have serum GH concentrations of less than 20mu/l (l0ng/ml). The effect is more rapid if the initiating serum GH level prior to radiotherapy is lower. Thus, if GH levels are greater than 50mu/1 (25ng/ml) the time taken for there to be a fall of the average serum GH to less than 5mu/l (2.5ng/ml) is longer: six years versus four years if pretreatment serum GH is below 50mu/l.
