Several studies have established that in treated acromegaly mortality is only normalized in patients achieving a serum GH concentration below 5 mU/l. Few studies however, have addressed results of radiotherapy using this strict criterion. The aim of our study was to assess the efficacy of postoperative radiotherapy in reducing serum GH concentration below 5 mU/l.

Forty acromegalic patients who underwent radiotherapy for postoperative persistent disease after transsphenoidal surgery were studied. Mean time of follow‐up after radiotherapy was 123 ± 11.1 months. A serum GH concentration of <5 mU/l was used to define remission.

Following surgery mean GH concentration decreased from 120.8 ± 21 mU/l to 24.4 ± 5.2 mU/l and mean GH concentration prior to radiotherapy, administered after a mean of 8.2 ± 2.7 (0.5–96) months postoperatively was 26.0 ± 5.1 mU/l.

Using individual regression plots, GH was reduced to 50% by radiotherapy after a mean of 27 ± 5 months (range 3–105 months). The observed mean GH concentration as measured at follow‐up visits was reduced by 55% (−36–95%) after the first year, by 65% (−29–97.3%) after 2 years and by 78% (−29–99%) after 5 years. We could not demonstrate a significant correlation between a normalized GH concentration on the one hand and tumour size, preradiotherapy GH concentration or duration of follow‐up on the other hand. During follow‐up, GH‐suppressive medical therapy was used in 11 patients, five of whom were still using medication at the end of follow‐up. Eight patients had a follow‐up of less than 5 years and three of them had a serum GH concentration of <5 mU/l at their latest follow‐up visit (38%). At 5 years of follow‐up after radiotherapy, 24 out of 32 patients had a GH level of <5 mU/l without medication (75%). At 10 years following irradiation, 16 of 21 patients (76%) had a GH level of <5 mU/l without medical treatment. At 15 years follow‐up, a serum GH concentration of <5 mU/l was observed in 13 out of 15 patients (87%) without GH suppressive medication. At the latest follow‐up visit, serum GH concentration below 5 mU/l was present in 75% of patients (30 out of 40) without medical therapy after a mean of 10.4 ± 0.9 years, but five patients required octreotide up to the end of follow‐up. Twenty‐seven out of 37 patients with available IGF‐I data had normal IGF‐I at the end of follow‐up (73%). Fifty percent of patients needed substitution therapy for (partial) hypopituitarism after 10 years and 75% after 15 years of radiotherapy.

In our group of patients who were incompletely cured by surgery, but had a significant postoperative decrease of serum GH concentrations, radiotherapy was able to achieve ‘safe’ serum GH concentrations in the majority in the long term.