Tyrosine positron emission tomography and protein synthesis rate in pituitary adenoma: different effects of surgery and radiation therapy

INTRODUCTION: Positron emission tomography (PET) using amino acid tracers is able to establish biochemical tumour characterization in vivo. The use of PET in the follow-up of non-functioning pituitary adenomas (NFA) and growth hormone producing pituitary adenomas (GHA) after surgery and radiation treatment is not yet clear.

METHODS: To determine the value of PET before and after transsphenoidal neurosurgery in NFA and GHA, we investigated 12 patients with pituitary adenoma (9 NFA and 3 GHA) before and 4 months after surgery with magnetic resonance imaging (MRI) and tyrosine PET (TYR-PET). Three years after radiation therapy TYR-PET was used to document residual activity in 6 of these patients (4 NFA- and 2 GHA). Tumour size was quantified by computerized MRI measurements. In TYR-PET, tumour activity was assessed by computerized measurements of the hot spot and by determination of protein synthesis rate (PSR).

RESULTS: In response to surgery, MRI showed a median tumour volume reduction of 58% (P<0.01). TYR-PET demonstrated 62% volume reduction (P<0.02), but no change in PSR (P>0.30). After radiation therapy the MRI-volumes of the residual pituitary adenomas did not change but the volume of the hot spot on TYR-PET-imaging was reduced by 58% (P=0.02), and PSR decreased in 5 of 6 patients (P=0.12).

CONCLUSION: Amino acid PET tumour activity is reduced parallel with MRI volume changes after surgery. The decrease in TYR-PET activity after radiation therapy, despite unaltered MRI tumour volume, supports the concept that it is possible to follow biological tumour activity with this technique. The diagnostic merit of this tracer technique, predicting pituitary adenoma re-growth, needs to be validated in a large prospective study.