Endocrine Abstracts (2010) 21 OC2.3

11C-Metomidate positron emission tomography (PET) scanning for Conn's syndrome

Timothy Burton, Anand Annamalai, Nick Bird, Mark Gurnell & Morris Brown

University of Cambridge, Cambridge, UK.

Primary hyperaldosteronism usually results from an aldosterone-secreting adenoma of the adrenal cortex (Conn’s adenoma) or bilateral adrenal hyperplasia. Identification of the anatomical adrenal lesion causing hyperaldosteronism typically involves CT or MR scanning, with lateralisation of aldosterone production confirmed by adrenal vein sampling (AVS). The latter is a technically difficult and invasive procedure, but current non-invasive alternatives (e.g. radiolabelled iodocholesterol scintigraphy) lack sensitivity and specificity. 11C-metomidate, a potent inhibitor of the enzyme 11-β hydroxylase, which is over-expressed in adrenocortical adenomas, has recently been used as a radiotracer in positron emission tomography (PET), and shows significant uptake into both normal adrenal tissue and Conn’s adenomas.

We hypothesised that 11C-metomidate uptake by normal adrenal tissue would be suppressed by both dexamethasone and fludrocortisone. In the zona fasciculata 11-β hydroxylase expression is controlled by adrenocorticotrophic hormone and thereby ‘switched off’ by dexamethasone. The zona glomerulosa contains all the components of the classical mineralocorticoid response pathway, and normal aldosterone secretion is suppressed by the mineralocorticoid agonist fludrocortisone.

Five patients with Conn’s adenomas (aged 58.8±3.0 years) underwent three PET-CT scans at least 1 week apart: (i) without pre-treatment; (ii) pre-treatment with dexamethasone (0.5 mg qds for 3 days) alone; (iii) pre-treatment with dexamethasone and fludrocortisone (400 μg od for 3 days). Maximum standardised uptake values (SUVmax) of tracer were calculated for adenoma and normal adrenal tissue and the ratio of SUVmax in tumour to normal adrenal was calculated. Pre-treatment with dexamethasone increased the ratio of SUVmax in tumour to normal by 20±8% (P=0.037). Combined pre-treatment with dexamethasone and fludrocortisone increased the ratio of SUVmax in tumour to normal by 16±9% (P=0.072). In conclusion, early experience using a new non-invasive technique confirms that it is possible to visualise sub-centimetre adrenal adenomas and differentiate functioning and non-functioning nodules within the same gland.

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