How hybrid SPECT/CT systems changed imaging work-up in detecting parathyroid adenoma (PA)

Abstract

The improvements achieved in imaging techniques in the last two decades have deeply changed the approach to the diagnosis of primary pauci-symptomatic hyperparathyroidism (PHPT). Initially, planar 99mTc-Sestamibi scintigraphy both adopting dual-phase or double-tracer subtraction protocol, both with Iodine-123 or 99mTc-pertechnetate, was the only possibility to assess the hyperfunction of a parathyroid adenoma (PA). Later, in 1980s, the tomography technology applied to scintigraphy, called SPECT (single-photon emission tomography), enabled 3-dimensional reconstruction and improved tumor to background contrast. The further step was the assembly of hybrid cameras capable of simultaneous computed tomography (CT) and SPECT acquisition. The first commercial SPECT/CT scanner, introduced in 1999, was able to permit anatomical localization of disease, define its extension, and improve discrimination of physiological versus pathological uptake. This first generation of hybrid systems had the CT transmission part highly limited in their spatial resolution, scan speed, and signal-to-noise performance. On the contrary, last-generation scanners build multislices CT having similar performance to that obtained with conventional diagnostic CT, thus permitting also the administration of CT-contrast media. Superior diagnostic accuracy of SPECT/CT over planar and SPECT imaging has been constantly reported since it improves diagnostic confidence and accord between specialists, thus impacting the patientmanagement course [1]. Some authors suggest the combination of functional imaging with neck ultrasound (US) in order to better visualize PA located within or behind the thyroid gland [2–4]. However, US principal constraints are the limited field-of-view (affecting low or mediastinal PA detection) and the fact that it is highly operator-dependent, and results may differ a lot among one and another operator. In this view, the use of objective and standardize imaging techniques, such as SPECT/CT, is highly appreciated in the endocrine community. In my experience and as recommended by EANM parathyroid guidelines of which I was the coordinator, the ideal work-up consisted of (a) double tracer subtraction 99mTc-pertechnetate ? 99mTc-Sestamibi plus potassium perchlorate protocol (which speed Sestamibi wash-out by the thyroid tissue after thyroid imaging [5]) lasting 40 min, (b) neck US, and (c) SPECT/CT acquisition with large field covering neck & mediastinum [6]. In my experience (data unpublished), the ratio of Sestamibi PA/thyroid uptake, measured by region-of-interest (ROI) method (manually positioned over the highest area of focus radiotracer uptake) using a last-generation hybrid SPECT/CT camera, should increase up to 70–155 % thanks to the attenuation correction and allowed a better detection of deep PAs. Different experiences in detecting PHPT by the means of other techniques have been investigated and worth to be cited. For example, the use of positron emission tomography/computed tomography (PET/CT) with 11CMethionine (MET) in patients with suspected parathyroid adenoma was deepened by Caldarella and co-worker in the only one meta-analysis published on this topic [7]. Rationale of the use of MET as specific tracer for detection of PHPT is that it is one of the amino acids comprising parathyroid hormone and therefore closely related with biosynthesis of parathyroid hormone. The pooled sensitivity of MET-PET was found to be good (81 %), however, D. Rubello (&) A. M. Maffione Department of Nuclear Medicine–PET/CT Oncologic & Endocrine Sections, Rovigo Hospital, Via Tre Martiri 140, 45100 Rovigo, Italy e-mail: domenico.rubello@libero.it