Functional imaging of thymic disorders

Human thymomas are rare tumours which usually develop in the chest. The diagnosis via guided biopsy, the evaluation of the extent of the tumour and its boundaries with adjacent organs, the choice of the appropriate therapeutic option, and the assessment of response to treatment are usually made with computed tomography (CT) alone or in combination with magnetic resonance imaging (MRI). More recently, radiopharmaceuticals and nuclear medicine procedures have been used increasingly in the imaging and functional characterization of benign and malignant thymic disorders. Two groups of radiopharmaceuticals have been used. The first includes several oncotropic tracers, such as 201Tl-chloride, 99mTc-sestamibi and 18F-fluorodeoxyglucose, which are significantly concentrated in thymic tumours. Their uptake correlates with tumour grades and cellularity. The second class includes two radioligands: [(111)In-DTPA-D-Phe1]-octreotide (DTPA, diethylenetriamine penta-acetic acid) and [(111)In-DTPA-Arg1]-substance P, which bind to specific receptors. [(111)In-DTPA-Arg1]-substance P binds to its receptors that are largely expressed in the thymus of patients with autoimmune diseases. [(111)In-DTPA-D-Phe1]-octreotide recognizes the somatostatin receptor subtype 2. In patients with active disease investigated in our institution [(111)In-DTPA-D-Phe1]-octreotide has been shown to concentrate in the majority of thymoma deposits. Conversely, it is not concentrated in adult patients with benign lymphofollicular thymic hyperplasia. This finding has had a significant impact on the management of patients with myasthenia gravis as it differentiates early-stage thymoma from benign hyperplasia, unlike CT and MRI, which often fail to distinguish between the two. In addition to its role in diagnostic imaging, somatostatin receptor scintigraphy also enables us to select patients with advanced or metastatic thymoma unresponsive to conventional therapies, who might benefit from a somatostatin analogue-based treatment, serving thus as a link between diagnosis and therapy. In this article, we discuss and analyse the results of functional imaging with different radiopharmaceuticals, primarily those that we have obtained with [(111)In-DTPA-D-Phe1]-octreotide.