Essential Endocrinology and Diabetes. Richard I. G. Holt
Simple X‐rays, CT and MRI depict tissues and organs but provide limited insight into the cells that compose these structures or their function. In later life, many organs develop benign tumours of little or no significance. For instance, incidental adrenal tumours (incidentalomas) can affect ∼5% of the population after ∼40 years. In a person with hypertension, it would be important to distinguish these from a phaeochromocytoma that could be the curable cause of elevated blood pressure (Chapter 6). Uptake markers (or ‘tracers’) specific to a particular cell type can provide valuable clues. For instance, meta‐iodobenzylguanidine (mIBG) acts as an analogue of norepinephrine and is taken up by adrenal medulla cells. When labelled with radioactive iodine‐123 (I123) it can be used to distinguish a phaeochromocytoma from other tumours (Figure 4.9). At higher doses, it can even be used as targeted therapy, when instead of marking cells, it kills them. I123 or technetium‐99m pertechnetate can also be used to delineate different causes of hyperthyroidism (Chapter 8) when taken up by the thyroid gland. In Graves disease, the uptake is homogeneous; with a solitary ‘toxic’ adenoma, the uptake is restricted to the relevant nodule.
Figure 4.6 Ultrasound of a polycystic ovary. The presence of multiple small cysts (one shown by the arrow) is consistent with, but not required for, the diagnosis of polycystic ovarian syndrome (Chapter 7). Ultrasound does help to exclude the single mass of an androgen‐secreting tumour (Chapter 7).
Image kindly provided by Dr Sue Ingamells, University of Southampton.
Figure 4.7 Abdominal computed tomography (CT) with contrast. This patient presented with Cushing syndrome (see Figure 6.9). The right adrenal mass on the CT (arrow) was a cortisol‐secreting adenoma.
Figure 4.8 Magnetic resonance imaging of a pituitary tumour. (a) T1‐weighted sagittal image. (b) T2‐weighted sagittal image (cerebrospinal fluid appears white). (c) T1‐weighted frontal image. A large irregularly shaped pituitary tumour (*) has compressed the pituitary stalk (not visible) and raised and tilted the optic chiasm (large arrow) such that it appears draped on top of the tumour sloping down to the right. The tumour has also extended bilaterally into the cavernous sinus to encase partially the internal carotid arteries (small arrow marks the right internal carotid artery).
Figure 4.9 mIBG uptake by a phaeochromocytoma. A whole‐body I123 mIBG scan with imaging from the front and back shows a right phaeochromocytoma with pulmonary and bony metastases. This imaging is helpful to investigate potential metastatic disease prior to adrenalectomy.
Image kindly provided by Dr Val Lewington, Royal Marsden Hospital.
Positron emission tomography (PET)
Positron emission tomography (PET) is a form of functional imaging which is widely used to assess metabolism in neoplasia and allows the identification of tumours that may be overlooked by conventional imaging. Cancer cells often have accelerated glucose metabolism and more readily take up glucose than surrounding healthy cells. This process can be visualized by using the radiotracer is 2‐[18F] fluoro‐2‐deoxyglucose [(18F)‐FDG] which crosses the cell membrane and is phosphorylated to become FDG‐6‐phosphate. This is resistant to further metabolic processes and can be imaged. When the kinetic energy of [(18F)‐FDG] is dispersed as a positron, this particle travels a short distance and interacts with an electron to release two photons which can be detected by a pair of detectors located on opposite sides of the patient. PET images are obtained simultaneously with CT images to match metabolic changes to specific anatomy.
A number of hormone precursors and amino acids are labelled with 11C and used successfully in the management of parathyroid, adrenal and pituitary tumours but the short‐life of these tracers have limited the clinical application. Newer tracers with longer half‐lives, such as Gallium‐68, are now being applied to neuroendocrine tumours, including phaeochromocytoma.
Diagnosing or excluding endocrine disorders relies on measuring the concentration of hormones and metabolites
Immunoassays provide accurate, reliable laboratory measurement of many hormones and metabolites
Techniques involving mass spectrometry are increasingly being used to measure hormones and metabolites
Cellular and molecular biology can increasingly provide patient‐specific diagnoses of congenital disorders or endocrine neoplasia syndromes; this information can predict and influence patient outcome and management
Imaging investigations localize endocrine disorders and assist surgical intervention
‘Incidentalomas’ are common and conscientious effort is needed to correlate a biochemical endocrine abnormality to a tumour identified on imaging
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