Pet

One of the areas of growth in nuclear medicine is that of positron emission tomography (PET).

This is based on the concept that many biological tracers are difficult to prepare in a radiolabelled form using conventional isotopes, often because they are small molecules and the insertion of a large radioactive atom, such as Tc-99m, I-123, or I-131, changes its behavior significantly. However, there are several low-atomic-number isotopes of potential value, which share the characteristic that they decay by giving off a positron. This particle is a positively charged electron, which only travels a millimetre or so before it meets a (more conventional) negatively charged electron. The two fuse and give off annihilation radiation consisting of two 511 keV gamma rays. These two particles are given off at almost exactly 180° to each other. If these are detected in a suitable machine, it is possible to reconstruct the 3-D distribution of the original activity in the subject. Nowadays, the majority of new machines also have a computed tomography (CT) scanner incorporated (PET/CT), which gives two advantages. First, it allows for accurate correction of the PET images for the effect of attenuation by the body tissues, and the necessary data can be acquired in two minutes, rather than the 20 minutes necessary with older methods. Second because the two sets of images are intrinsically aligned, it allows for accurate localization of sites of increased uptake.

The most widely used tracer in applications of interest to urogenital imaging is F18-fluorodeoxyglucose (FDG). This is a substituted sugar, which is taken into cells through glucose transporters (mainly glucose transporter type 1), then metabolized by hexokinase, but it cannot pass further along the metabolic chain. Its concentration therefore reflects the metabolic activity of the cells. Many tumors have increased glucose uptake and, therefore, concentrate FDG. Normal uptake is seen in tissues that predominantly rely on glucose for their metabolic activity—especially the brain and heart—and also in muscle. It is excreted via the kidneys into urine, which may present problems. The tracer has to be prepared at a center that has a cyclotron, but these are now relatively widespread, and there are networks for the distribution of the radiotracer that has a half-life of just under two hours.

In testicular cancer, there is usually avid uptake of FDG, more so with semi-nomas than nonseminomas, and it appears to be an accurate method of staging, although probably not more so than CT. However, it is considerably more accurate than CT in restaging after chemotherapy, although early imaging after chemotherapy (within two weeks) may produce some false positives as a result of increase in inflammation. If necessary, it can be used for targeting biopsy. It is also more accurate than CT in detecting recurrence.

In cervical cancer, FDG-PET seems to be more accurate than CT or magnetic resonance imaging (MRI) in detecting lymph node metastases; and in a multivariate analysis, the most significant determinant of progression-free survival is the presence of positive para-aortic lymph nodes on an FDG-PET scan. It also appears to be the most accurate method of detecting recurrent and metastatic disease.

In ovarian cancer, PET has been shown to be accurate in restaging for lesions more than 5 mm in size but may miss smaller lesions and disseminated disease.

FDG-PET has not yet been shown to be of value in prostate or bladder cancer, partially because of physiological excretion via the urinary tract and also because of relatively low uptake. Studies are in progress using labeled amino acids, which seem promising.

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