Radionuclide imaging in oncology and infection

Published on 12/06/2015 by admin

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Last modified 12/06/2015

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Radionuclide imaging in oncology and infection

Positron emission tomography imaging

Positron emission tomography (PET) imaging is a technique used to detect and accurately stage malignant disease, to differentiate benign and malignant tissue, and to assess response to treatment. Until recently, PET imaging availability was restricted due to high capital cost and logistics of radiopharmaceutical supply. It uses short-lived cyclotron-produced radionuclides such as 18Fluorine, 11Carbon, 13Nitrogen and 15Oxygen with half-lives of 110, 20, 10 and 2 min respectively. 18Fluorine is the only one of these that has a half-life long enough to allow it to be produced off-site. This does permit 2-[18F]fluoro-2-deoxy-d-glucose (18F-FDG), the single most important PET radiopharmaceutical, to be used by sites without their own cyclotron.

The widespread acceptance of PET as a major advance is due to two major factors:

1. There is increased recognition in the literature of the role of the main PET tracer 18F-FDG, a glucose analogue that is taken up in tissue in proportion to cellular glucose metabolism. This is particularly useful for tumour imaging, since most tumours have increased glucose metabolism and will concentrate FDG. Malignant cells are characterized by increased glucose transporter molecules at the cell surface. FDG is phosphorylated by the enzyme hexokinase to a polar intermediate which does not cross cell membranes well and is, therefore, trapped in the cell. Hexokinase levels and activity are increased in malignant cells. The reverse reaction (glucose-6-phosphatase) is slow and the enzyme is commonly deficient in cancer cells.

2. Integrated PET CT scanners are now widely available and are the standard of care. These units have separate PET and CT scanners installed in the same gantry. The patient undergoes a conventional CT scan (usually performed with low exposure factors to reduce radiation dose) immediately followed by a PET scan without moving, on the same table-top. This allows fusion of the anatomical information from CT with the functional data from the PET scan, and hence accurate anatomical localization of metabolically active disease and recognition of normal anatomical and physiological uptake. The density data from the CT scan are also used to correct the PET data for differential attenuation of the emitted photons within the patient.

Normal physiological uptake is seen in organs that are hypermetabolic and big glucose users, especially the brain and the heart, or active or recently active skeletal muscle. Variable uptake is seen in the gut and there is normal excreted urinary activity in the urinary tract. One confounding factor for interpretation may be normal physiological uptake in brown fat – particularly in the neck and paraspinal regions. Differentiation of this normal activity from pathology is greatly aided by the image registration afforded by combined PET CT scanners.

However, FDG is not specific for cancer cells as any hypermetabolic cell will show increased uptake of FDG such as those in sites of inflammation or infection, so interpretation with reference to full clinical details and other imaging is important to avoid false-positive scans.

As the only PET pharmaceutical currently widely available and widely used, this section is restricted to FDG imaging. Other pharmaceuticals tagged with carbon-11 or other short-lived isotopes are restricted to sites with a dedicated cyclotron. Other fluorine-labelled pharmaceuticals such as 18F-choline are, however, being produced and have a role in imaging of prostate cancer metastases for example.

2-[18F]fluoro-2-deoxy-d-glucose (18F-FDG) PET scanning

Indications (oncology)

Specific

The following tumours are commonly imaged for staging and follow-up purposes and in cases of diagnostic difficulty:

A full list of tumour-specific indications can be found in the 2012 guidance document produced by the Royal Colleges of Physicians and Radiologists listed under further reading.

Technique

1. Up to the UK limit of 400 MBq 18FDG intravenously (i.v.) (10 mSv effective dose (ED)) is administered.

2. To reduce muscle uptake of FDG, patients should remain in a relaxed environment such as lying in a darkened room (without talking if head and neck area are being imaged) between injection and scan.

3. Image at 1 h post injection.

4. Imaging is preferred with the arms above the head to reduce beam hardening artifact on the CT.

5. CT:

6. PET:

7. In some instances a diagnostic standard dose CT with i.v. contrast may be acquired as well, but in routine practice a diagnostic scan will usually be already available. A scan performed with i.v. contrast may result in attenuation artefacts on the reconstructed PET images if used for attenuation correction.

Gallium radionuclide tumour imaging

This is rarely used, having almost entirely been superseded by cross-sectional techniques and PET scanning.1 The main disadvantages are the high radiation dose, the extended nature of the investigation, its non-specific nature, and difficulties in interpretation in the abdomen due to normal bowel activity.

Aftercare

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