Main Menu

Login

Registered Users



Please note that a password-login is required to access some of the links above. To register and obtain a password please contact us!
PET and Cancer
ABOUT PET

Positron Emission Tomography is a medical scanning technique which is used to investigate normal and abnormal processes in the body. The images produced by a PET scanner reflect how the tissue is functioning at the molecular level. Most other types of scanner, e.g. CT (Computer Tomography) or MRI (Magnetic Resonance Imaging), show images that reflect the anatomy of the body, whereas a PET image may reflect, for example, the amount of oxygen that is reaching a tumour or whether a chemotherapy drug has acted upon a particular cellular pathway. For this reason PET is generally classified as a ‘molecular imaging’ technique.

PET works by injecting a very small amount of a radiolabelled compound into the body, usually intravenously. The compound is selected to be taken up by a particular process, for example, a radiolabelled glucose analogue (fluorodeoxyglucose or FDG) is taken up by tissue in proportion to its rate of metabolism, and in particular by most tumours. Radiation emitted from the radiotracer escapes from the body and is detected by the PET scanner which then constructs a 3D image of the distribution of the radiotracer. Regions that have accumulated tracer (e.g. a tumour) show up as ‘hot spots’ on the PET image. The radioactive label has a half-life of just a couple of minutes to a couple of hours and so rapidly disappears from the body. The radiation dose associated with a PET scan is roughly equivalent to several years natural background radiation.

FDG is the most widely used PET radiopharmaceutical. The scanning session starts with injection of FDG, the patient must then remain still for ~60 minutes for the tracer to distribute within the body. The patient then lies in the PET scanner for ~20 minutes whilst the image is obtained.

‘Positron Emission Tomography’ refers to the fact that the radioactive label which allows the injected compound to be imaged, emits a particle called a positron. The idea of imaging positron labelled compounds goes back to the early 1950s. One reason that PET has taken so long to become widely available is that it is complex and hence expensive. In addition to the PET scanner, a particle accelerator (‘cyclotron’) is required to produce the positron emitting radionuclide, and a radiochemistry laboratory is needed to attach this radioactive tag (eg 18F) to the relevant compound (eg glucose). Both routine and research PET activities therefore are truly multidisciplinary requiring chemists, physicists, computer experts, physicians, pharmacists, biologists, radiography and technical expertise.

Nearly all PET scanners in use today have a CT scanner incorporated into the same gantry. The CT scanner  uses x-rays to obtain a high resolution anatomical image of the body. A CT scan, which only takes ~30 seconds, is performed immediately prior to the PET scan. The anatomical CT scan allows ‘hot spots’ and other features seen on the PET image to be located within the body.


A. Pre Treatment Images (CT, PET and fused PET-CT image, from left to right)
 B. Post Treatment Images (CT, PET and fused PET-CT image, from left to right)
Pre Treatment FDG PET-CT scan Prost Treatment FDG PET-CT scan
Images A. and B. Patient with high grade Non Hodgkin's Lymphoma treated with chemotherapy. Pre treatment images (A.) and images after two cycles of chemotherapy (B.) showing complete metabolic response despite extensive residual mediastinal lymphadenopathy on CT. Physiologic uptake is seen in the heart and bladder.


FDG is far and away the most widely used PET tracer being used for nearly all clinical PET scans. Many other tracers are under development and are likely to see widespread use, both in the clinic and for research, within the next ~5 years. These include:

  • 18F-FLT (tumour proliferation rate)
  • 11C-Methionine (protein synthesis e.g. for brain tumours)
  • 18F-Fluoride (e.g. bone cancer)
  • 64Cu-ATSM (measures tumour hypoxia)

Validating new radiotracers such as these and making them widely available is one of the main challenges that needs to be addressed in order to realise the full potential of PET imaging.
 

ABOUT CANCER

Overview

Different cell types with different functions and structures are grouped together to form different organs and tissues of the body. In principle, any of these cell types can become cancerous. Under normal, healthy conditions, cell growth and cell division follow strict rules to regulate these processes. Due to various reasons this control can be disrupted and cells may start growing and dividing rapidly, resulting in a collection or ‘lump’ of cancer cells called a tumour.


Benign vs. malignant tumours

A tumour is not necessarily dangerous. In general ‘benign’ tumours do not cause harm as these types of tumours usually stay in the same place, grow slowly and often stop growing at a certain time point mainly due to space limitations. Yet, other tumours can cause problems due to their rapid growth and their invasion into normal surrounding tissues, destroying other body structures by e.g. causing compression. These tumour types are called ‘malignant’. By spreading into blood vessels or the lymphatic system these cancer cells may even get carried around in the body, eventually forming new tumours in other parts of the body (metastases).


Causes of cancer

The magnitude of cell growth and cell division is controlled by the activity of certain genes. A gene is a length of DNA (deoxyribonucleic acid) that codes for a protein that performs a specific task within the cell. Genes can be switched on or off. Under normal healthy conditions, certain genes make sure that cells grow and reproduce in a very controlled and well-planned manner. However, genetic damage may occur so these genes cannot fulfil their proper function in regulating cell growth anymore, allowing cells to grow rapidly in an uncontrolled manner. Genes cannot directly cause cancer and cancer as such cannot be inherited, however a higher risk of getting a particular type of cancer can be passed on. There is no single cause of cancer; it depends on many factors both inherited and environmental.


For more information please visit:

Cancerbackup
Cancer Research UK
Department of Health
 
CLINICAL APPLICATIONS OF PET IN CANCER

PET-CT scanning is now established as having a major role in the management of patients with cancer. PET is used in the following ways:

  • Initial diagnosis (benign/malign disease, grade of malignancy)
  • Staging (assessing the extent of disease)
  • Monitoring of response to therapy
  • Residual vs. active disease after chemotherapy/ radiotherapy
  • Recurrence/ restaging of disease after therapy
  • Identification of primary site of the tumour
  • Treatment planning for radiotherapy is likely to become a major application

Currently evidence for the benefit of using PET is strong in several cancer types, notably lung cancer, lymphoma and colorectal cancer. There are also encouraging results in a large number of other tumour types and this list is likely to grow dramatically.
 

RESEARCH APPLICATIONS OF PET IN CANCER

Overview

PET has great potential to improve the management of patients with cancer and the evaluation of new anti-cancer drugs with the aim of making new therapies available in the clinic more rapidly. Most PET research in cancer falls into one of the following areas:

  • Assessment of the utility of PET in patient management
  • Facilitating the development of new anti-cancer drugs
  • Translational research Development of new PET technology and radiopharmaceuticals



Assessment of the utility of PET in patient management

PET already has an established role in the management of patients with a wide variety of cancers. The areas where it is most useful are in staging of disease and in monitoring of response to therapy. PET is also likely to become used to allow more accurate planning of radiotherapy treatment. These applications are evolving rapidly as access to PET scanning becomes more widely available and as new tracers other than FDG are developed. Studies are necessary to evaluate the efficacy of these new indications and to define the role of PET in the patient pathway. These studies usually require large numbers of patients and are conducted as multi-centre trials across many sites.



PET in the development of new anti-cancer drugs

PET can be used in several ways to improve the efficiency of  the time-consuming and expensive process of taking a new anti-cancer drug from the laboratory through to the clinic. New drugs can be labelled with a PET radionuclide to image their biodistribution and kinetics at a very early stage on a small number of volunteers – this may enable an unsuitable compound to be rejected at an early stage without having to go through large clinical trials. Well understood PET tracers, such as FDG, can then be used in later stage trials on large numbers patients to determine how effective the drug is. PET is able to provide this information much more rapidly than currently used methods. Trials of new drugs are usually performed by or in collaboration with a pharmaceutical company. Early stage trials are usually more technically complex, requiring only a small number of subjects, and are performed at a small number of PET units. Later stage trials require large numbers of patients and are usually performed at a much larger number of sites.



PET in translational research

Because PET can be used to obtain information about tumour metabolism and the effects of new treatments in both animal models of disease and in human patients it is proving useful in speeding up the ‘translation’ of studies from the laboratory into the clinic.



Development of PET technology and radiopharmaceuticals

PET scanner technology and the radiopharmaceuticals that are administered to the patient are themselves undergoing very rapid development at present. This work is performed both in academia and in industry. Current developments include, for example, the development of combined PET & MRI scanners and radiopharmaceuticals that can image tumour hypoxia and other properties that can be used to characterise tumours non-invasively.
 
 


 
 
Design by Sabine / Script by Joomla!