Imaging cancer using positron emission tomography
Positron emission tomography (PET) is a modern imaging technique that has been widely available throughout hospitals and cancer clinics since the 1990s. PET provides very different information to the better-known imaging methods X-ray and CT, which “capture” tumours by showing the internal structure of the body, similar to a photograph. Rather, PET is used to image bodily processes, which may not function as normal in the presence of a forming tumour.
Before the PET scan, the patient is injected with a radioactive substance, which is often referred to as a “radiotracer”. Once the radiotracer has reached the site of interest, the patient is scanned for between 15 and 75 minutes, and the emitted radiation is used to generate a series of 3D images. From these images, doctors obtain clinically useful information about a bodily function. Radiotracers are similar to drug substances: there are many different forms available, and they are selected based on the disease type (e.g. a specific cancer), and the associated process that you wish to monitor. For example, most tumours take up more glucose than healthy cells, so a radioactive form of a glucose molecule (known as FDG) can be used to image tumours. Other biological processes that have been imaged using PET radiotracers include:
- Oxygen deprivation (a feature of many solid tumours)
- Increased formation of critical cell components, such as cell membranes
- Increased presence of proteins on the surface of a tumour (e.g. prostate-specific membrane antigens are found at high levels on cancerous prostate tissue)
- Changes in hormone production (observed in breast cancer)
- Spread of cancer to the bone.
Improving patient care through non-invasive diagnosis
PET is a highly valuable tool for cancer diagnosis. At very early stages of disease, changes occur at a molecular level before a detectable mass of tumour forms. PET can spot these changes, and since most PET scanners have integrated CT capabilities, the location of the abnormality can be detected earlier, compared with using CT alone.
Currently, surgical biopsy is the most common method used by doctors to diagnose a cancer subtype. Doctors must then process and analyse the excised tissue sample, by observing the cells through a microscope and testing the sample for the presence of specific proteins. With the development of new radiotracers, PET can now also be used to identify and measure traits belonging to a particular cancer subtype. The process of having a PET scan takes only a few hours, involves minimal patient discomfort (a simple injection), and the patient can leave on the same day. Furthermore, the results are almost immediately available to the doctor, which speeds up diagnosis considerably.
After diagnosis, PET continues to be useful throughout the cancer journey, for staging, selecting treatment, monitoring response to treatment, detecting mutations, and for assessing disease-free status or recurrence. Nevertheless, some patients express concern regarding radiation exposure. Exposure from a single PET scan is low, equivalent to 3 years of natural radiation exposure (i.e. from the sun, or underground sources of radon). Doctors generally agree that the benefits of accurate and rapid diagnosis far outweigh the risks.
Discovering new PET radiotracers to image pancreatic cancer
The PET Radiochemistry team at The Institute of Cancer Research in London work on the discovery of new radiotracers to image biological processes that have not yet been monitored by PET in a clinical setting. My PhD research is focused on creating radiotracers that recognise proteins that are highly expressed in pancreatic tumour cells. These radiotracers are designed to accumulate within the pancreatic tumour, to enable faster diagnosis of this particularly aggressive form of cancer. A common problem associated with pancreatic cancer is treatment resistance. Since the effectiveness of some pancreatic cancer drugs depends on the presence of known proteins, we also hope to create radiotracers to predict whether or not the patient will respond to a chosen treatment.
Outlook for the clinical use of PET
The availability of PET facilities is increasing throughout hospitals and research centres worldwide. Currently, over 90% of clinical PET scans use the same PET radiotracer (FDG), which is due mostly to a lack of clinically-approved alternatives. However, huge strides have been made in the field of PET research over the past decade, and with that, we expect to see more diversity in the clinical use of PET. Furthermore, with the discovery that common cancers may be identified by blood tests in the future, PET would provide the perfect complimentary approach to confirm cancer diagnosis, without invasive testing.