Mesothelioma is one of the more difficult cancers to diagnose. Unlike other cancerous tumours, which form solid round shapes, mesothelioma is similar to ‘cling wrap’ around the lung.
It is a cancer of the pleura, the lining of the lung, which means the cancer grows inside the lining, essentially wrapping the lung. Because of the way it grows, it is more difficult for doctors to work out exactly where the mesothelioma is, how advanced it is and what treatment might work best. Dr Roslyn Francis, Nuclear Medicine Physician at Sir Charles Gairdner Hospital, is at the forefront of research into one of the newer diagnostic tools being used to diagnose mesothelioma.
PET, or positron emission tomography, scanning, is being used in conjunction with traditional tests, such as biopsies, to provide more insight into treatment of this disease. A biopsy involves a piece of the tissue being removed to test under a microscope for cancerous cells.
Doctors generally use CT or CAT (computerised tomography) scans to diagnose solid cancers because they can be easily detected by the beams it sends out. A CT scan is an x-ray machine which sends out several beams from different angles, as opposed to a normal x-ray which sends out one beam.
Unfortunately there are some limitations with CT scans in assessing mesothelioma because of the way it grows. However, it provides doctors with a better assessment of what stage the disease is at than PET scans and remains the main diagnostic tool used for mesothelioma.
Dr Francis explains that PET scans have been a positive breakthrough in mesothelioma because they can show how the mesothelioma is behaving, rather than just that the disease is present. PET is a nuclear imaging technique which produces a three-dimensional picture of functional processes in the body.
“The difference between PET and CT scans is that we inject radioactive tracers into the body for the PET scan to image. Glucose with a radioactive tag attached is injected into the blood stream and then patients have to wait for about an hour before we can take the pictures under the camera.
“The PET camera then detects where the radioactive glucose is being used in the body and puts this together as a 3D image of the body. With a CT scan, radiation is beamed through the body and a picture is taken of the anatomy.
The PET scan shows function and what we are looking at is glucose metabolism,” Dr Francis said.
“The cells in active tumours are very dependent on glucose and they will take it up. The pictures taken on the scan show these areas as ‘hot’ areas.
“We can see exactly where the cancer is,” she said.
“More precisely, it gives us an overall idea of the active tumour. With a CT scan you can see thickening but you can’t always see whether it is fluid or active tumour.
“The PET scan results are more precise to the extent and bulk of the disease, allowing us to measure all of the active tumour area,” Dr Francis said.
She said PET scans can therefore be useful in addition to CT scans, in providing extra information.
Dr Francis has been involved with a recent study of PET scanning as a diagnosic tool, involving 93 patients. The study, which began in 2003, showed PET scans were particularly useful in assessing overall burden of disease and in predicting patient outcomes.
“If we do a scan and (it shows) they don’t have very much disease, then they are going to do a lot better than people who have a lot of tumour. So the PET scan can give a better estimate of outcome.
“Some people just want to know whether they have six months or 2 to 3 years (to live) and knowing this can also help inform treatment options.”
PET scans are also being used to monitor patients undergoing treatment to measure more precisely how they are responding and whether changes to treatment are needed.
“We have found the FDG PET scans are very good at assessing response to treatment.
“If you give chemotherapy, and take the scan before and after, you can tell even after three weeks which patients are responding.
“If people are not responding, you might review their trea
tment. If they are responding, they might feel better about going on with the treatment,” Dr Francis said.
Surgery is one form of treatment that may only be helpful in limited cases where the mesothelioma is very isolated. PET scans can help provide doctors with more certainty about whether surgery could be beneficial.
By Wendy Pryer