PET scans for prostate cancer
PET scans for prostate cancer are typically used when a patient has had first line treatment (surgery or radiotherapy) and after this treatment, his PSA level starts to rise. A rising PSA sometimes indicates that treatment has not achieved a full cancer clearance and so further assessments are required to identify the location and stage of the residual disease.
Traditionally, a bone scan is carried out to identify any metastases (cancer which has spread beyond the primary site). However, some men will have metastatic disease which has not yet progressed sufficiently in order to be identified. It may be present in lymph nodes and if cancer has spread to soft tissue and bones, this is at a microscopic level which it isn’t evident in a bone scan.
This is where a PET (Positron Emission Tomography) scan can be very helpful because it provides a more detailed picture, including lymph nodes and microscopic changes in the soft tissue and bones.
How does it work?
Dr Peter Guest, consultant radiologist at the Queen Elizabeth Hospital, Birmingham, who works in association with BPC, explains: “PET scans allow us to understand how cells behave on a biological level, which would not be evident in CT or MRI scans (no matter how advanced), because they depend on size changes to detect cancer.”
During a PET scan, a substance called a radiotracer is injected into your body. It is known that cancer cells respond differently to radiotracers than normal cells (cancer cells use the radiotracer much more quickly than normal cells). The challenge, in the field of prostate cancer, has been to find the best radiotracer because prostate cancer cells do not react to the type of radiotracer commonly used for other cancers.
Dr Guest explains: “Some major cancer centres in the UK have started using a different radiopharmaceutical, called choline, which works better for prostate cancer. It works by looking at cell membrane turnover.
“There is also a different isotope called Gallium 68, combined with a complex organic compound which binds itself to membrane (PSMA) expressed by prostate cancer cells. The quality of the results has been superb – they have genuinely shown disease in lymph nodes which have otherwise appeared normal.”
To date (January 2018) about 30 BPC patients have had a Gallium 68 PET scan, initially travelling to Germany where this type of radiotracer was first used, but it is now also available in London.
What is the clinical benefit?
Alan Doherty, consultant urologist at BPC, comments: “Our experience is that for a specific group of patients, this is a very useful test: these are patients for whom we suspect there is some residual prostate cancer, but traditional tests don’t tell us where or how extensive.
“We have found that the Gallium 68 PET scan can sometimes answer this question and importantly, that this information is clinically significant. It means we can target the second line treatment (radiotherapy) in a precise and selective way and act at an early stage, rather than waiting for the metastases to grow and spread so they are identifiable.”
What do clinical studies tell us?
There is growing evidence to indicate that the Gallium 68 PET scan is both accurate and clinically significant.
In a study featured in The Journal of Nuclear Medicine, Australian (December 2017), researchers demonstrate that PET scans can identify which of these prostate cancer patients would benefit from salvage radiation treatment (SRT).
“The research is novel because it looks at the impact of PSMA PET/CT on patient responses to treatment, not just on whether the PET scan results in changed management,” explains Louise Emmett, MD, of the St. Vincent’s Hospital, Sydney, Australia. She elaborates, “In the study, these patients underwent imaging with a PSMA PET scan and had treatment based on the results of the scan findings. The study then followed how these men were treated, and whether the treatment was effective.”
The study demonstrates that PSMA PET can independently predict treatment response to SRT. Men with negative or fossa-confined PSMA have the highest treatment response to SRT, while men with cancerous nodes or distant disease have a poor response. In particular, a negative PSMA PET predicts a high response to SRT.
Emmett points out, “The results of the study show that PSMA PET is more predictive of a treatment response than PSA level, surgical margins or seminal vesical involvement.”
“At BPC, we continue to carefully scrutinise new developments in this rapidly changing field of prostate cancer and where appropriate, support access to diagnostics and treatments which seem to benefit our patients.”