It’s unfortunate that prostate surgery does not always mean the end of prostate cancer. Rising PSA levels after surgery mean that the cancer is probably on the way back. Some men with recurrent prostate cancer can be effectively cured by radiotherapy directed to the prostate bed. But those whose disease has spread beyond this area would do better to have more extensive treatment. A new Australian study has compared three different types of scan to ask which is best at directing treatment decisions for men with rising PSA after surgery.
Recurrent prostate cancer
Most men with localised prostate cancer are effectively cured by their treatment. They receive radiotherapy or surgery to destroy the prostate and its cancer. It the cancer has not spread out of the prostate, then it won’t return. But unfortunately, initial treatment doesn’t work for every man. When surgery fails to cure prostate cancer, this has happened because not all the cancer cells were removed. Sometimes, a whole prostate is successfully removed, but some of the cancer cells escape and are left behind. New tumours eventually grow from these escaping cells. This is known as recurrent prostate cancer. These new prostate tumours either grow close to the region where the prostate was situated (called the prostate bed, or prostate fossa), or they grow in distant regions in the body.
Salvage radiotherapy
A rise in PSA after treatment is usually the first indication of recurrent prostate cancer. Rising PSA levels after surgery are therefore a very concerning sign. An effective cure is still possible at this stage if the new tumours are located at the prostate bed, where the prostate used to be. For these men, radiotherapy is directed at the prostate bed to kill off the cancerous cells growing there. This treatment is known as salvage radiation therapy to the prostate fossa (herein referred to as salvage radiotherapy).
Salvage radiotherapy is useful for men if the new tumours are growing at the prostate site. But for others whose cancer has moved further away, to the pelvic lymph nodes or bones, it’s not so useful. Salvage radiotherapy takes time to work, and it has side effects. So for men who won’t benefit, it would be better for them to start more extensive treatments such as hormone therapy (androgen deprivation therapy; ADT) and/or radiotherapy to other pelvic areas.
Scans for men with recurrent prostate cancer
If PSA levels are rising, how do we know where the new tumours are growing? Once they are large enough, they can be seen on scans. But salvage radiotherapy for recurrent prostate cancer needs to be given early, when the new tumours are difficult to detect. The increased sensitivity of the latest imaging techniques helps to detect new tumours when they are small. This should help to detect recurrent tumours early, improving treatment decisions. Knowing the position of the new tumours will help men to choose the most appropriate treatment.
A new Australian study has compared three different types of scan used to detect recurrent prostate tumours after surgery. The study was conducted by a team led by A/Prof Louise Emmett, a radiation oncologist at St Vincent’s Hospital in Sydney.
The scans investigated were multiparametric MRI (mpMRI) and two different types of PET scan. PET scans use a radioactive dye (called a radiotracer) that is introduced into the body. The radiotracer sticks to specific organs or tissues, depending on the target chosen. It’s detected by a scanning machine that detects radioactivity. The amount of radiation is very low and it has a short half-life. MRI is a non-radioactive scan that doesn’t use X-rays, so it has minimal effects on the body. MRI applies a strong magnetic field to the body. The scan detects different types of tissue as they look different under the magnetic field.
The three types of scan compared in the Australian trial were:
FCH-PET (F18 fluoro-methyl-choline PET/CT). This type of PET scan detects regions of the body where cells are consuming a lot of glucose sugar. Cancer cells grow and divide very quickly, using a lot of glucose.
PSMA-PET (Ga-68 HBED-CC PSMA-11 PET) This PET scan detects prostate cancer cells, based on the presence of a protein called PSMA on the cell surface. Approximately 95% of prostate cancer cells have PSMA on their surface. About 100 times more PSMA is present on the surface of prostate cancer cells than other cell types.
Pelvic-mpMRI (pelvic multi-parametric magnetic resonance imaging) This scan makes an image of the whole pelvis region under a strong magnetic field. Cancerous cells are detected because they look different to normal cells when the magnetic field is applied.
Comparison of scans
A/Prof Emmett’s study recruited men from 8 sites across Australia, Canada and the UK. 91 men joined the study. They had rising PSA levels after surgery for localised prostate cancer. They also had at least one high-risk feature, such as Gleason score higher than 7 or a PSA doubling time of less than 10 months. Each of the men had a pelvic mpMRI and an FCH PET within 2 weeks. Men from Australia also had a PSMA-PET scan. Results from the study were:
- Recurrent prostate cancer was most likely to be detected by the PET scans. PSMA-PET detected prostate tumours in 42% of men, FCH-PET in 32% and pelvic MRI in 28%.
- Pelvic-mpMRI and PSMA-PET had the highest chance of detecting prostate cancer that returned to the prostate bed (where the prostate used to be).
- PSMA-PET and FCH-PET were more sensitive than pelvic-mpMRI for detecting tumours outside of the prostate bed.
An important consideration for whether to use scans is whether the results of the scan make any difference to the treatment plan. If the scan makes no difference to which treatments are chosen, then why have the expensive scan?
The plan for future treatment was changed by the scan results finding tumours for 46% of men based on FCH-PET and 24% based on pelvic-mpMRI. Men who had PSMA-PET as well FCH-PET had an even higher chance of a change in treatment decisions. These changes included where radiation was applied, the dose used, and whether to use ADT. There were also changes to treatment choices for men whose scans were clear. Previously they had planned radiotherapy to the prostate bed. After a clear scan, this was considered unnecessary at the time.
Overall, this study showed that the PET scans were most sensitive for identifying men whose cancer had spread outside of the prostate bed. Success rates of the treatments in this trial also indicated that PET scans could more successfully identify men who would benefit from salvage radiotherapy, than pelvic-mpMRI.
PSMA-PET scans are commonly performed in Australia, but less commonly in the US or UK. We are lucky that PSMA-PET technology is readily available and popular in Australia for men with recurrent prostate cancer. Unfortunately, PSMA-PET scans remain expensive. Studies like this one, and the PCFA-funded ProPSMA trial, will hopefully contribute to the evidence required to convince the Australia Government to subsidise the costs of these useful scans.
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