Prostate cancer is the second-leading cause of death among men in the United States. Imaging for the disease, especially when it comes to recurrences, has traditionally been challenging, but recent advances give hope for better outcomes.
While prostate cancer is consistently one of the top three cancers in the nation, there are very few effective imaging solutions. Blood work, specifically through monitoring the prostate-specific antigen (PSA) level, delivers the majority of diagnoses. Any deviation from normal is an indicator, and may lead to surgery, cryotherapy, or radiation, as necessary.
These developments in prostate imaging could help to provide more reliable solutions. Here’s a brief outlook showing the current and future states of prostate imaging.
PET/CT vs. PET/MR for Prostate Cancer
Many people look to MRI because they want to reduce exposure to radiation, but that shouldn’t be the only consideration as the actual radiation exposure is relatively negligible. One good reason to opt for MRI is that you get all of the tissue and functional characterization from it, along with the specificity of the PET agents for prostate cancer.
Not all patients are suitable for MRI imaging, particularly those with metallic implants, so PET/CT is a good alternative for those patients. Studies have shown that both are great options for imaging, so it may come down to what is available for the patient.
Axumin for Prostate Imaging
Axumin (Fluciclovine F18) injection was added to the National Comprehensive Cancer Network (NCCN) Clinical Practice Guidelines in Oncology for Prostate Cancer (Version 1.2018) in 2018. The updated NCCN Guidelines state that Fluciclovine F18positron emission tomography/computed tomography (PET/CT) or PET/magnetic resonance imaging (MRI) scans should be considered in the clinical workup of patients with biochemical recurrence of their prostate cancer.
Specifically, Axumin is indicated for use in PET imaging to identify any suspected sites of prostate cancer recurrence in men who have elevated blood levels of prostate-specific antigen (PSA) following prior treatment.
Recurrent disease is typically detected by a rise in PSA levels, but often the location and extent of the disease cannot be detected by conventional imaging. Axumin is a game-changer in this sense, as it could detect disease early enough to treat it successfully.
Additionally, with conventional imaging, a prostatic lesion must be large enough to be seen, whereas Axumin allows changes to be seen on the cellular level. Axumin’s ability to uncover small changes allows for much earlier detection compared to other modalities.
FDG for Prostate Imaging
Fluorodeoxyglucose (FDG) is currently the most commonly used PET tracer for a wide range of cancers, but its applications are limited for prostate cancer.
Since FDG is sugar based, it helps to identify areas of higher metabolic activity on the cellular level. These areas of increased uptake with increased cellular metabolism are typically caused by malignancies, infections or areas of the body that are actively healing.
In the case of prostate cancer, the cellular metabolism is typically much slower than other cancers. In cases where the metabolic rate is much slower or average in comparison to the rest of the body, there is no focal point that indicates to the physician that the prostate cancer is present and may result in a false-negative result.
Overall, FDG has low sensitivity for early stages of the disease, but has been reported to be sensitive in patients with higher PSA levels and more advanced cancer.
NaF for Prostate Imaging
Metastatic bone disease is a common problem for patients with recurring prostate cancer. NaF18 has been around as a bone-seeking tracer since the 1970s, but was not widely used due to the relative scarcity of PET. It has had a resurgence now that PET is more widely available.
A bone is a living, metabolically active tissue that is constantly breaking down and rebuilding itself. Areas that are impacted by cancer have a much higher rate of destruction/repair occurring.
NaF contains properties that are similar to what is used in the bone repair process throughout the body. Physicians utilize NaF to identify high density areas where the osteoblastic (repair) is occurring. These focal points where the NaF has accumulated are typically indicative of bone metastases.
NaF studies have found it has excellent diagnostic performance in the detection of bone metastases in staging and restaging of high-risk prostate cancer patients.
Current options for PET agents include choline and fluciclovine. These have excellent sensitivity for prostate imaging, but may not be specific enough to pick up the lesions from recurring cancers at an early stage.
The next agent being trialed in the U.S. (which is already in use in countries like Germany and Australia) is PSMA (Prostate Specific Membrane Antigen). Providers here hope that it will be available soon. PSMA isn’t just one agent, it is a component of the cell’s surface that is upregulated in prostate cancer. The PET tracers that detect it are very sensitive and specific for prostate cancer.
A PSMA study involves a PET camera and small molecule positron-emitting radiopharmaceutical that binds to the extracellular portion of the prostate-specific membrane antigen. These PSMA PET studies are used in patients with previously treated prostate cancer that now have clinical suspicion of recurrent prostate cancer.
Numerous studies have found PSMA-PET to be superior at detecting prostate cancer compared with CT and bone scans. Here is what one comprehensive study found:
“Dr. Wolfgang Fendler, of the University of California Los Angeles (UCLA), reported results from a phase 2/3 clinical trial testing 68Ga-PSMA-11 PET plus CT imaging in 250 patients experiencing a rising PSA after previous treatment with radical prostatectomy or radiation therapy.
68Ga-PSMA-11 PET/CT imaging detected at least one lesion in 79% of the patients, including in 41% of patients with PSA levels <0.5 ng/ml, and increasing to over 80% of patients with PSA levels ≥ 1 ng/ml and nearly 100% of patients with PSA levels ≥ 5 ng/ml. In 33 patients, surgery or biopsy on the lesions detected by PSMA-PET imaging confirmed 85% of these to be prostate cancer (85% accurate). When other methods of confirmation were additionally assessed, accuracy was found to be as high as 91%. In 23 patients, salvage surgery or radiotherapy was used to treat sites identified on PSMA-PET imaging, of whom 78% had a PSA drop of 50% or more, and 30% had PSA decreases to undetectable levels.”
PSMA PET MRI helps to characterize the prostate and tissue around it to determine treatment. The sensitivity of PSMA also helps to examine any metastatic disease around the area. It’s a quicker process for patients too.
The Future of Prostate Imaging
A prediction from Mark Moyad, M.D. of the Prostate Cancer Research Institute is that we will get much better at having patient-specific imaging choices. Each of the imaging options outlined here has applications for different stages of prostate cancer, and further research is underway.
For example, below are some of the current areas of research:
- New imaging agents for PET scanning of the prostate
- Hybrid imaging in which PSMA PET studies are combined with other imaging technologies such as computed tomography (CT) to improve image accuracy and to offer more targeted treatment
- The use of radioimmunotherapy
- New molecular imaging techniques that will:
- Predict the aggressiveness of a tumor
- Predict the outcome of treatment
- Detect genetic markers of the disease
- Assist physicians in developing even more tailored treatment plans.
We are hopeful that there will be more tools available in the arsenal for detection and monitoring of prostate cancer in the future.