PSMA PET vs Whole-body MRI for Prostate Cancer

Following Men’s Health Week, we’ve seen a significant increase in enquiries about prostate cancer imaging, particularly PSMA PET scans. As awareness of prostate cancer screening and early detection continues to grow, many men are asking how PSMA PET compares with other imaging techniques, including whole-body MRI, and which imaging test is best for their situation.

Like most things in medicine, the answer isn’t straightforward and ultimately depends on each individuals clinical indications, circumstances, values and preferences. However, in most cases the two modalities provide different but complementary information.

To help individuals better understand these imaging options and play an active role in their healthcare decisions, we’ve written this article to explain the key differences between PSMA PET and whole-body MRI, including how each scan works, their strengths and limitations, and the emerging role of whole-body MRI in the diagnosis, staging and long-term management of prostate cancer.

What is a PET scan?

PET (Positron Emission Tomography) is a type of nuclear medicine imaging that uses a small amount of radioactive tracer to detect certain types of biological activity within the body. The tracer accumulates in specific tissues depending on its intended target, allowing doctors to identify disease that may not yet be visible on conventional (structural) imaging. While PET is also used for non-cancer conditions (e.g. sarcoidosis or Alzheimer’s disease), the focus of this article is on the use of PET in oncology settings.

When cancer take up a radio-tracer, it is referred to as tracer avidity. How “avid” something is a quantifiable measure, with areas demonstrating increased avidity often appearing as “hot spots” on the scan. The more avid, or the hotter the spot, the increased likelihood of malignancy.

The most common radio-tracer used is known as FDG, or more precisely 18F-FDG, which is a radioactive form of glucose. Cancers are more metabolically active than the surrounding tissue and consume glucose faster, leading to an elevated accumulation of radioactive tracer compared to benign, normal tissues. However, not all cancers have sufficient metabolic activity for measurable avidity with FDG. In these cancer such as lobular breast cancer or prostate cancer, these tumour types are better detected by newer radio-tracers that detect cell-surface receptors (e.g. Estrogen receptor) or membrane proteins (e.g. PSMA), specific to the cancer itself. Because PET is essentially a functional map of increase avidity, it is often paired with CT scan for anatomical detail to localises areas of increased avidity.

What is PSMA PET?

In prostate cancer, PSMA PET is a specialised type of PET scan that uses a radio-tracer to target prostate-specific membrane antigen (PSMA), a protein found in high concentrations on the surface of most prostate cancer cells. After the tracer is injected into the vein, it binds to PSMA-expressing cells, allowing areas of prostate cancer throughout the body to be visualised on the PET scan. PSMA is also found in low concentrations in other healthy tissues of the body such as the salivary glands, kidneys, and in the small bowel. As a result, these tissues also show tracer uptake on PSMA PET scans and are considered normal by the reporting radiologist.

What is Whole-body MRI?

Whole-body MRI provides detailed anatomical and functional imaging of the body without using ionising radiation. Rather than detecting tumour biology, MRI evaluates tissue structure and physiology using multiple imaging sequences, including diffusion-weighted imaging (DWI), which is highly sensitive to changes in tissue cellularity commonly seen in cancer. Unlike PET imaging, whole-body MRI is not tumour biology dependent.

Whole-body MRI is particularly effective for assessing both boney and soft tissues, making it increasingly valuable in the assessment of metastatic cancer.

Which scan is better: PSMA PET or Whole-body MRI?

PSMA PET is currently the preferred imaging modality for staging and detecting recurrent prostate cancer in many clinical settings. However, comprehensive assessment at diagnosis and follow up often involves additional scan types including, CT and nuclear medicine bone scan. The increased imaging burden associated with a prostate cancer diagnosis has led to a growing interest in whether whole-body MRI can consolidate several conventional imaging tests into a single, comprehensive radiation free examination.

Whole-body MRI is already the preferred approach for some cancers such as multiple myeloma and metastatic bone disease such as sarcoma, with increasing body of evidence suggesting whole-body MRI may be as good as, if not superior to traditional diagnostic work ups in breast, especially lobular breast cancer subtype, melanoma, and prostate cancer. This 2024 review by Lecouvet and colleagues discusses the future of the modality including hurdles yet to overcome.

Figure: Metastatic Prostate Cancer on Whole-body MRI (A) and PSMA-PET (B). Same Patient Comparison

One aspect that makes whole-body MRI particularly appealing, is that unlike PSMA PET, it’s ability to detect disease is not dependent of the expression (“production”) of a specific receptor or antigen. Tumour biology can evolve over time, particularly following systemic therapy with selective pressure, with some lesions demonstrating reduced or non-uniform PSMA expression. In these situations, it is possible that the PSMA PET tracer uptake may no longer fully reflect the true disease burden. In contrast, whole-body MRI evaluates tissue structure and cellularity rather than receptor expression. As such, it may provide a more consistent assessment of disease through out the course of treatment. For this reason, whole-body MRI and PSMA PET are increasingly viewed as complementary.

Can whole-body MRI replace PSMA PET?

From the perspective of current consensus, no, not yet. At this time, these two tests are considered complementary and work together, and are often used together. While early research is encouraging, further prospective studies are required to determine if whole-body MRI can replace PSMA PET in selected clinical settings.

That said, we understand the appeal of a single, radiation-free imaging option. Individuals who feel that whole-body MRI better aligns with their personal preferences, circumstances and values to discuss with their treating doctor whether this scan may be beneficial. This may be particularly relevant for individuals who are concerned about the cumulative effects of radiation exposure or suspect non-PSMA expressing prostate cancer.

One strategy increasingly adopted by clinicians is to perform a baseline PET followed shortly afterwards by a whole-body MRI. This allows PET-avid lesions to be correlated with their appearance on MRI, creating a comprehensive imaging baseline. Future surveillance can then often be performed using whole-body MRI every 6–12 months, reducing cumulative radiation exposure while allowing previously identified lesions to be monitored over time.

Key take aways

PET is a nuclear medicine imaging technology, while the radioactive tracer determines what the scan detects.
PSMA PET is a specialised PET scan designed specifically for prostate cancer.
Whole-body MRI provides comprehensive anatomical and functional imaging without ionising radiation.
PSMA PET and whole-body MRI provide complementary information rather than competing information.
For many patients requiring long-term surveillance, a baseline PSMA PET followed by serial whole-body MRI may reduce radiation exposure while maintaining comprehensive disease monitoring.