A PSMA PET scan highlights areas where prostate-specific membrane antigen, a protein found on prostate cancer cells, is concentrated in the body. Reading the scan means understanding which bright spots are normal tissue, which are suspicious, and how radiologists score what they see. Whether you’re looking at your own images or trying to decode the written report, here’s what each element means.
What the Scan Actually Shows
PSMA PET scans work by injecting a small amount of radioactive tracer that binds to PSMA, a protein expressed at high levels on most prostate cancer cells. Two tracers are currently FDA-approved: Ga-68 PSMA-11, approved in December 2020, and Pylarify (piflufolastat F-18), approved shortly after. Both do the same basic job. Wherever the tracer accumulates, the PET scanner detects it and produces a bright signal on the image.
The scan is typically fused with a CT scan, so you see two layers of information at once: the PET layer showing tracer activity (often displayed in color) and the CT layer showing anatomy in grayscale. This combination lets radiologists pin a hot spot to a specific bone, lymph node, or organ rather than guessing at its location.
Normal Organs That Light Up
One of the most important things to understand is that many organs naturally express PSMA and will glow brightly on the scan even in a completely healthy person. This normal background pattern, called physiological biodistribution, can look alarming if you don’t know what to expect.
In a study of 229 patients, the organs with the highest normal tracer uptake, measured by SUVmax (a standard intensity score), were:
- Kidneys: SUVmax around 47, the brightest signal on the scan because the tracer is filtered and excreted through the urinary system
- Bladder: SUVmax around 29, again because the tracer collects in urine
- Salivary glands (submandibular and parotid): SUVmax around 17
- Tear glands (lacrimal glands): SUVmax around 13
- Small intestine: SUVmax around 13
- Spleen: SUVmax around 9
- Liver: SUVmax around 8
So if your scan shows intense bright areas in the kidneys, bladder, salivary glands, or tear glands, that is completely expected. These are not signs of cancer spread. The liver and spleen will also show moderate activity. Radiologists already know to ignore these areas and focus on uptake that appears where it shouldn’t.
What SUVmax Means on Your Report
SUVmax stands for maximum standardized uptake value. It’s the number radiologists use to quantify how intensely a spot on the scan is absorbing the tracer. A higher SUVmax means more tracer concentration, which typically means more PSMA expression. Normal or enlarged prostate tissue without cancer shows low to moderate PSMA expression, while prostate cancer cells often show significantly higher uptake that stands out clearly against normal tissue.
One study found that using a SUVmax cutoff of 9.1 for the primary tumor achieved 78% sensitivity and 81% specificity for identifying high-risk disease. In practical terms, a lesion with a SUVmax well above that range is more concerning, while faint or low-level uptake is less likely to represent aggressive cancer. Your report will list SUVmax values for individual lesions, and your doctor compares these to the surrounding normal tissue to judge significance.
The Visual Scoring System
Rather than relying on SUVmax numbers alone, many radiologists also use a simple visual comparison system called the miPSMA expression score. It compares how bright a suspicious lesion is relative to two internal reference points everyone has: the blood pool (the large blood vessels, which show low-level background activity) and the liver.
- Score 0: No PSMA expression, or expression below blood-pool level. Essentially invisible.
- Score 1: Low expression, equal to or above blood pool but below liver. Faint signal.
- Score 2: Intermediate expression, equal to or above liver but below the parotid (salivary) gland. Moderately bright.
- Score 3: High expression, equal to or above the parotid gland. Very bright.
This system is useful because it doesn’t depend on exact numbers, which can vary between scanners and tracers. If your report mentions a lesion with “expression above liver,” that’s a Score 2, which warrants attention. A lesion brighter than the parotid gland (Score 3) is highly suspicious.
PSMA-RADS: The Standardized Classification
Many radiology reports use a framework called PSMA-RADS to classify findings on a 1-to-5 scale, similar to how breast imaging uses BI-RADS. Each number carries a specific meaning:
- PSMA-RADS-1: Definitively benign. This includes lesions with no uptake that are clearly normal (1A) and lesions that do show uptake but are still definitively benign, like the salivary glands (1B).
- PSMA-RADS-2: Almost certainly benign. Low-level tracer activity in a location that would be unusual for prostate cancer to spread to.
- PSMA-RADS-3: Equivocal, meaning uncertain. This is the “gray zone” category, and it’s further subdivided. A 3A finding means faint uptake in soft tissue (like a lymph node) in a location where prostate cancer could plausibly spread. A 3B is an unclear bone lesion. A 3C is a spot with high uptake that might represent a different type of cancer entirely, not prostate. A 3D is a lesion that looks anatomically suspicious on CT but doesn’t actually take up the tracer. Findings in this category often require additional imaging or biopsy to clarify.
- PSMA-RADS-4: High likelihood of prostate cancer. The lesion has significant tracer uptake in a typical location for prostate cancer spread, but the CT portion of the scan doesn’t show a clear structural abnormality yet. Think of it as the tracer detecting cancer before the tumor is large enough to see on CT.
- PSMA-RADS-5: Almost certainly prostate cancer. Both the PET and CT findings point the same direction: high tracer uptake plus a visible anatomic lesion, such as an enlarged lymph node or a bone abnormality, in a location consistent with prostate cancer.
If your report lists a PSMA-RADS score, the number itself tells you the bottom line. Scores of 1 or 2 are reassuring. A score of 3 means more investigation is needed. Scores of 4 or 5 indicate cancer is likely or almost certain at that location.
Common False Positives
Not every bright spot is cancer. PSMA is expressed by some non-cancerous conditions, and knowing the common false positives can prevent unnecessary alarm. In bone, areas that light up but turn out to be benign are often caused by conditions like Paget’s disease (a bone remodeling disorder), fibrous dysplasia, healing fractures, or hyperplastic bone marrow. These are sometimes called unspecific bone uptakes (UBUs), and when biopsied, they consistently turn out to be benign.
Benign conditions like ganglia (nerve clusters), hemangiomas (clusters of blood vessels), and areas of prior surgery or inflammation can also show tracer uptake. This is one reason the PSMA-RADS system exists: it forces radiologists to weigh both the tracer intensity and the anatomic location before calling something cancer. A bright spot in an unusual location for prostate cancer metastasis gets flagged as equivocal rather than definitively malignant.
How Sensitive the Scan Is
PSMA PET is dramatically more sensitive than older imaging methods. Compared to traditional bone scans, PSMA PET/CT has a pooled sensitivity of 98% versus 83% for detecting bone metastases, and a specificity of 97% versus 68%. In practical terms, PSMA PET correctly identified bone metastases in 22.3% of patients whose conventional bone scan was negative. The reverse almost never happened: bone scans caught metastases that PSMA PET missed in only 1.9% of cases.
For detecting cancer recurrence after treatment, the scan’s detection rate depends heavily on your PSA level. At very low PSA levels (below 0.2 ng/mL), the scan still finds the source of recurrence about 67% of the time. Once PSA rises above 1.0 ng/mL, the detection rate exceeds 90%. This means that even with a barely detectable PSA rise, a PSMA PET has a reasonable chance of locating where the cancer has returned.
Key Terms You’ll See in Your Report
Radiology reports use specialized language that can be confusing. Here are the terms you’re most likely to encounter:
- Focal uptake: Tracer activity concentrated in a single, well-defined spot rather than spread diffusely. Focal uptake draws more concern than vague, scattered activity.
- Tracer avid / PSMA avid: The lesion absorbs the tracer actively. A “PSMA-avid lymph node” is a lymph node that lit up on the scan.
- Physiological uptake: Normal tracer activity in organs that naturally express PSMA (kidneys, salivary glands, liver). This is expected and not a sign of disease.
- Attenuation correction: A technical adjustment the scanner makes using the CT data to produce accurate PET images. You may see references to “attenuation-corrected” and “non-attenuation-corrected” images in the report. This is about image processing, not about your results.
- Index lesion: The most prominent or significant lesion identified, often the one with the highest SUVmax or the one driving treatment decisions.
- Osseous: Related to bone. “Osseous metastasis” means cancer that has spread to bone.
When reading your report, focus on the impression or conclusion section at the end. This is where the radiologist synthesizes everything into a clear summary: which findings are normal, which are suspicious, and what the PSMA-RADS classification is for each notable lesion. The detailed body of the report catalogs individual findings organ by organ, but the impression section tells you what it all means together.