A PET scan for multiple myeloma shows areas of active disease throughout the body by detecting cancer cells that are consuming sugar at an abnormally high rate. The scan highlights bone lesions, tumors inside and outside the bone marrow, and helps doctors gauge how aggressive the disease is and whether treatment is working. It’s one of several imaging tools used at diagnosis, during treatment, and after treatment to track the disease.
How the Scan Detects Myeloma
PET scans use a radioactive sugar tracer (FDG) that gets injected into your bloodstream. Cancer cells, including the malignant plasma cells in myeloma, burn through glucose faster than normal cells. The tracer collects in those high-metabolism spots, and the scanner picks them up as bright areas, called “hot spots,” on the image.
The scan is almost always combined with a CT scan (PET/CT), so your doctor sees both the metabolic activity and the physical structure of bones and tissues in one image. This combination is what makes it so useful for myeloma: it can reveal bone damage and active cancer at the same time, across the entire skeleton and soft tissues in a single session.
What It Reveals at Diagnosis
At the time of diagnosis, a PET scan shows several things that directly shape treatment decisions. The most common finding is focal lesions, which are distinct spots of active disease in the bones. These appear as bright areas against the dimmer background of normal bone marrow. The scan also picks up lytic lesions, places where myeloma has already destroyed bone tissue.
Beyond the skeleton, PET/CT can detect extramedullary disease, meaning myeloma that has spread outside the bone marrow into soft tissues and organs. The liver is the organ most commonly involved, followed by the pancreas, stomach, and areas of the peritoneum. Finding extramedullary disease changes the picture significantly because it typically signals more aggressive behavior.
The intensity of tracer uptake at each lesion is reported as a number called the SUVmax. Higher values generally indicate more metabolically active, aggressive disease. In one large study, most newly diagnosed patients had a baseline SUVmax above 4.2, and the small percentage who fell below that threshold tended to have a less aggressive form of the disease.
PET Scans for Smoldering Myeloma
Smoldering myeloma is a precursor condition that sometimes progresses to active myeloma and sometimes stays quiet for years. PET scans play a specific role here: finding even one focal lesion on PET dramatically changes the risk outlook. In a prospective study of 120 smoldering myeloma patients, 16% had a positive PET scan. Those with positive scans had a 58% probability of progressing to active myeloma within two years, compared to 33% for those with negative scans. Their median time to progression was just 1.1 years versus 4.5 years.
This makes PET a practical tool for deciding how closely to monitor someone with smoldering myeloma and whether early treatment might be warranted.
What PET Misses Compared to MRI
PET scans are powerful, but they have a notable blind spot: diffuse bone marrow infiltration. This is when myeloma cells spread evenly through the marrow rather than forming distinct lumps. In a prospective head-to-head comparison, whole-body MRI detected diffuse disease in 82% of myeloma patients, while PET/CT caught it in only 17%. Fewer than a quarter of cases with diffuse infiltration on MRI were visible on PET.
For focal lesions, the gap is narrower but still present. MRI detected at least one focal lesion in 83% of participants compared to 60% for PET/CT. Every focal lesion that PET found was also visible on MRI. Notably, all ten patients in the study who had no focal disease still showed diffuse marrow involvement on MRI, but only four of those ten showed anything on PET.
This is why current diagnostic guidelines from the International Myeloma Working Group recognize MRI focal lesions as a criterion that can help define active myeloma. In practice, many myeloma centers use both imaging methods at different points in care: PET for its whole-body metabolic snapshot and treatment monitoring, MRI for its superior sensitivity to marrow involvement.
Tracking Treatment Response
One of the most valuable uses of PET in myeloma is measuring how well treatment is working. Because the scan reflects real-time metabolic activity, it can show response to chemotherapy surprisingly early, with studies detecting meaningful drops in tracer uptake as soon as seven days after starting treatment.
Doctors compare the SUVmax between scans to quantify the change. A decrease of 25% or more in SUVmax after initial treatment has been linked to significantly better outcomes. In the IMAJEM study, patients whose SUVmax dropped by more than 25% had not yet reached their median progression-free survival at follow-up, while those with smaller decreases had a median of 22.6 months before their disease progressed.
After treatment, a PET scan that goes completely “cold,” showing no abnormal tracer uptake, is a strong positive sign. Research on patients before and after stem cell transplant found that those whose scans normalized had consistently better time to progression regardless of other test results. When PET results are combined with sensitive blood-based tests for leftover cancer cells, doctors get an even clearer picture of how deep the remission runs.
False Positives and Limitations
Not every bright spot on a PET scan is cancer. The tracer accumulates wherever cells are metabolically active, and several non-cancerous conditions can mimic myeloma activity. Infections are one of the most common culprits, especially after chemotherapy when the immune system is compromised. Inflammatory cells like activated macrophages absorb the tracer just as eagerly as cancer cells do.
Bone fractures, particularly insufficiency fractures that happen in weakened bones, also light up on PET. These can be tricky to distinguish from new myeloma lesions, though they tend to appear in characteristic locations and are often bilateral. Osteonecrosis, a known complication of the high-dose steroids commonly used in myeloma treatment, is another source of false positives.
On the other side, false negatives happen too. Some myeloma subtypes simply don’t consume glucose at the same elevated rate and won’t light up as expected. This is another reason PET results are always interpreted alongside blood work, bone marrow biopsies, and sometimes MRI.
Preparing for Your PET Scan
Preparation is straightforward but important for getting accurate results. You’ll need to fast for at least four hours before the scan, avoiding all food, sugary drinks, and anything other than plain water. If you’re receiving IV glucose or nutritional fluids, those need to stop at least four hours ahead of the tracer injection as well.
The reason for fasting is practical: eating raises your blood sugar and insulin levels, which causes normal cells throughout your body to absorb more glucose. That creates background “noise” on the scan that can obscure the cancer signal. Your blood sugar will be checked before the injection, and if it’s too high, the scan may need to be rescheduled. After the tracer is injected, you’ll rest quietly for about an hour while it circulates, then the scan itself typically takes 20 to 30 minutes.